The Tor Browser: media/libpng/libpng-manual.txt@6474c204b198 (annotated)

media/libpng/libpng-manual.txt@6474c204b198 (annotated)

media/libpng/libpng-manual.txt

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 libpng-manual.txt - A description on how to use and modify libpng
  libpng version 1.6.9 - February 6, 2014
  Updated and distributed by Glenn Randers-Pehrson
  <glennrp at users.sourceforge.net>
  Copyright (c) 1998-2014 Glenn Randers-Pehrson
  This document is released under the libpng license.
  For conditions of distribution and use, see the disclaimer
  and license in png.h
  Based on:
  libpng versions 0.97, January 1998, through 1.6.9 - February 6, 2014
  Updated and distributed by Glenn Randers-Pehrson
  Copyright (c) 1998-2014 Glenn Randers-Pehrson
  libpng 1.0 beta 6  version 0.96 May 28, 1997
  Updated and distributed by Andreas Dilger
  Copyright (c) 1996, 1997 Andreas Dilger
  libpng 1.0 beta 2 - version 0.88  January 26, 1996
  For conditions of distribution and use, see copyright
  notice in png.h. Copyright (c) 1995, 1996 Guy Eric
  Schalnat, Group 42, Inc.
  Updated/rewritten per request in the libpng FAQ
  Copyright (c) 1995, 1996 Frank J. T. Wojcik
  December 18, 1995 & January 20, 1996
  TABLE OF CONTENTS
     I. Introduction
    II. Structures
   III. Reading
    IV. Writing
     V. Simplified API
    VI. Modifying/Customizing libpng
   VII. MNG support
  VIII. Changes to Libpng from version 0.88
    IX. Changes to Libpng from version 1.0.x to 1.2.x
     X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
    XI. Changes to Libpng from version 1.4.x to 1.5.x
   XII. Changes to Libpng from version 1.5.x to 1.6.x
  XIII. Detecting libpng
   XIV. Source code repository
    XV. Coding style
   XVI. Y2K Compliance in libpng
 I. Introduction
 This file describes how to use and modify the PNG reference library
 (known as libpng) for your own use.  In addition to this
 file, example.c is a good starting point for using the library, as
 it is heavily commented and should include everything most people
 will need.  We assume that libpng is already installed; see the
 INSTALL file for instructions on how to install libpng.
 For examples of libpng usage, see the files "example.c", "pngtest.c",
 and the files in the "contrib" directory, all of which are included in
 the libpng distribution.
 Libpng was written as a companion to the PNG specification, as a way
 of reducing the amount of time and effort it takes to support the PNG
 file format in application programs.
 The PNG specification (second edition), November 2003, is available as
 a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
 <http://www.w3.org/TR/2003/REC-PNG-20031110/
 The W3C and ISO documents have identical technical content.
 The PNG-1.2 specification is available at
 <http://www.libpng.org/pub/png/documents/>.  It is technically equivalent
 to the PNG specification (second edition) but has some additional material.
 The PNG-1.0 specification is available
 as RFC 2083 <http://www.libpng.org/pub/png/documents/> and as a
 W3C Recommendation <http://www.w3.org/TR/REC.png.html>.
 Some additional chunks are described in the special-purpose public chunks
 documents at <http://www.libpng.org/pub/png/documents/>.
 Other information
 about PNG, and the latest version of libpng, can be found at the PNG home
 page, <http://www.libpng.org/pub/png/>.
 Most users will not have to modify the library significantly; advanced
 users may want to modify it more.  All attempts were made to make it as
 complete as possible, while keeping the code easy to understand.
 Currently, this library only supports C.  Support for other languages
 is being considered.
 Libpng has been designed to handle multiple sessions at one time,
 to be easily modifiable, to be portable to the vast majority of
 machines (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy
 to use.  The ultimate goal of libpng is to promote the acceptance of
 the PNG file format in whatever way possible.  While there is still
 work to be done (see the TODO file), libpng should cover the
 majority of the needs of its users.
 Libpng uses zlib for its compression and decompression of PNG files.
 Further information about zlib, and the latest version of zlib, can
 be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
 The zlib compression utility is a general purpose utility that is
 useful for more than PNG files, and can be used without libpng.
 See the documentation delivered with zlib for more details.
 You can usually find the source files for the zlib utility wherever you
 find the libpng source files.
 Libpng is thread safe, provided the threads are using different
 instances of the structures.  Each thread should have its own
 png_struct and png_info instances, and thus its own image.
 Libpng does not protect itself against two threads using the
 same instance of a structure.
 II. Structures
 There are two main structures that are important to libpng, png_struct
 and png_info.  Both are internal structures that are no longer exposed
 in the libpng interface (as of libpng 1.5.0).
 The png_info structure is designed to provide information about the
 PNG file.  At one time, the fields of png_info were intended to be
 directly accessible to the user.  However, this tended to cause problems
 with applications using dynamically loaded libraries, and as a result
 a set of interface functions for png_info (the png_get_*() and png_set_*()
 functions) was developed, and direct access to the png_info fields was
 deprecated..
 The png_struct structure is the object used by the library to decode a
 single image.  As of 1.5.0 this structure is also not exposed.
 Almost all libpng APIs require a pointer to a png_struct as the first argument.
 Many (in particular the png_set and png_get APIs) also require a pointer
 to png_info as the second argument.  Some application visible macros
 defined in png.h designed for basic data access (reading and writing
 integers in the PNG format) don't take a png_info pointer, but it's almost
 always safe to assume that a (png_struct*) has to be passed to call an API
 function.
 You can have more than one png_info structure associated with an image,
 as illustrated in pngtest.c, one for information valid prior to the
 IDAT chunks and another (called "end_info" below) for things after them.
 The png.h header file is an invaluable reference for programming with libpng.
 And while I'm on the topic, make sure you include the libpng header file:
 #include <png.h>
 and also (as of libpng-1.5.0) the zlib header file, if you need it:
 #include <zlib.h>
 Types
 The png.h header file defines a number of integral types used by the
 APIs.  Most of these are fairly obvious; for example types corresponding
 to integers of particular sizes and types for passing color values.
 One exception is how non-integral numbers are handled.  For application
 convenience most APIs that take such numbers have C (double) arguments;
 however, internally PNG, and libpng, use 32 bit signed integers and encode
 the value by multiplying by 100,000.  As of libpng 1.5.0 a convenience
 macro PNG_FP_1 is defined in png.h along with a type (png_fixed_point)
 which is simply (png_int_32).
 All APIs that take (double) arguments also have a matching API that
 takes the corresponding fixed point integer arguments.  The fixed point
 API has the same name as the floating point one with "_fixed" appended.
 The actual range of values permitted in the APIs is frequently less than
 the full range of (png_fixed_point) (-21474 to +21474).  When APIs require
 a non-negative argument the type is recorded as png_uint_32 above.  Consult
 the header file and the text below for more information.
 Special care must be take with sCAL chunk handling because the chunk itself
 uses non-integral values encoded as strings containing decimal floating point
 numbers.  See the comments in the header file.
 Configuration
 The main header file function declarations are frequently protected by C
 preprocessing directives of the form:
     #ifdef PNG_feature_SUPPORTED
     declare-function
     #endif
     ...
     #ifdef PNG_feature_SUPPORTED
     use-function
     #endif
 The library can be built without support for these APIs, although a
 standard build will have all implemented APIs.  Application programs
 should check the feature macros before using an API for maximum
 portability.  From libpng 1.5.0 the feature macros set during the build
 of libpng are recorded in the header file "pnglibconf.h" and this file
 is always included by png.h.
 If you don't need to change the library configuration from the default, skip to
 the next section ("Reading").
 Notice that some of the makefiles in the 'scripts' directory and (in 1.5.0) all
 of the build project files in the 'projects' directory simply copy
 scripts/pnglibconf.h.prebuilt to pnglibconf.h.  This means that these build
 systems do not permit easy auto-configuration of the library - they only
 support the default configuration.
 The easiest way to make minor changes to the libpng configuration when
 auto-configuration is supported is to add definitions to the command line
 using (typically) CPPFLAGS.  For example:
 CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC
 will change the internal libpng math implementation for gamma correction and
 other arithmetic calculations to fixed point, avoiding the need for fast
 floating point support.  The result can be seen in the generated pnglibconf.h -
 make sure it contains the changed feature macro setting.
 If you need to make more extensive configuration changes - more than one or two
 feature macro settings - you can either add -DPNG_USER_CONFIG to the build
 command line and put a list of feature macro settings in pngusr.h or you can set
 DFA_XTRA (a makefile variable) to a file containing the same information in the
 form of 'option' settings.
 A. Changing pnglibconf.h
 A variety of methods exist to build libpng.  Not all of these support
 reconfiguration of pnglibconf.h.  To reconfigure pnglibconf.h it must either be
 rebuilt from scripts/pnglibconf.dfa using awk or it must be edited by hand.
 Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
 pnglibconf.h and changing the lines defining the supported features, paying
 very close attention to the 'option' information in scripts/pnglibconf.dfa
 that describes those features and their requirements.  This is easy to get
 wrong.
 B. Configuration using DFA_XTRA
 Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a later
 variant such as 'nawk' or 'gawk', is available.  The configure build will
 automatically find an appropriate awk and build pnglibconf.h.
 The scripts/pnglibconf.mak file contains a set of make rules for doing the
 same thing if configure is not used, and many of the makefiles in the scripts
 directory use this approach.
 When rebuilding simply write a new file containing changed options and set
 DFA_XTRA to the name of this file.  This causes the build to append the new file
 to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file should contain lines
 of the following forms:
 everything = off
 This turns all optional features off.  Include it at the start of pngusr.dfa to
 make it easier to build a minimal configuration.  You will need to turn at least
 some features on afterward to enable either reading or writing code, or both.
 option feature on
 option feature off
 Enable or disable a single feature.  This will automatically enable other
 features required by a feature that is turned on or disable other features that
 require a feature which is turned off.  Conflicting settings will cause an error
 message to be emitted by awk.
 setting feature default value
 Changes the default value of setting 'feature' to 'value'.  There are a small
 number of settings listed at the top of pnglibconf.h, they are documented in the
 source code.  Most of these values have performance implications for the library
 but most of them have no visible effect on the API.  Some can also be overridden
 from the API.
 This method of building a customized pnglibconf.h is illustrated in
 contrib/pngminim/*.  See the "$(PNGCONF):" target in the makefile and
 pngusr.dfa in these directories.
 C. Configuration using PNG_USR_CONFIG
 If -DPNG_USR_CONFIG is added to the CFLAGS when pnglibconf.h is built the file
 pngusr.h will automatically be included before the options in
 scripts/pnglibconf.dfa are processed.  Your pngusr.h file should contain only
 macro definitions turning features on or off or setting settings.
 Apart from the global setting "everything = off" all the options listed above
 can be set using macros in pngusr.h:
 #define PNG_feature_SUPPORTED
 is equivalent to:
 option feature on
 #define PNG_NO_feature
 is equivalent to:
 option feature off
 #define PNG_feature value
 is equivalent to:
 setting feature default value
 Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
 pngusr file you supply override the contents of scripts/pnglibconf.dfa
 If confusing or incomprehensible behavior results it is possible to
 examine the intermediate file pnglibconf.dfn to find the full set of
 dependency information for each setting and option.  Simply locate the
 feature in the file and read the C comments that precede it.
 This method is also illustrated in the contrib/pngminim/* makefiles and
 pngusr.h.
 III. Reading
 We'll now walk you through the possible functions to call when reading
 in a PNG file sequentially, briefly explaining the syntax and purpose
 of each one.  See example.c and png.h for more detail.  While
 progressive reading is covered in the next section, you will still
 need some of the functions discussed in this section to read a PNG
 file.
 Setup
 You will want to do the I/O initialization(*) before you get into libpng,
 so if it doesn't work, you don't have much to undo.  Of course, you
 will also want to insure that you are, in fact, dealing with a PNG
 file.  Libpng provides a simple check to see if a file is a PNG file.
 To use it, pass in the first 1 to 8 bytes of the file to the function
 png_sig_cmp(), and it will return 0 (false) if the bytes match the
 corresponding bytes of the PNG signature, or nonzero (true) otherwise.
 Of course, the more bytes you pass in, the greater the accuracy of the
 prediction.
 If you are intending to keep the file pointer open for use in libpng,
 you must ensure you don't read more than 8 bytes from the beginning
 of the file, and you also have to make a call to png_set_sig_bytes_read()
 with the number of bytes you read from the beginning.  Libpng will
 then only check the bytes (if any) that your program didn't read.
 (*): If you are not using the standard I/O functions, you will need
 to replace them with custom functions.  See the discussion under
 Customizing libpng.
     FILE *fp = fopen(file_name, "rb");
     if (!fp)
     {
        return (ERROR);
     }
     fread(header, 1, number, fp);
     is_png = !png_sig_cmp(header, 0, number);
     if (!is_png)
     {
        return (NOT_PNG);
     }
 Next, png_struct and png_info need to be allocated and initialized.  In
 order to ensure that the size of these structures is correct even with a
 dynamically linked libpng, there are functions to initialize and
 allocate the structures.  We also pass the library version, optional
 pointers to error handling functions, and a pointer to a data struct for
 use by the error functions, if necessary (the pointer and functions can
 be NULL if the default error handlers are to be used).  See the section
 on Changes to Libpng below regarding the old initialization functions.
 The structure allocation functions quietly return NULL if they fail to
 create the structure, so your application should check for that.
     png_structp png_ptr = png_create_read_struct
         (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
         user_error_fn, user_warning_fn);
     if (!png_ptr)
        return (ERROR);
     png_infop info_ptr = png_create_info_struct(png_ptr);
     if (!info_ptr)
     {
        png_destroy_read_struct(&png_ptr,
            (png_infopp)NULL, (png_infopp)NULL);
        return (ERROR);
     }
 If you want to use your own memory allocation routines,
 use a libpng that was built with PNG_USER_MEM_SUPPORTED defined, and use
 png_create_read_struct_2() instead of png_create_read_struct():
     png_structp png_ptr = png_create_read_struct_2
         (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
         user_error_fn, user_warning_fn, (png_voidp)
         user_mem_ptr, user_malloc_fn, user_free_fn);
 The error handling routines passed to png_create_read_struct()
 and the memory alloc/free routines passed to png_create_struct_2()
 are only necessary if you are not using the libpng supplied error
 handling and memory alloc/free functions.
 When libpng encounters an error, it expects to longjmp back
 to your routine.  Therefore, you will need to call setjmp and pass
 your png_jmpbuf(png_ptr).  If you read the file from different
 routines, you will need to update the longjmp buffer every time you enter
 a new routine that will call a png_*() function.
 See your documentation of setjmp/longjmp for your compiler for more
 information on setjmp/longjmp.  See the discussion on libpng error
 handling in the Customizing Libpng section below for more information
 on the libpng error handling.  If an error occurs, and libpng longjmp's
 back to your setjmp, you will want to call png_destroy_read_struct() to
 free any memory.
     if (setjmp(png_jmpbuf(png_ptr)))
     {
        png_destroy_read_struct(&png_ptr, &info_ptr,
            &end_info);
        fclose(fp);
        return (ERROR);
     }
 Pass (png_infopp)NULL instead of &end_info if you didn't create
 an end_info structure.
 If you would rather avoid the complexity of setjmp/longjmp issues,
 you can compile libpng with PNG_NO_SETJMP, in which case
 errors will result in a call to PNG_ABORT() which defaults to abort().
 You can #define PNG_ABORT() to a function that does something
 more useful than abort(), as long as your function does not
 return.
 Now you need to set up the input code.  The default for libpng is to
 use the C function fread().  If you use this, you will need to pass a
 valid FILE * in the function png_init_io().  Be sure that the file is
 opened in binary mode.  If you wish to handle reading data in another
 way, you need not call the png_init_io() function, but you must then
 implement the libpng I/O methods discussed in the Customizing Libpng
 section below.
     png_init_io(png_ptr, fp);
 If you had previously opened the file and read any of the signature from
 the beginning in order to see if this was a PNG file, you need to let
 libpng know that there are some bytes missing from the start of the file.
     png_set_sig_bytes(png_ptr, number);
 You can change the zlib compression buffer size to be used while
 reading compressed data with
     png_set_compression_buffer_size(png_ptr, buffer_size);
 where the default size is 8192 bytes.  Note that the buffer size
 is changed immediately and the buffer is reallocated immediately,
 instead of setting a flag to be acted upon later.
 If you want CRC errors to be handled in a different manner than
 the default, use
     png_set_crc_action(png_ptr, crit_action, ancil_action);
 The values for png_set_crc_action() say how libpng is to handle CRC errors in
 ancillary and critical chunks, and whether to use the data contained
 therein.  Note that it is impossible to "discard" data in a critical
 chunk.
 Choices for (int) crit_action are
    PNG_CRC_DEFAULT      0  error/quit
    PNG_CRC_ERROR_QUIT   1  error/quit
    PNG_CRC_WARN_USE     3  warn/use data
    PNG_CRC_QUIET_USE    4  quiet/use data
    PNG_CRC_NO_CHANGE    5  use the current value
 Choices for (int) ancil_action are
    PNG_CRC_DEFAULT      0  error/quit
    PNG_CRC_ERROR_QUIT   1  error/quit
    PNG_CRC_WARN_DISCARD 2  warn/discard data
    PNG_CRC_WARN_USE     3  warn/use data
    PNG_CRC_QUIET_USE    4  quiet/use data
    PNG_CRC_NO_CHANGE    5  use the current value
 Setting up callback code
 You can set up a callback function to handle any unknown chunks in the
 input stream. You must supply the function
     read_chunk_callback(png_structp png_ptr,
          png_unknown_chunkp chunk);
     {
        /* The unknown chunk structure contains your
           chunk data, along with similar data for any other
           unknown chunks: */
            png_byte name[5];
            png_byte *data;
            png_size_t size;
        /* Note that libpng has already taken care of
           the CRC handling */
        /* put your code here.  Search for your chunk in the
           unknown chunk structure, process it, and return one
           of the following: */
        return (-n); /* chunk had an error */
        return (0); /* did not recognize */
        return (n); /* success */
     }
 (You can give your function another name that you like instead of
 "read_chunk_callback")
 To inform libpng about your function, use
     png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
         read_chunk_callback);
 This names not only the callback function, but also a user pointer that
 you can retrieve with
     png_get_user_chunk_ptr(png_ptr);
 If you call the png_set_read_user_chunk_fn() function, then all unknown
 chunks which the callback does not handle will be saved when read.  You can
 cause them to be discarded by returning '1' ("handled") instead of '0'.  This
 behavior will change in libpng 1.7 and the default handling set by the
 png_set_keep_unknown_chunks() function, described below, will be used when the
 callback returns 0.  If you want the existing behavior you should set the global
 default to PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
 versions of libpng and with 1.7.  Libpng 1.6 issues a warning if you keep the
 default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns 0.
 At this point, you can set up a callback function that will be
 called after each row has been read, which you can use to control
 a progress meter or the like.  It's demonstrated in pngtest.c.
 You must supply a function
     void read_row_callback(png_structp png_ptr,
        png_uint_32 row, int pass);
     {
       /* put your code here */
     }
 (You can give it another name that you like instead of "read_row_callback")
 To inform libpng about your function, use
     png_set_read_status_fn(png_ptr, read_row_callback);
 When this function is called the row has already been completely processed and
 the 'row' and 'pass' refer to the next row to be handled.  For the
 non-interlaced case the row that was just handled is simply one less than the
 passed in row number, and pass will always be 0.  For the interlaced case the
 same applies unless the row value is 0, in which case the row just handled was
 the last one from one of the preceding passes.  Because interlacing may skip a
 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
 need to know what the last pass is record (row,pass) from the callback and use
 the last recorded value each time.
 As with the user transform you can find the output row using the
 PNG_ROW_FROM_PASS_ROW macro.
 Unknown-chunk handling
 Now you get to set the way the library processes unknown chunks in the
 input PNG stream. Both known and unknown chunks will be read.  Normal
 behavior is that known chunks will be parsed into information in
 various info_ptr members while unknown chunks will be discarded. This
 behavior can be wasteful if your application will never use some known
 chunk types. To change this, you can call:
     png_set_keep_unknown_chunks(png_ptr, keep,
         chunk_list, num_chunks);
     keep       - 0: default unknown chunk handling
 : ignore; do not keep
 : keep only if safe-to-copy
 : keep even if unsafe-to-copy
                You can use these definitions:
                  PNG_HANDLE_CHUNK_AS_DEFAULT   0
                  PNG_HANDLE_CHUNK_NEVER        1
                  PNG_HANDLE_CHUNK_IF_SAFE      2
                  PNG_HANDLE_CHUNK_ALWAYS       3
     chunk_list - list of chunks affected (a byte string,
                  five bytes per chunk, NULL or '\0' if
                  num_chunks is positive; ignored if
                  numchunks <= 0).
     num_chunks - number of chunks affected; if 0, all
                  unknown chunks are affected.  If positive,
                  only the chunks in the list are affected,
                  and if negative all unknown chunks and
                  all known chunks except for the IHDR,
                  PLTE, tRNS, IDAT, and IEND chunks are
                  affected.
 Unknown chunks declared in this way will be saved as raw data onto a
 list of png_unknown_chunk structures.  If a chunk that is normally
 known to libpng is named in the list, it will be handled as unknown,
 according to the "keep" directive.  If a chunk is named in successive
 instances of png_set_keep_unknown_chunks(), the final instance will
 take precedence.  The IHDR and IEND chunks should not be named in
 chunk_list; if they are, libpng will process them normally anyway.
 If you know that your application will never make use of some particular
 chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.
 Here is an example of the usage of png_set_keep_unknown_chunks(),
 where the private "vpAg" chunk will later be processed by a user chunk
 callback function:
     png_byte vpAg[5]={118, 112,  65, 103, (png_byte) '\0'};
     #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
       png_byte unused_chunks[]=
       {
 ,  73,  83,  84, (png_byte) '\0',   /* hIST */
 ,  84,  88, 116, (png_byte) '\0',   /* iTXt */
 ,  67,  65,  76, (png_byte) '\0',   /* pCAL */
 ,  67,  65,  76, (png_byte) '\0',   /* sCAL */
 ,  80,  76,  84, (png_byte) '\0',   /* sPLT */
 ,  73,  77,  69, (png_byte) '\0',   /* tIME */
       };
     #endif
     ...
     #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
       /* ignore all unknown chunks
        * (use global setting "2" for libpng16 and earlier):
        */
       png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
       /* except for vpAg: */
       png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);
       /* also ignore unused known chunks: */
       png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
          (int)(sizeof unused_chunks)/5);
     #endif
 User limits
 The PNG specification allows the width and height of an image to be as
 large as 2^31-1 (0x7fffffff), or about 2.147 billion rows and columns.
 Since very few applications really need to process such large images,
 we have imposed an arbitrary 1-million limit on rows and columns.
 Larger images will be rejected immediately with a png_error() call. If
 you wish to change this limit, you can use
    png_set_user_limits(png_ptr, width_max, height_max);
 to set your own limits, or use width_max = height_max = 0x7fffffffL
 to allow all valid dimensions (libpng may reject some very large images
 anyway because of potential buffer overflow conditions).
 You should put this statement after you create the PNG structure and
 before calling png_read_info(), png_read_png(), or png_process_data().
 When writing a PNG datastream, put this statement before calling
 png_write_info() or png_write_png().
 If you need to retrieve the limits that are being applied, use
    width_max = png_get_user_width_max(png_ptr);
    height_max = png_get_user_height_max(png_ptr);
 The PNG specification sets no limit on the number of ancillary chunks
 allowed in a PNG datastream.  You can impose a limit on the total number
 of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored, with
    png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);
 where 0x7fffffffL means unlimited.  You can retrieve this limit with
    chunk_cache_max = png_get_chunk_cache_max(png_ptr);
 You can also set a limit on the amount of memory that a compressed chunk
 other than IDAT can occupy, with
    png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);
 and you can retrieve the limit with
    chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);
 Any chunks that would cause either of these limits to be exceeded will
 be ignored.
 Information about your system
 If you intend to display the PNG or to incorporate it in other image data you
 need to tell libpng information about your display or drawing surface so that
 libpng can convert the values in the image to match the display.
 From libpng-1.5.4 this information can be set before reading the PNG file
 header.  In earlier versions png_set_gamma() existed but behaved incorrectly if
 called before the PNG file header had been read and png_set_alpha_mode() did not
 exist.
 If you need to support versions prior to libpng-1.5.4 test the version number
 as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow the procedures
 described in the appropriate manual page.
 You give libpng the encoding expected by your system expressed as a 'gamma'
 value.  You can also specify a default encoding for the PNG file in
 case the required information is missing from the file.  By default libpng
 assumes that the PNG data matches your system, to keep this default call:
    png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/);
 or you can use the fixed point equivalent:
    png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
       PNG_FP_1/screen_gamma);
 If you don't know the gamma for your system it is probably 2.2 - a good
 approximation to the IEC standard for display systems (sRGB).  If images are
 too contrasty or washed out you got the value wrong - check your system
 documentation!
 Many systems permit the system gamma to be changed via a lookup table in the
 display driver, a few systems, including older Macs, change the response by
 default.  As of 1.5.4 three special values are available to handle common
 situations:
    PNG_DEFAULT_sRGB: Indicates that the system conforms to the
                      IEC 61966-2-1 standard.  This matches almost
                      all systems.
    PNG_GAMMA_MAC_18: Indicates that the system is an older
                      (pre Mac OS 10.6) Apple Macintosh system with
                      the default settings.
    PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
                      that the system expects data with no gamma
                      encoding.
 You would use the linear (unencoded) value if you need to process the pixel
 values further because this avoids the need to decode and re-encode each
 component value whenever arithmetic is performed.  A lot of graphics software
 uses linear values for this reason, often with higher precision component values
 to preserve overall accuracy.
 The second thing you may need to tell libpng about is how your system handles
 alpha channel information.  Some, but not all, PNG files contain an alpha
 channel.  To display these files correctly you need to compose the data onto a
 suitable background, as described in the PNG specification.
 Libpng only supports composing onto a single color (using png_set_background;
 see below).  Otherwise you must do the composition yourself and, in this case,
 you may need to call png_set_alpha_mode:
    #if PNG_LIBPNG_VER >= 10504
       png_set_alpha_mode(png_ptr, mode, screen_gamma);
    #else
       png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
    #endif
 The screen_gamma value is the same as the argument to png_set_gamma; however,
 how it affects the output depends on the mode.  png_set_alpha_mode() sets the
 file gamma default to 1/screen_gamma, so normally you don't need to call
 png_set_gamma.  If you need different defaults call png_set_gamma() before
 png_set_alpha_mode() - if you call it after it will override the settings made
 by png_set_alpha_mode().
 The mode is as follows:
     PNG_ALPHA_PNG: The data is encoded according to the PNG specification.  Red,
 green and blue, or gray, components are gamma encoded color
 values and are not premultiplied by the alpha value.  The
 alpha value is a linear measure of the contribution of the
 pixel to the corresponding final output pixel.
 You should normally use this format if you intend to perform
 color correction on the color values; most, maybe all, color
 correction software has no handling for the alpha channel and,
 anyway, the math to handle pre-multiplied component values is
 unnecessarily complex.
 Before you do any arithmetic on the component values you need
 to remove the gamma encoding and multiply out the alpha
 channel.  See the PNG specification for more detail.  It is
 important to note that when an image with an alpha channel is
 scaled, linear encoded, pre-multiplied component values must
 be used!
 The remaining modes assume you don't need to do any further color correction or
 that if you do, your color correction software knows all about alpha (it
 probably doesn't!)
     PNG_ALPHA_STANDARD:  The data libpng produces
 is encoded in the standard way
 assumed by most correctly written graphics software.
 The gamma encoding will be removed by libpng and the
 linear component values will be pre-multiplied by the
 alpha channel.
 With this format the final image must be re-encoded to
 match the display gamma before the image is displayed.
 If your system doesn't do that, yet still seems to
 perform arithmetic on the pixels without decoding them,
 it is broken - check out the modes below.
 With PNG_ALPHA_STANDARD libpng always produces linear
 component values, whatever screen_gamma you supply.  The
 screen_gamma value is, however, used as a default for
 the file gamma if the PNG file has no gamma information.
 If you call png_set_gamma() after png_set_alpha_mode() you
 will override the linear encoding.  Instead the
 pre-multiplied pixel values will be gamma encoded but
 the alpha channel will still be linear.  This may
 actually match the requirements of some broken software,
 but it is unlikely.
 While linear 8-bit data is often used it has
 insufficient precision for any image with a reasonable
 dynamic range.  To avoid problems, and if your software
 supports it, use png_set_expand_16() to force all
 components to 16 bits.
     PNG_ALPHA_OPTIMIZED: This mode is the same
 as PNG_ALPHA_STANDARD except that
 completely opaque pixels are gamma encoded according to
 the screen_gamma value.  Pixels with alpha less than 1.0
 will still have linear components.
 Use this format if you have control over your
 compositing software and so don't do other arithmetic
 (such as scaling) on the data you get from libpng.  Your
 compositing software can simply copy opaque pixels to
 the output but still has linear values for the
 non-opaque pixels.
 In normal compositing, where the alpha channel encodes
 partial pixel coverage (as opposed to broad area
 translucency), the inaccuracies of the 8-bit
 representation of non-opaque pixels are irrelevant.
 You can also try this format if your software is broken;
 it might look better.
     PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD;
 however, all component values,
 including the alpha channel are gamma encoded.  This is
 an appropriate format to try if your software, or more
 likely hardware, is totally broken, i.e., if it performs
 linear arithmetic directly on gamma encoded values.
 In most cases of broken software or hardware the bug in the final display
 manifests as a subtle halo around composited parts of the image.  You may not
 even perceive this as a halo; the composited part of the image may simply appear
 separate from the background, as though it had been cut out of paper and pasted
 on afterward.
 If you don't have to deal with bugs in software or hardware, or if you can fix
 them, there are three recommended ways of using png_set_alpha_mode():
    png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
        screen_gamma);
 You can do color correction on the result (libpng does not currently
 support color correction internally).  When you handle the alpha channel
 you need to undo the gamma encoding and multiply out the alpha.
    png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
        screen_gamma);
    png_set_expand_16(png_ptr);
 If you are using the high level interface, don't call png_set_expand_16();
 instead pass PNG_TRANSFORM_EXPAND_16 to the interface.
 With this mode you can't do color correction, but you can do arithmetic,
 including composition and scaling, on the data without further processing.
    png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
        screen_gamma);
 You can avoid the expansion to 16-bit components with this mode, but you
 lose the ability to scale the image or perform other linear arithmetic.
 All you can do is compose the result onto a matching output.  Since this
 mode is libpng-specific you also need to write your own composition
 software.
 If you don't need, or can't handle, the alpha channel you can call
 png_set_background() to remove it by compositing against a fixed color.  Don't
 call png_set_strip_alpha() to do this - it will leave spurious pixel values in
 transparent parts of this image.
    png_set_background(png_ptr, &background_color,
        PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);
 The background_color is an RGB or grayscale value according to the data format
 libpng will produce for you.  Because you don't yet know the format of the PNG
 file, if you call png_set_background at this point you must arrange for the
 format produced by libpng to always have 8-bit or 16-bit components and then
 store the color as an 8-bit or 16-bit color as appropriate.  The color contains
 separate gray and RGB component values, so you can let libpng produce gray or
 RGB output according to the input format, but low bit depth grayscale images
 must always be converted to at least 8-bit format.  (Even though low bit depth
 grayscale images can't have an alpha channel they can have a transparent
 color!)
 You set the transforms you need later, either as flags to the high level
 interface or libpng API calls for the low level interface.  For reference the
 settings and API calls required are:
 -bit values:
    PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
    png_set_expand(png_ptr); png_set_scale_16(png_ptr);
    If you must get exactly the same inaccurate results
    produced by default in versions prior to libpng-1.5.4,
    use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
    instead.
 -bit values:
    PNG_TRANSFORM_EXPAND_16
    png_set_expand_16(png_ptr);
 In either case palette image data will be expanded to RGB.  If you just want
 color data you can add PNG_TRANSFORM_GRAY_TO_RGB or png_set_gray_to_rgb(png_ptr)
 to the list.
 Calling png_set_background before the PNG file header is read will not work
 prior to libpng-1.5.4.  Because the failure may result in unexpected warnings or
 errors it is therefore much safer to call png_set_background after the head has
 been read.  Unfortunately this means that prior to libpng-1.5.4 it cannot be
 used with the high level interface.
 The high-level read interface
 At this point there are two ways to proceed; through the high-level
 read interface, or through a sequence of low-level read operations.
 You can use the high-level interface if (a) you are willing to read
 the entire image into memory, and (b) the input transformations
 you want to do are limited to the following set:
     PNG_TRANSFORM_IDENTITY      No transformation
     PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
 -bit accurately
     PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
 -bit less accurately
     PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
     PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                 samples to bytes
     PNG_TRANSFORM_PACKSWAP      Change order of packed
                                 pixels to LSB first
     PNG_TRANSFORM_EXPAND        Perform set_expand()
     PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
     PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                 sBIT depth
     PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                 to BGRA
     PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                 to AG
     PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                 to transparency
     PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
     PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                 to RGB (or GA to RGBA)
     PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits
 (This excludes setting a background color, doing gamma transformation,
 quantizing, and setting filler.)  If this is the case, simply do this:
     png_read_png(png_ptr, info_ptr, png_transforms, NULL)
 where png_transforms is an integer containing the bitwise OR of some
 set of transformation flags.  This call is equivalent to png_read_info(),
 followed the set of transformations indicated by the transform mask,
 then png_read_image(), and finally png_read_end().
 (The final parameter of this call is not yet used.  Someday it might point
 to transformation parameters required by some future input transform.)
 You must use png_transforms and not call any png_set_transform() functions
 when you use png_read_png().
 After you have called png_read_png(), you can retrieve the image data
 with
    row_pointers = png_get_rows(png_ptr, info_ptr);
 where row_pointers is an array of pointers to the pixel data for each row:
    png_bytep row_pointers[height];
 If you know your image size and pixel size ahead of time, you can allocate
 row_pointers prior to calling png_read_png() with
    if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
       png_error (png_ptr,
           "Image is too tall to process in memory");
    if (width > PNG_UINT_32_MAX/pixel_size)
       png_error (png_ptr,
           "Image is too wide to process in memory");
    row_pointers = png_malloc(png_ptr,
        height*(sizeof (png_bytep)));
    for (int i=0; i<height, i++)
       row_pointers[i]=NULL;  /* security precaution */
    for (int i=0; i<height, i++)
       row_pointers[i]=png_malloc(png_ptr,
           width*pixel_size);
    png_set_rows(png_ptr, info_ptr, &row_pointers);
 Alternatively you could allocate your image in one big block and define
 row_pointers[i] to point into the proper places in your block.
 If you use png_set_rows(), the application is responsible for freeing
 row_pointers (and row_pointers[i], if they were separately allocated).
 If you don't allocate row_pointers ahead of time, png_read_png() will
 do it, and it'll be free'ed by libpng when you call png_destroy_*().
 The low-level read interface
 If you are going the low-level route, you are now ready to read all
 the file information up to the actual image data.  You do this with a
 call to png_read_info().
     png_read_info(png_ptr, info_ptr);
 This will process all chunks up to but not including the image data.
 This also copies some of the data from the PNG file into the decode structure
 for use in later transformations.  Important information copied in is:
 ) The PNG file gamma from the gAMA chunk.  This overwrites the default value
 provided by an earlier call to png_set_gamma or png_set_alpha_mode.
 ) Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
 damages the information provided by an earlier call to png_set_background
 resulting in unexpected behavior.  Libpng-1.5.4 no longer does this.
 ) The number of significant bits in each component value.  Libpng uses this to
 optimize gamma handling by reducing the internal lookup table sizes.
 ) The transparent color information from a tRNS chunk.  This can be modified by
 a later call to png_set_tRNS.
 Querying the info structure
 Functions are used to get the information from the info_ptr once it
 has been read.  Note that these fields may not be completely filled
 in until png_read_end() has read the chunk data following the image.
     png_get_IHDR(png_ptr, info_ptr, &width, &height,
        &bit_depth, &color_type, &interlace_type,
        &compression_type, &filter_method);
     width          - holds the width of the image
                      in pixels (up to 2^31).
     height         - holds the height of the image
                      in pixels (up to 2^31).
     bit_depth      - holds the bit depth of one of the
                      image channels.  (valid values are
 , 2, 4, 8, 16 and depend also on
                      the color_type.  See also
                      significant bits (sBIT) below).
     color_type     - describes which color/alpha channels
                          are present.
                      PNG_COLOR_TYPE_GRAY
                         (bit depths 1, 2, 4, 8, 16)
                      PNG_COLOR_TYPE_GRAY_ALPHA
                         (bit depths 8, 16)
                      PNG_COLOR_TYPE_PALETTE
                         (bit depths 1, 2, 4, 8)
                      PNG_COLOR_TYPE_RGB
                         (bit_depths 8, 16)
                      PNG_COLOR_TYPE_RGB_ALPHA
                         (bit_depths 8, 16)
                      PNG_COLOR_MASK_PALETTE
                      PNG_COLOR_MASK_COLOR
                      PNG_COLOR_MASK_ALPHA
     interlace_type - (PNG_INTERLACE_NONE or
                      PNG_INTERLACE_ADAM7)
     compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                      for PNG 1.0)
     filter_method  - (must be PNG_FILTER_TYPE_BASE
                      for PNG 1.0, and can also be
                      PNG_INTRAPIXEL_DIFFERENCING if
                      the PNG datastream is embedded in
                      a MNG-1.0 datastream)
     Any or all of interlace_type, compression_type, or
     filter_method can be NULL if you are
     not interested in their values.
     Note that png_get_IHDR() returns 32-bit data into
     the application's width and height variables.
     This is an unsafe situation if these are 16-bit
     variables.  In such situations, the
     png_get_image_width() and png_get_image_height()
     functions described below are safer.
     width            = png_get_image_width(png_ptr,
                          info_ptr);
     height           = png_get_image_height(png_ptr,
                          info_ptr);
     bit_depth        = png_get_bit_depth(png_ptr,
                          info_ptr);
     color_type       = png_get_color_type(png_ptr,
                          info_ptr);
     interlace_type   = png_get_interlace_type(png_ptr,
                          info_ptr);
     compression_type = png_get_compression_type(png_ptr,
                          info_ptr);
     filter_method    = png_get_filter_type(png_ptr,
                          info_ptr);
     channels = png_get_channels(png_ptr, info_ptr);
     channels       - number of channels of info for the
                      color type (valid values are 1 (GRAY,
                      PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
 (RGB_ALPHA or RGB + filler byte))
     rowbytes = png_get_rowbytes(png_ptr, info_ptr);
     rowbytes       - number of bytes needed to hold a row
     signature = png_get_signature(png_ptr, info_ptr);
     signature      - holds the signature read from the
                      file (if any).  The data is kept in
                      the same offset it would be if the
                      whole signature were read (i.e. if an
                      application had already read in 4
                      bytes of signature before starting
                      libpng, the remaining 4 bytes would
                      be in signature[4] through signature[7]
                      (see png_set_sig_bytes())).
 These are also important, but their validity depends on whether the chunk
 has been read.  The png_get_valid(png_ptr, info_ptr, PNG_INFO_<chunk>) and
 png_get_<chunk>(png_ptr, info_ptr, ...) functions return non-zero if the
 data has been read, or zero if it is missing.  The parameters to the
 png_get_<chunk> are set directly if they are simple data types, or a
 pointer into the info_ptr is returned for any complex types.
 The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks
 is simply returned to give the application information about how the
 image was encoded.  Libpng itself only does transformations using the file
 gamma when combining semitransparent pixels with the background color, and,
 since libpng-1.6.0, when converting between 8-bit sRGB and 16-bit linear pixels
 within the simplified API.  Libpng also uses the file gamma when converting
 RGB to gray, beginning with libpng-1.0.5, if the application calls
 png_set_rgb_to_gray()).
     png_get_PLTE(png_ptr, info_ptr, &palette,
                      &num_palette);
     palette        - the palette for the file
                      (array of png_color)
     num_palette    - number of entries in the palette
     png_get_gAMA(png_ptr, info_ptr, &file_gamma);
     png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);
     file_gamma     - the gamma at which the file is
                      written (PNG_INFO_gAMA)
     int_file_gamma - 100,000 times the gamma at which the
                      file is written
     png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
                      &red_y, &green_x, &green_y, &blue_x, &blue_y)
     png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
                      &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
                      &blue_Z)
     png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
                      &int_white_y, &int_red_x, &int_red_y,
                      &int_green_x, &int_green_y, &int_blue_x,
                      &int_blue_y)
     png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
                      &int_red_Z, &int_green_X, &int_green_Y,
                      &int_green_Z, &int_blue_X, &int_blue_Y,
                      &int_blue_Z)
     {white,red,green,blue}_{x,y}
                      A color space encoding specified using the
                      chromaticities of the end points and the
                      white point. (PNG_INFO_cHRM)
     {red,green,blue}_{X,Y,Z}
                      A color space encoding specified using the
                      encoding end points - the CIE tristimulus
                      specification of the intended color of the red,
                      green and blue channels in the PNG RGB data.
                      The white point is simply the sum of the three
                      end points. (PNG_INFO_cHRM)
     png_get_sRGB(png_ptr, info_ptr, &srgb_intent);
     file_srgb_intent - the rendering intent (PNG_INFO_sRGB)
                      The presence of the sRGB chunk
                      means that the pixel data is in the
                      sRGB color space.  This chunk also
                      implies specific values of gAMA and
                      cHRM.
     png_get_iCCP(png_ptr, info_ptr, &name,
        &compression_type, &profile, &proflen);
     name             - The profile name.
     compression_type - The compression type; always
                        PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                        You may give NULL to this argument to
                        ignore it.
     profile          - International Color Consortium color
                        profile data. May contain NULs.
     proflen          - length of profile data in bytes.
     png_get_sBIT(png_ptr, info_ptr, &sig_bit);
     sig_bit        - the number of significant bits for
                      (PNG_INFO_sBIT) each of the gray,
                      red, green, and blue channels,
                      whichever are appropriate for the
                      given color type (png_color_16)
     png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                      &num_trans, &trans_color);
     trans_alpha    - array of alpha (transparency)
                      entries for palette (PNG_INFO_tRNS)
     num_trans      - number of transparent entries
                      (PNG_INFO_tRNS)
     trans_color    - graylevel or color sample values of
                      the single transparent color for
                      non-paletted images (PNG_INFO_tRNS)
     png_get_hIST(png_ptr, info_ptr, &hist);
                      (PNG_INFO_hIST)
     hist           - histogram of palette (array of
                      png_uint_16)
     png_get_tIME(png_ptr, info_ptr, &mod_time);
     mod_time       - time image was last modified
                     (PNG_VALID_tIME)
     png_get_bKGD(png_ptr, info_ptr, &background);
     background     - background color (of type
                      png_color_16p) (PNG_VALID_bKGD)
                      valid 16-bit red, green and blue
                      values, regardless of color_type
     num_comments   = png_get_text(png_ptr, info_ptr,
                      &text_ptr, &num_text);
     num_comments   - number of comments
     text_ptr       - array of png_text holding image
                      comments
     text_ptr[i].compression - type of compression used
                  on "text" PNG_TEXT_COMPRESSION_NONE
                            PNG_TEXT_COMPRESSION_zTXt
                            PNG_ITXT_COMPRESSION_NONE
                            PNG_ITXT_COMPRESSION_zTXt
     text_ptr[i].key   - keyword for comment.  Must contain
 -79 characters.
     text_ptr[i].text  - text comments for current
                          keyword.  Can be empty.
     text_ptr[i].text_length - length of text string,
                  after decompression, 0 for iTXt
     text_ptr[i].itxt_length - length of itxt string,
                  after decompression, 0 for tEXt/zTXt
     text_ptr[i].lang  - language of comment (empty
                          string for unknown).
     text_ptr[i].lang_key  - keyword in UTF-8
                          (empty string for unknown).
     Note that the itxt_length, lang, and lang_key
     members of the text_ptr structure only exist when the
     library is built with iTXt chunk support.  Prior to
     libpng-1.4.0 the library was built by default without
     iTXt support. Also note that when iTXt is supported,
     they contain NULL pointers when the "compression"
     field contains PNG_TEXT_COMPRESSION_NONE or
     PNG_TEXT_COMPRESSION_zTXt.
     num_text       - number of comments (same as
                      num_comments; you can put NULL here
                      to avoid the duplication)
     Note while png_set_text() will accept text, language,
     and translated keywords that can be NULL pointers, the
     structure returned by png_get_text will always contain
     regular zero-terminated C strings.  They might be
     empty strings but they will never be NULL pointers.
     num_spalettes = png_get_sPLT(png_ptr, info_ptr,
        &palette_ptr);
     num_spalettes  - number of sPLT chunks read.
     palette_ptr    - array of palette structures holding
                      contents of one or more sPLT chunks
                      read.
     png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
        &unit_type);
     offset_x       - positive offset from the left edge
                      of the screen (can be negative)
     offset_y       - positive offset from the top edge
                      of the screen (can be negative)
     unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
     png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
        &unit_type);
     res_x          - pixels/unit physical resolution in
                      x direction
     res_y          - pixels/unit physical resolution in
                      x direction
     unit_type      - PNG_RESOLUTION_UNKNOWN,
                      PNG_RESOLUTION_METER
     png_get_sCAL(png_ptr, info_ptr, &unit, &width,
        &height)
     unit        - physical scale units (an integer)
     width       - width of a pixel in physical scale units
     height      - height of a pixel in physical scale units
                  (width and height are doubles)
     png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
        &height)
     unit        - physical scale units (an integer)
     width       - width of a pixel in physical scale units
                   (expressed as a string)
     height      - height of a pixel in physical scale units
                  (width and height are strings like "2.54")
     num_unknown_chunks = png_get_unknown_chunks(png_ptr,
        info_ptr, &unknowns)
     unknowns          - array of png_unknown_chunk
                         structures holding unknown chunks
     unknowns[i].name  - name of unknown chunk
     unknowns[i].data  - data of unknown chunk
     unknowns[i].size  - size of unknown chunk's data
     unknowns[i].location - position of chunk in file
     The value of "i" corresponds to the order in which the
     chunks were read from the PNG file or inserted with the
     png_set_unknown_chunks() function.
     The value of "location" is a bitwise "or" of
          PNG_HAVE_IHDR  (0x01)
          PNG_HAVE_PLTE  (0x02)
          PNG_AFTER_IDAT (0x08)
 The data from the pHYs chunk can be retrieved in several convenient
 forms:
     res_x = png_get_x_pixels_per_meter(png_ptr,
        info_ptr)
     res_y = png_get_y_pixels_per_meter(png_ptr,
        info_ptr)
     res_x_and_y = png_get_pixels_per_meter(png_ptr,
        info_ptr)
     res_x = png_get_x_pixels_per_inch(png_ptr,
        info_ptr)
     res_y = png_get_y_pixels_per_inch(png_ptr,
        info_ptr)
     res_x_and_y = png_get_pixels_per_inch(png_ptr,
        info_ptr)
     aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
        info_ptr)
     Each of these returns 0 [signifying "unknown"] if
        the data is not present or if res_x is 0;
        res_x_and_y is 0 if res_x != res_y
     Note that because of the way the resolutions are
        stored internally, the inch conversions won't
        come out to exactly even number.  For example,
 dpi is stored as 0.28346 pixels/meter, and
        when this is retrieved it is 71.9988 dpi, so
        be sure to round the returned value appropriately
        if you want to display a reasonable-looking result.
 The data from the oFFs chunk can be retrieved in several convenient
 forms:
     x_offset = png_get_x_offset_microns(png_ptr, info_ptr);
     y_offset = png_get_y_offset_microns(png_ptr, info_ptr);
     x_offset = png_get_x_offset_inches(png_ptr, info_ptr);
     y_offset = png_get_y_offset_inches(png_ptr, info_ptr);
     Each of these returns 0 [signifying "unknown" if both
        x and y are 0] if the data is not present or if the
        chunk is present but the unit is the pixel.  The
        remark about inexact inch conversions applies here
        as well, because a value in inches can't always be
        converted to microns and back without some loss
        of precision.
 For more information, see the
 PNG specification for chunk contents.  Be careful with trusting
 rowbytes, as some of the transformations could increase the space
 needed to hold a row (expand, filler, gray_to_rgb, etc.).
 See png_read_update_info(), below.
 A quick word about text_ptr and num_text.  PNG stores comments in
 keyword/text pairs, one pair per chunk, with no limit on the number
 of text chunks, and a 2^31 byte limit on their size.  While there are
 suggested keywords, there is no requirement to restrict the use to these
 strings.  It is strongly suggested that keywords and text be sensible
 to humans (that's the point), so don't use abbreviations.  Non-printing
 symbols are not allowed.  See the PNG specification for more details.
 There is also no requirement to have text after the keyword.
 Keywords should be limited to 79 Latin-1 characters without leading or
 trailing spaces, but non-consecutive spaces are allowed within the
 keyword.  It is possible to have the same keyword any number of times.
 The text_ptr is an array of png_text structures, each holding a
 pointer to a language string, a pointer to a keyword and a pointer to
 a text string.  The text string, language code, and translated
 keyword may be empty or NULL pointers.  The keyword/text
 pairs are put into the array in the order that they are received.
 However, some or all of the text chunks may be after the image, so, to
 make sure you have read all the text chunks, don't mess with these
 until after you read the stuff after the image.  This will be
 mentioned again below in the discussion that goes with png_read_end().
 Input transformations
 After you've read the header information, you can set up the library
 to handle any special transformations of the image data.  The various
 ways to transform the data will be described in the order that they
 should occur.  This is important, as some of these change the color
 type and/or bit depth of the data, and some others only work on
 certain color types and bit depths.
 Transformations you request are ignored if they don't have any meaning for a
 particular input data format.  However some transformations can have an effect
 as a result of a previous transformation.  If you specify a contradictory set of
 transformations, for example both adding and removing the alpha channel, you
 cannot predict the final result.
 The color used for the transparency values should be supplied in the same
 format/depth as the current image data.  It is stored in the same format/depth
 as the image data in a tRNS chunk, so this is what libpng expects for this data.
 The color used for the background value depends on the need_expand argument as
 described below.
 Data will be decoded into the supplied row buffers packed into bytes
 unless the library has been told to transform it into another format.
 For example, 4 bit/pixel paletted or grayscale data will be returned
 pixels/byte with the leftmost pixel in the high-order bits of the
 byte, unless png_set_packing() is called.  8-bit RGB data will be stored
 in RGB RGB RGB format unless png_set_filler() or png_set_add_alpha()
 is called to insert filler bytes, either before or after each RGB triplet.
 -bit RGB data will be returned RRGGBB RRGGBB, with the most significant
 byte of the color value first, unless png_set_scale_16() is called to
 transform it to regular RGB RGB triplets, or png_set_filler() or
 png_set_add alpha() is called to insert filler bytes, either before or
 after each RRGGBB triplet.  Similarly, 8-bit or 16-bit grayscale data can
 be modified with png_set_filler(), png_set_add_alpha(), png_set_strip_16(),
 or png_set_scale_16().
 The following code transforms grayscale images of less than 8 to 8 bits,
 changes paletted images to RGB, and adds a full alpha channel if there is
 transparency information in a tRNS chunk.  This is most useful on
 grayscale images with bit depths of 2 or 4 or if there is a multiple-image
 viewing application that wishes to treat all images in the same way.
     if (color_type == PNG_COLOR_TYPE_PALETTE)
         png_set_palette_to_rgb(png_ptr);
     if (png_get_valid(png_ptr, info_ptr,
         PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);
     if (color_type == PNG_COLOR_TYPE_GRAY &&
         bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);
 The first two functions are actually aliases for png_set_expand(), added
 in libpng version 1.0.4, with the function names expanded to improve code
 readability.  In some future version they may actually do different
 things.
 As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
 added.  It expands the sample depth without changing tRNS to alpha.
 As of libpng version 1.5.2, png_set_expand_16() was added.  It behaves as
 png_set_expand(); however, the resultant channels have 16 bits rather than 8.
 Use this when the output color or gray channels are made linear to avoid fairly
 severe accuracy loss.
    if (bit_depth < 16)
       png_set_expand_16(png_ptr);
 PNG can have files with 16 bits per channel.  If you only can handle
 bits per channel, this will strip the pixels down to 8-bit.
     if (bit_depth == 16)
 #if PNG_LIBPNG_VER >= 10504
        png_set_scale_16(png_ptr);
 #else
        png_set_strip_16(png_ptr);
 #endif
 (The more accurate "png_set_scale_16()" API became available in libpng version
 .5.4).
 If you need to process the alpha channel on the image separately from the image
 data (for example if you convert it to a bitmap mask) it is possible to have
 libpng strip the channel leaving just RGB or gray data:
     if (color_type & PNG_COLOR_MASK_ALPHA)
        png_set_strip_alpha(png_ptr);
 If you strip the alpha channel you need to find some other way of dealing with
 the information.  If, instead, you want to convert the image to an opaque
 version with no alpha channel use png_set_background; see below.
 As of libpng version 1.5.2, almost all useful expansions are supported, the
 major ommissions are conversion of grayscale to indexed images (which can be
 done trivially in the application) and conversion of indexed to grayscale (which
 can be done by a trivial manipulation of the palette.)
 In the following table, the 01 means grayscale with depth<8, 31 means
 indexed with depth<8, other numerals represent the color type, "T" means
 the tRNS chunk is present, A means an alpha channel is present, and O
 means tRNS or alpha is present but all pixels in the image are opaque.
   FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
    TO
 -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
 [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
 1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
 T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
 O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
 C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
 T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
 O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
 [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
 T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
 O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
 A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
 O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
 A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
 O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +
 Within the matrix,
      "+" identifies entries where 'from' and 'to' are the same.
      "-" means the transformation is not supported.
      "." means nothing is necessary (a tRNS chunk can just be ignored).
      "t" means the transformation is obtained by png_set_tRNS.
      "A" means the transformation is obtained by png_set_add_alpha().
      "X" means the transformation is obtained by png_set_expand().
      "1" means the transformation is obtained by
          png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
          if there is no transparency in the original or the final
          format).
      "C" means the transformation is obtained by png_set_gray_to_rgb().
      "G" means the transformation is obtained by png_set_rgb_to_gray().
      "P" means the transformation is obtained by
          png_set_expand_palette_to_rgb().
      "p" means the transformation is obtained by png_set_packing().
      "Q" means the transformation is obtained by png_set_quantize().
      "T" means the transformation is obtained by
          png_set_tRNS_to_alpha().
      "B" means the transformation is obtained by
          png_set_background(), or png_strip_alpha().
 When an entry has multiple transforms listed all are required to cause the
 right overall transformation.  When two transforms are separated by a comma
 either will do the job.  When transforms are enclosed in [] the transform should
 do the job but this is currently unimplemented - a different format will result
 if the suggested transformations are used.
 In PNG files, the alpha channel in an image
 is the level of opacity.  If you need the alpha channel in an image to
 be the level of transparency instead of opacity, you can invert the
 alpha channel (or the tRNS chunk data) after it's read, so that 0 is
 fully opaque and 255 (in 8-bit or paletted images) or 65535 (in 16-bit
 images) is fully transparent, with
     png_set_invert_alpha(png_ptr);
 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
 they can, resulting in, for example, 8 pixels per byte for 1 bit
 files.  This code expands to 1 pixel per byte without changing the
 values of the pixels:
     if (bit_depth < 8)
        png_set_packing(png_ptr);
 PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
 stored in a PNG image have been "scaled" or "shifted" up to the next
 higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
 to 8 bits/sample in the range [0, 255]).  However, it is also possible
 to convert the PNG pixel data back to the original bit depth of the
 image.  This call reduces the pixels back down to the original bit depth:
     png_color_8p sig_bit;
     if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
        png_set_shift(png_ptr, sig_bit);
 PNG files store 3-color pixels in red, green, blue order.  This code
 changes the storage of the pixels to blue, green, red:
     if (color_type == PNG_COLOR_TYPE_RGB ||
         color_type == PNG_COLOR_TYPE_RGB_ALPHA)
        png_set_bgr(png_ptr);
 PNG files store RGB pixels packed into 3 or 6 bytes. This code expands them
 into 4 or 8 bytes for windowing systems that need them in this format:
     if (color_type == PNG_COLOR_TYPE_RGB)
        png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);
 where "filler" is the 8 or 16-bit number to fill with, and the location is
 either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
 you want the filler before the RGB or after.  This transformation
 does not affect images that already have full alpha channels.  To add an
 opaque alpha channel, use filler=0xff or 0xffff and PNG_FILLER_AFTER which
 will generate RGBA pixels.
 Note that png_set_filler() does not change the color type.  If you want
 to do that, you can add a true alpha channel with
     if (color_type == PNG_COLOR_TYPE_RGB ||
        color_type == PNG_COLOR_TYPE_GRAY)
        png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);
 where "filler" contains the alpha value to assign to each pixel.
 This function was added in libpng-1.2.7.
 If you are reading an image with an alpha channel, and you need the
 data as ARGB instead of the normal PNG format RGBA:
     if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
        png_set_swap_alpha(png_ptr);
 For some uses, you may want a grayscale image to be represented as
 RGB.  This code will do that conversion:
     if (color_type == PNG_COLOR_TYPE_GRAY ||
         color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
        png_set_gray_to_rgb(png_ptr);
 Conversely, you can convert an RGB or RGBA image to grayscale or grayscale
 with alpha.
     if (color_type == PNG_COLOR_TYPE_RGB ||
         color_type == PNG_COLOR_TYPE_RGB_ALPHA)
        png_set_rgb_to_gray(png_ptr, error_action,
           double red_weight, double green_weight);
     error_action = 1: silently do the conversion
     error_action = 2: issue a warning if the original
                       image has any pixel where
                       red != green or red != blue
     error_action = 3: issue an error and abort the
                       conversion if the original
                       image has any pixel where
                       red != green or red != blue
     red_weight:       weight of red component
     green_weight:     weight of green component
                       If either weight is negative, default
                       weights are used.
 In the corresponding fixed point API the red_weight and green_weight values are
 simply scaled by 100,000:
     png_set_rgb_to_gray(png_ptr, error_action,
        png_fixed_point red_weight,
        png_fixed_point green_weight);
 If you have set error_action = 1 or 2, you can
 later check whether the image really was gray, after processing
 the image rows, with the png_get_rgb_to_gray_status(png_ptr) function.
 It will return a png_byte that is zero if the image was gray or
 if there were any non-gray pixels.  Background and sBIT data
 will be silently converted to grayscale, using the green channel
 data for sBIT, regardless of the error_action setting.
 The default values come from the PNG file cHRM chunk if present; otherwise, the
 defaults correspond to the ITU-R recommendation 709, and also the sRGB color
 space, as recommended in the Charles Poynton's Colour FAQ,
 <http://www.poynton.com/>, in section 9:
    <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>
     Y = 0.2126 * R + 0.7152 * G + 0.0722 * B
 Previous versions of this document, 1998 through 2002, recommended a slightly
 different formula:
     Y = 0.212671 * R + 0.715160 * G + 0.072169 * B
 Libpng uses an integer approximation:
     Y = (6968 * R + 23434 * G + 2366 * B)/32768
 The calculation is done in a linear colorspace, if the image gamma
 can be determined.
 The png_set_background() function has been described already; it tells libpng to
 composite images with alpha or simple transparency against the supplied
 background color.  For compatibility with versions of libpng earlier than
 libpng-1.5.4 it is recommended that you call the function after reading the file
 header, even if you don't want to use the color in a bKGD chunk, if one exists.
 If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid),
 you may use this color, or supply another color more suitable for
 the current display (e.g., the background color from a web page).  You
 need to tell libpng how the color is represented, both the format of the
 component values in the color (the number of bits) and the gamma encoding of the
 color.  The function takes two arguments, background_gamma_mode and need_expand
 to convey this information; however, only two combinations are likely to be
 useful:
     png_color_16 my_background;
     png_color_16p image_background;
     if (png_get_bKGD(png_ptr, info_ptr, &image_background))
        png_set_background(png_ptr, image_background,
            PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
     else
        png_set_background(png_ptr, &my_background,
            PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);
 The second call was described above - my_background is in the format of the
 final, display, output produced by libpng.  Because you now know the format of
 the PNG it is possible to avoid the need to choose either 8-bit or 16-bit
 output and to retain palette images (the palette colors will be modified
 appropriately and the tRNS chunk removed.)  However, if you are doing this,
 take great care not to ask for transformations without checking first that
 they apply!
 In the first call the background color has the original bit depth and color type
 of the PNG file.  So, for palette images the color is supplied as a palette
 index and for low bit greyscale images the color is a reduced bit value in
 image_background->gray.
 If you didn't call png_set_gamma() before reading the file header, for example
 if you need your code to remain compatible with older versions of libpng prior
 to libpng-1.5.4, this is the place to call it.
 Do not call it if you called png_set_alpha_mode(); doing so will damage the
 settings put in place by png_set_alpha_mode().  (If png_set_alpha_mode() is
 supported then you can certainly do png_set_gamma() before reading the PNG
 header.)
 This API unconditionally sets the screen and file gamma values, so it will
 override the value in the PNG file unless it is called before the PNG file
 reading starts.  For this reason you must always call it with the PNG file
 value when you call it in this position:
    if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
       png_set_gamma(png_ptr, screen_gamma, file_gamma);
    else
       png_set_gamma(png_ptr, screen_gamma, 0.45455);
 If you need to reduce an RGB file to a paletted file, or if a paletted
 file has more entries then will fit on your screen, png_set_quantize()
 will do that.  Note that this is a simple match quantization that merely
 finds the closest color available.  This should work fairly well with
 optimized palettes, but fairly badly with linear color cubes.  If you
 pass a palette that is larger than maximum_colors, the file will
 reduce the number of colors in the palette so it will fit into
 maximum_colors.  If there is a histogram, libpng will use it to make
 more intelligent choices when reducing the palette.  If there is no
 histogram, it may not do as good a job.
    if (color_type & PNG_COLOR_MASK_COLOR)
    {
       if (png_get_valid(png_ptr, info_ptr,
           PNG_INFO_PLTE))
       {
          png_uint_16p histogram = NULL;
          png_get_hIST(png_ptr, info_ptr,
              &histogram);
          png_set_quantize(png_ptr, palette, num_palette,
             max_screen_colors, histogram, 1);
       }
       else
       {
          png_color std_color_cube[MAX_SCREEN_COLORS] =
             { ... colors ... };
          png_set_quantize(png_ptr, std_color_cube,
             MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
             NULL,0);
       }
    }
 PNG files describe monochrome as black being zero and white being one.
 The following code will reverse this (make black be one and white be
 zero):
    if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
       png_set_invert_mono(png_ptr);
 This function can also be used to invert grayscale and gray-alpha images:
    if (color_type == PNG_COLOR_TYPE_GRAY ||
        color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
       png_set_invert_mono(png_ptr);
 PNG files store 16-bit pixels in network byte order (big-endian,
 ie. most significant bits first).  This code changes the storage to the
 other way (little-endian, i.e. least significant bits first, the
 way PCs store them):
     if (bit_depth == 16)
        png_set_swap(png_ptr);
 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
 need to change the order the pixels are packed into bytes, you can use:
     if (bit_depth < 8)
        png_set_packswap(png_ptr);
 Finally, you can write your own transformation function if none of
 the existing ones meets your needs.  This is done by setting a callback
 with
     png_set_read_user_transform_fn(png_ptr,
         read_transform_fn);
 You must supply the function
     void read_transform_fn(png_structp png_ptr, png_row_infop
         row_info, png_bytep data)
 See pngtest.c for a working example.  Your function will be called
 after all of the other transformations have been processed.  Take care with
 interlaced images if you do the interlace yourself - the width of the row is the
 width in 'row_info', not the overall image width.
 If supported, libpng provides two information routines that you can use to find
 where you are in processing the image:
    png_get_current_pass_number(png_structp png_ptr);
    png_get_current_row_number(png_structp png_ptr);
 Don't try using these outside a transform callback - firstly they are only
 supported if user transforms are supported, secondly they may well return
 unexpected results unless the row is actually being processed at the moment they
 are called.
 With interlaced
 images the value returned is the row in the input sub-image image.  Use
 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
 The discussion of interlace handling above contains more information on how to
 use these values.
 You can also set up a pointer to a user structure for use by your
 callback function, and you can inform libpng that your transform
 function will change the number of channels or bit depth with the
 function
     png_set_user_transform_info(png_ptr, user_ptr,
         user_depth, user_channels);
 The user's application, not libpng, is responsible for allocating and
 freeing any memory required for the user structure.
 You can retrieve the pointer via the function
 png_get_user_transform_ptr().  For example:
     voidp read_user_transform_ptr =
         png_get_user_transform_ptr(png_ptr);
 The last thing to handle is interlacing; this is covered in detail below,
 but you must call the function here if you want libpng to handle expansion
 of the interlaced image.
     number_of_passes = png_set_interlace_handling(png_ptr);
 After setting the transformations, libpng can update your png_info
 structure to reflect any transformations you've requested with this
 call.
     png_read_update_info(png_ptr, info_ptr);
 This is most useful to update the info structure's rowbytes
 field so you can use it to allocate your image memory.  This function
 will also update your palette with the correct screen_gamma and
 background if these have been given with the calls above.  You may
 only call png_read_update_info() once with a particular info_ptr.
 After you call png_read_update_info(), you can allocate any
 memory you need to hold the image.  The row data is simply
 raw byte data for all forms of images.  As the actual allocation
 varies among applications, no example will be given.  If you
 are allocating one large chunk, you will need to build an
 array of pointers to each row, as it will be needed for some
 of the functions below.
 Remember: Before you call png_read_update_info(), the png_get_*()
 functions return the values corresponding to the original PNG image.
 After you call png_read_update_info the values refer to the image
 that libpng will output.  Consequently you must call all the png_set_
 functions before you call png_read_update_info().  This is particularly
 important for png_set_interlace_handling() - if you are going to call
 png_read_update_info() you must call png_set_interlace_handling() before
 it unless you want to receive interlaced output.
 Reading image data
 After you've allocated memory, you can read the image data.
 The simplest way to do this is in one function call.  If you are
 allocating enough memory to hold the whole image, you can just
 call png_read_image() and libpng will read in all the image data
 and put it in the memory area supplied.  You will need to pass in
 an array of pointers to each row.
 This function automatically handles interlacing, so you don't
 need to call png_set_interlace_handling() (unless you call
 png_read_update_info()) or call this function multiple times, or any
 of that other stuff necessary with png_read_rows().
    png_read_image(png_ptr, row_pointers);
 where row_pointers is:
    png_bytep row_pointers[height];
 You can point to void or char or whatever you use for pixels.
 If you don't want to read in the whole image at once, you can
 use png_read_rows() instead.  If there is no interlacing (check
 interlace_type == PNG_INTERLACE_NONE), this is simple:
     png_read_rows(png_ptr, row_pointers, NULL,
         number_of_rows);
 where row_pointers is the same as in the png_read_image() call.
 If you are doing this just one row at a time, you can do this with
 a single row_pointer instead of an array of row_pointers:
     png_bytep row_pointer = row;
     png_read_row(png_ptr, row_pointer, NULL);
 If the file is interlaced (interlace_type != 0 in the IHDR chunk), things
 get somewhat harder.  The only current (PNG Specification version 1.2)
 interlacing type for PNG is (interlace_type == PNG_INTERLACE_ADAM7);
 a somewhat complicated 2D interlace scheme, known as Adam7, that
 breaks down an image into seven smaller images of varying size, based
 on an 8x8 grid.  This number is defined (from libpng 1.5) as
 PNG_INTERLACE_ADAM7_PASSES in png.h
 libpng can fill out those images or it can give them to you "as is".
 It is almost always better to have libpng handle the interlacing for you.
 If you want the images filled out, there are two ways to do that.  The one
 mentioned in the PNG specification is to expand each pixel to cover
 those pixels that have not been read yet (the "rectangle" method).
 This results in a blocky image for the first pass, which gradually
 smooths out as more pixels are read.  The other method is the "sparkle"
 method, where pixels are drawn only in their final locations, with the
 rest of the image remaining whatever colors they were initialized to
 before the start of the read.  The first method usually looks better,
 but tends to be slower, as there are more pixels to put in the rows.
 If, as is likely, you want libpng to expand the images, call this before
 calling png_start_read_image() or png_read_update_info():
     if (interlace_type == PNG_INTERLACE_ADAM7)
        number_of_passes
            = png_set_interlace_handling(png_ptr);
 This will return the number of passes needed.  Currently, this is seven,
 but may change if another interlace type is added.  This function can be
 called even if the file is not interlaced, where it will return one pass.
 You then need to read the whole image 'number_of_passes' times.  Each time
 will distribute the pixels from the current pass to the correct place in
 the output image, so you need to supply the same rows to png_read_rows in
 each pass.
 If you are not going to display the image after each pass, but are
 going to wait until the entire image is read in, use the sparkle
 effect.  This effect is faster and the end result of either method
 is exactly the same.  If you are planning on displaying the image
 after each pass, the "rectangle" effect is generally considered the
 better looking one.
 If you only want the "sparkle" effect, just call png_read_rows() as
 normal, with the third parameter NULL.  Make sure you make pass over
 the image number_of_passes times, and you don't change the data in the
 rows between calls.  You can change the locations of the data, just
 not the data.  Each pass only writes the pixels appropriate for that
 pass, and assumes the data from previous passes is still valid.
     png_read_rows(png_ptr, row_pointers, NULL,
         number_of_rows);
 If you only want the first effect (the rectangles), do the same as
 before except pass the row buffer in the third parameter, and leave
 the second parameter NULL.
     png_read_rows(png_ptr, NULL, row_pointers,
         number_of_rows);
 If you don't want libpng to handle the interlacing details, just call
 png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the images.
 Each of the images is a valid image by itself; however, you will almost
 certainly need to distribute the pixels from each sub-image to the
 correct place.  This is where everything gets very tricky.
 If you want to retrieve the separate images you must pass the correct
 number of rows to each successive call of png_read_rows().  The calculation
 gets pretty complicated for small images, where some sub-images may
 not even exist because either their width or height ends up zero.
 libpng provides two macros to help you in 1.5 and later versions:
    png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
    png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);
 Respectively these tell you the width and height of the sub-image
 corresponding to the numbered pass.  'pass' is in in the range 0 to 6 -
 this can be confusing because the specification refers to the same passes
 as 1 to 7!  Be careful, you must check both the width and height before
 calling png_read_rows() and not call it for that pass if either is zero.
 You can, of course, read each sub-image row by row.  If you want to
 produce optimal code to make a pixel-by-pixel transformation of an
 interlaced image this is the best approach; read each row of each pass,
 transform it, and write it out to a new interlaced image.
 If you want to de-interlace the image yourself libpng provides further
 macros to help that tell you where to place the pixels in the output image.
 Because the interlacing scheme is rectangular - sub-image pixels are always
 arranged on a rectangular grid - all you need to know for each pass is the
 starting column and row in the output image of the first pixel plus the
 spacing between each pixel.  As of libpng 1.5 there are four macros to
 retrieve this information:
    png_uint_32 x = PNG_PASS_START_COL(pass);
    png_uint_32 y = PNG_PASS_START_ROW(pass);
    png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
    png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);
 These allow you to write the obvious loop:
    png_uint_32 input_y = 0;
    png_uint_32 output_y = PNG_PASS_START_ROW(pass);
    while (output_y < output_image_height)
    {
       png_uint_32 input_x = 0;
       png_uint_32 output_x = PNG_PASS_START_COL(pass);
       while (output_x < output_image_width)
       {
          image[output_y][output_x] =
              subimage[pass][input_y][input_x++];
          output_x += xStep;
       }
       ++input_y;
       output_y += yStep;
    }
 Notice that the steps between successive output rows and columns are
 returned as shifts.  This is possible because the pixels in the subimages
 are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the original
 image.  In practice you may need to directly calculate the output coordinate
 given an input coordinate.  libpng provides two further macros for this
 purpose:
    png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
    png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);
 Finally a pair of macros are provided to tell you if a particular image
 row or column appears in a given pass:
    int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
    int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);
 Bear in mind that you will probably also need to check the width and height
 of the pass in addition to the above to be sure the pass even exists!
 With any luck you are convinced by now that you don't want to do your own
 interlace handling.  In reality normally the only good reason for doing this
 is if you are processing PNG files on a pixel-by-pixel basis and don't want
 to load the whole file into memory when it is interlaced.
 libpng includes a test program, pngvalid, that illustrates reading and
 writing of interlaced images.  If you can't get interlacing to work in your
 code and don't want to leave it to libpng (the recommended approach), see
 how pngvalid.c does it.
 Finishing a sequential read
 After you are finished reading the image through the
 low-level interface, you can finish reading the file.  If you are
 interested in comments or time, which may be stored either before or
 after the image data, you should pass the separate png_info struct if
 you want to keep the comments from before and after the image
 separate.
     png_infop end_info = png_create_info_struct(png_ptr);
     if (!end_info)
     {
        png_destroy_read_struct(&png_ptr, &info_ptr,
            (png_infopp)NULL);
        return (ERROR);
     }
    png_read_end(png_ptr, end_info);
 If you are not interested, you should still call png_read_end()
 but you can pass NULL, avoiding the need to create an end_info structure.
    png_read_end(png_ptr, (png_infop)NULL);
 If you don't call png_read_end(), then your file pointer will be
 left pointing to the first chunk after the last IDAT, which is probably
 not what you want if you expect to read something beyond the end of
 the PNG datastream.
 When you are done, you can free all memory allocated by libpng like this:
    png_destroy_read_struct(&png_ptr, &info_ptr,
        &end_info);
 or, if you didn't create an end_info structure,
    png_destroy_read_struct(&png_ptr, &info_ptr,
        (png_infopp)NULL);
 It is also possible to individually free the info_ptr members that
 point to libpng-allocated storage with the following function:
     png_free_data(png_ptr, info_ptr, mask, seq)
     mask - identifies data to be freed, a mask
            containing the bitwise OR of one or
            more of
              PNG_FREE_PLTE, PNG_FREE_TRNS,
              PNG_FREE_HIST, PNG_FREE_ICCP,
              PNG_FREE_PCAL, PNG_FREE_ROWS,
              PNG_FREE_SCAL, PNG_FREE_SPLT,
              PNG_FREE_TEXT, PNG_FREE_UNKN,
            or simply PNG_FREE_ALL
     seq  - sequence number of item to be freed
            (-1 for all items)
 This function may be safely called when the relevant storage has
 already been freed, or has not yet been allocated, or was allocated
 by the user and not by libpng,  and will in those cases do nothing.
 The "seq" parameter is ignored if only one item of the selected data
 type, such as PLTE, is allowed.  If "seq" is not -1, and multiple items
 are allowed for the data type identified in the mask, such as text or
 sPLT, only the n'th item in the structure is freed, where n is "seq".
 The default behavior is only to free data that was allocated internally
 by libpng.  This can be changed, so that libpng will not free the data,
 or so that it will free data that was allocated by the user with png_malloc()
 or png_calloc() and passed in via a png_set_*() function, with
     png_data_freer(png_ptr, info_ptr, freer, mask)
     freer  - one of
                PNG_DESTROY_WILL_FREE_DATA
                PNG_SET_WILL_FREE_DATA
                PNG_USER_WILL_FREE_DATA
     mask   - which data elements are affected
              same choices as in png_free_data()
 This function only affects data that has already been allocated.
 You can call this function after reading the PNG data but before calling
 any png_set_*() functions, to control whether the user or the png_set_*()
 function is responsible for freeing any existing data that might be present,
 and again after the png_set_*() functions to control whether the user
 or png_destroy_*() is supposed to free the data.  When the user assumes
 responsibility for libpng-allocated data, the application must use
 png_free() to free it, and when the user transfers responsibility to libpng
 for data that the user has allocated, the user must have used png_malloc()
 or png_calloc() to allocate it.
 If you allocated your row_pointers in a single block, as suggested above in
 the description of the high level read interface, you must not transfer
 responsibility for freeing it to the png_set_rows or png_read_destroy function,
 because they would also try to free the individual row_pointers[i].
 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
 separately, do not transfer responsibility for freeing text_ptr to libpng,
 because when libpng fills a png_text structure it combines these members with
 the key member, and png_free_data() will free only text_ptr.key.  Similarly,
 if you transfer responsibility for free'ing text_ptr from libpng to your
 application, your application must not separately free those members.
 The png_free_data() function will turn off the "valid" flag for anything
 it frees.  If you need to turn the flag off for a chunk that was freed by
 your application instead of by libpng, you can use
     png_set_invalid(png_ptr, info_ptr, mask);
     mask - identifies the chunks to be made invalid,
            containing the bitwise OR of one or
            more of
              PNG_INFO_gAMA, PNG_INFO_sBIT,
              PNG_INFO_cHRM, PNG_INFO_PLTE,
              PNG_INFO_tRNS, PNG_INFO_bKGD,
              PNG_INFO_hIST, PNG_INFO_pHYs,
              PNG_INFO_oFFs, PNG_INFO_tIME,
              PNG_INFO_pCAL, PNG_INFO_sRGB,
              PNG_INFO_iCCP, PNG_INFO_sPLT,
              PNG_INFO_sCAL, PNG_INFO_IDAT
 For a more compact example of reading a PNG image, see the file example.c.
 Reading PNG files progressively
 The progressive reader is slightly different from the non-progressive
 reader.  Instead of calling png_read_info(), png_read_rows(), and
 png_read_end(), you make one call to png_process_data(), which calls
 callbacks when it has the info, a row, or the end of the image.  You
 set up these callbacks with png_set_progressive_read_fn().  You don't
 have to worry about the input/output functions of libpng, as you are
 giving the library the data directly in png_process_data().  I will
 assume that you have read the section on reading PNG files above,
 so I will only highlight the differences (although I will show
 all of the code).
 png_structp png_ptr;
 png_infop info_ptr;
  /*  An example code fragment of how you would
      initialize the progressive reader in your
      application. */
  int
  initialize_png_reader()
  {
     png_ptr = png_create_read_struct
         (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
          user_error_fn, user_warning_fn);
     if (!png_ptr)
         return (ERROR);
     info_ptr = png_create_info_struct(png_ptr);
     if (!info_ptr)
     {
        png_destroy_read_struct(&png_ptr,
           (png_infopp)NULL, (png_infopp)NULL);
        return (ERROR);
     }
     if (setjmp(png_jmpbuf(png_ptr)))
     {
        png_destroy_read_struct(&png_ptr, &info_ptr,
           (png_infopp)NULL);
        return (ERROR);
     }
     /* This one's new.  You can provide functions
        to be called when the header info is valid,
        when each row is completed, and when the image
        is finished.  If you aren't using all functions,
        you can specify NULL parameters.  Even when all
        three functions are NULL, you need to call
        png_set_progressive_read_fn().  You can use
        any struct as the user_ptr (cast to a void pointer
        for the function call), and retrieve the pointer
        from inside the callbacks using the function
           png_get_progressive_ptr(png_ptr);
        which will return a void pointer, which you have
        to cast appropriately.
      */
     png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
         info_callback, row_callback, end_callback);
     return 0;
  }
  /* A code fragment that you call as you receive blocks
    of data */
  int
  process_data(png_bytep buffer, png_uint_32 length)
  {
     if (setjmp(png_jmpbuf(png_ptr)))
     {
        png_destroy_read_struct(&png_ptr, &info_ptr,
            (png_infopp)NULL);
        return (ERROR);
     }
     /* This one's new also.  Simply give it a chunk
        of data from the file stream (in order, of
        course).  On machines with segmented memory
        models machines, don't give it any more than
 K.  The library seems to run fine with sizes
        of 4K. Although you can give it much less if
        necessary (I assume you can give it chunks of
 byte, I haven't tried less then 256 bytes
        yet).  When this function returns, you may
        want to display any rows that were generated
        in the row callback if you don't already do
        so there.
      */
     png_process_data(png_ptr, info_ptr, buffer, length);
     /* At this point you can call png_process_data_skip if
        you want to handle data the library will skip yourself;
        it simply returns the number of bytes to skip (and stops
        libpng skipping that number of bytes on the next
        png_process_data call).
     return 0;
  }
  /* This function is called (as set by
     png_set_progressive_read_fn() above) when enough data
     has been supplied so all of the header has been
     read.
  */
  void
  info_callback(png_structp png_ptr, png_infop info)
  {
     /* Do any setup here, including setting any of
        the transformations mentioned in the Reading
        PNG files section.  For now, you _must_ call
        either png_start_read_image() or
        png_read_update_info() after all the
        transformations are set (even if you don't set
        any).  You may start getting rows before
        png_process_data() returns, so this is your
        last chance to prepare for that.
        This is where you turn on interlace handling,
        assuming you don't want to do it yourself.
        If you need to you can stop the processing of
        your original input data at this point by calling
        png_process_data_pause.  This returns the number
        of unprocessed bytes from the last png_process_data
        call - it is up to you to ensure that the next call
        sees these bytes again.  If you don't want to bother
        with this you can get libpng to cache the unread
        bytes by setting the 'save' parameter (see png.h) but
        then libpng will have to copy the data internally.
      */
  }
  /* This function is called when each row of image
     data is complete */
  void
  row_callback(png_structp png_ptr, png_bytep new_row,
     png_uint_32 row_num, int pass)
  {
     /* If the image is interlaced, and you turned
        on the interlace handler, this function will
        be called for every row in every pass.  Some
        of these rows will not be changed from the
        previous pass.  When the row is not changed,
        the new_row variable will be NULL.  The rows
        and passes are called in order, so you don't
        really need the row_num and pass, but I'm
        supplying them because it may make your life
        easier.
        If you did not turn on interlace handling then
        the callback is called for each row of each
        sub-image when the image is interlaced.  In this
        case 'row_num' is the row in the sub-image, not
        the row in the output image as it is in all other
        cases.
        For the non-NULL rows of interlaced images when
        you have switched on libpng interlace handling,
        you must call png_progressive_combine_row()
        passing in the row and the old row.  You can
        call this function for NULL rows (it will just
        return) and for non-interlaced images (it just
        does the memcpy for you) if it will make the
        code easier.  Thus, you can just do this for
        all cases if you switch on interlace handling;
      */
         png_progressive_combine_row(png_ptr, old_row,
           new_row);
     /* where old_row is what was displayed for
        previously for the row.  Note that the first
        pass (pass == 0, really) will completely cover
        the old row, so the rows do not have to be
        initialized.  After the first pass (and only
        for interlaced images), you will have to pass
        the current row, and the function will combine
        the old row and the new row.
        You can also call png_process_data_pause in this
        callback - see above.
     */
  }
  void
  end_callback(png_structp png_ptr, png_infop info)
  {
     /* This function is called after the whole image
        has been read, including any chunks after the
        image (up to and including the IEND).  You
        will usually have the same info chunk as you
        had in the header, although some data may have
        been added to the comments and time fields.
        Most people won't do much here, perhaps setting
        a flag that marks the image as finished.
      */
  }
 IV. Writing
 Much of this is very similar to reading.  However, everything of
 importance is repeated here, so you won't have to constantly look
 back up in the reading section to understand writing.
 Setup
 You will want to do the I/O initialization before you get into libpng,
 so if it doesn't work, you don't have anything to undo. If you are not
 using the standard I/O functions, you will need to replace them with
 custom writing functions.  See the discussion under Customizing libpng.
     FILE *fp = fopen(file_name, "wb");
     if (!fp)
        return (ERROR);
 Next, png_struct and png_info need to be allocated and initialized.
 As these can be both relatively large, you may not want to store these
 on the stack, unless you have stack space to spare.  Of course, you
 will want to check if they return NULL.  If you are also reading,
 you won't want to name your read structure and your write structure
 both "png_ptr"; you can call them anything you like, such as
 "read_ptr" and "write_ptr".  Look at pngtest.c, for example.
     png_structp png_ptr = png_create_write_struct
        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
         user_error_fn, user_warning_fn);
     if (!png_ptr)
        return (ERROR);
     png_infop info_ptr = png_create_info_struct(png_ptr);
     if (!info_ptr)
     {
        png_destroy_write_struct(&png_ptr,
            (png_infopp)NULL);
        return (ERROR);
     }
 If you want to use your own memory allocation routines,
 define PNG_USER_MEM_SUPPORTED and use
 png_create_write_struct_2() instead of png_create_write_struct():
     png_structp png_ptr = png_create_write_struct_2
        (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
         user_error_fn, user_warning_fn, (png_voidp)
         user_mem_ptr, user_malloc_fn, user_free_fn);
 After you have these structures, you will need to set up the
 error handling.  When libpng encounters an error, it expects to
 longjmp() back to your routine.  Therefore, you will need to call
 setjmp() and pass the png_jmpbuf(png_ptr).  If you
 write the file from different routines, you will need to update
 the png_jmpbuf(png_ptr) every time you enter a new routine that will
 call a png_*() function.  See your documentation of setjmp/longjmp
 for your compiler for more information on setjmp/longjmp.  See
 the discussion on libpng error handling in the Customizing Libpng
 section below for more information on the libpng error handling.
     if (setjmp(png_jmpbuf(png_ptr)))
     {
     png_destroy_write_struct(&png_ptr, &info_ptr);
        fclose(fp);
        return (ERROR);
     }
     ...
     return;
 If you would rather avoid the complexity of setjmp/longjmp issues,
 you can compile libpng with PNG_NO_SETJMP, in which case
 errors will result in a call to PNG_ABORT() which defaults to abort().
 You can #define PNG_ABORT() to a function that does something
 more useful than abort(), as long as your function does not
 return.
 Checking for invalid palette index on write was added at libpng
 .5.10.  If a pixel contains an invalid (out-of-range) index libpng issues
 a benign error.  This is enabled by default because this condition is an
 error according to the PNG specification, Clause 11.3.2, but the error can
 be ignored in each png_ptr with
    png_set_check_for_invalid_index(png_ptr, 0);
 If the error is ignored, or if png_benign_error() treats it as a warning,
 any invalid pixels are written as-is by the encoder, resulting in an
 invalid PNG datastream as output.  In this case the application is
 responsible for ensuring that the pixel indexes are in range when it writes
 a PLTE chunk with fewer entries than the bit depth would allow.
 Now you need to set up the output code.  The default for libpng is to
 use the C function fwrite().  If you use this, you will need to pass a
 valid FILE * in the function png_init_io().  Be sure that the file is
 opened in binary mode.  Again, if you wish to handle writing data in
 another way, see the discussion on libpng I/O handling in the Customizing
 Libpng section below.
     png_init_io(png_ptr, fp);
 If you are embedding your PNG into a datastream such as MNG, and don't
 want libpng to write the 8-byte signature, or if you have already
 written the signature in your application, use
     png_set_sig_bytes(png_ptr, 8);
 to inform libpng that it should not write a signature.
 Write callbacks
 At this point, you can set up a callback function that will be
 called after each row has been written, which you can use to control
 a progress meter or the like.  It's demonstrated in pngtest.c.
 You must supply a function
     void write_row_callback(png_structp png_ptr, png_uint_32 row,
        int pass);
     {
       /* put your code here */
     }
 (You can give it another name that you like instead of "write_row_callback")
 To inform libpng about your function, use
     png_set_write_status_fn(png_ptr, write_row_callback);
 When this function is called the row has already been completely processed and
 it has also been written out.  The 'row' and 'pass' refer to the next row to be
 handled.  For the
 non-interlaced case the row that was just handled is simply one less than the
 passed in row number, and pass will always be 0.  For the interlaced case the
 same applies unless the row value is 0, in which case the row just handled was
 the last one from one of the preceding passes.  Because interlacing may skip a
 pass you cannot be sure that the preceding pass is just 'pass-1', if you really
 need to know what the last pass is record (row,pass) from the callback and use
 the last recorded value each time.
 As with the user transform you can find the output row using the
 PNG_ROW_FROM_PASS_ROW macro.
 You now have the option of modifying how the compression library will
 run.  The following functions are mainly for testing, but may be useful
 in some cases, like if you need to write PNG files extremely fast and
 are willing to give up some compression, or if you want to get the
 maximum possible compression at the expense of slower writing.  If you
 have no special needs in this area, let the library do what it wants by
 not calling this function at all, as it has been tuned to deliver a good
 speed/compression ratio. The second parameter to png_set_filter() is
 the filter method, for which the only valid values are 0 (as of the
 July 1999 PNG specification, version 1.2) or 64 (if you are writing
 a PNG datastream that is to be embedded in a MNG datastream).  The third
 parameter is a flag that indicates which filter type(s) are to be tested
 for each scanline.  See the PNG specification for details on the specific
 filter types.
     /* turn on or off filtering, and/or choose
        specific filters.  You can use either a single
        PNG_FILTER_VALUE_NAME or the bitwise OR of one
        or more PNG_FILTER_NAME masks.
      */
     png_set_filter(png_ptr, 0,
        PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
        PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
        PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
        PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
        PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
        PNG_ALL_FILTERS);
 If an application wants to start and stop using particular filters during
 compression, it should start out with all of the filters (to ensure that
 the previous row of pixels will be stored in case it's needed later),
 and then add and remove them after the start of compression.
 If you are writing a PNG datastream that is to be embedded in a MNG
 datastream, the second parameter can be either 0 or 64.
 The png_set_compression_*() functions interface to the zlib compression
 library, and should mostly be ignored unless you really know what you are
 doing.  The only generally useful call is png_set_compression_level()
 which changes how much time zlib spends on trying to compress the image
 data.  See the Compression Library (zlib.h and algorithm.txt, distributed
 with zlib) for details on the compression levels.
     #include zlib.h
     /* Set the zlib compression level */
     png_set_compression_level(png_ptr,
         Z_BEST_COMPRESSION);
     /* Set other zlib parameters for compressing IDAT */
     png_set_compression_mem_level(png_ptr, 8);
     png_set_compression_strategy(png_ptr,
         Z_DEFAULT_STRATEGY);
     png_set_compression_window_bits(png_ptr, 15);
     png_set_compression_method(png_ptr, 8);
     png_set_compression_buffer_size(png_ptr, 8192)
     /* Set zlib parameters for text compression
      * If you don't call these, the parameters
      * fall back on those defined for IDAT chunks
      */
     png_set_text_compression_mem_level(png_ptr, 8);
     png_set_text_compression_strategy(png_ptr,
         Z_DEFAULT_STRATEGY);
     png_set_text_compression_window_bits(png_ptr, 15);
     png_set_text_compression_method(png_ptr, 8);
 Setting the contents of info for output
 You now need to fill in the png_info structure with all the data you
 wish to write before the actual image.  Note that the only thing you
 are allowed to write after the image is the text chunks and the time
 chunk (as of PNG Specification 1.2, anyway).  See png_write_end() and
 the latest PNG specification for more information on that.  If you
 wish to write them before the image, fill them in now, and flag that
 data as being valid.  If you want to wait until after the data, don't
 fill them until png_write_end().  For all the fields in png_info and
 their data types, see png.h.  For explanations of what the fields
 contain, see the PNG specification.
 Some of the more important parts of the png_info are:
     png_set_IHDR(png_ptr, info_ptr, width, height,
        bit_depth, color_type, interlace_type,
        compression_type, filter_method)
     width          - holds the width of the image
                      in pixels (up to 2^31).
     height         - holds the height of the image
                      in pixels (up to 2^31).
     bit_depth      - holds the bit depth of one of the
                      image channels.
                      (valid values are 1, 2, 4, 8, 16
                      and depend also on the
                      color_type.  See also significant
                      bits (sBIT) below).
     color_type     - describes which color/alpha
                      channels are present.
                      PNG_COLOR_TYPE_GRAY
                         (bit depths 1, 2, 4, 8, 16)
                      PNG_COLOR_TYPE_GRAY_ALPHA
                         (bit depths 8, 16)
                      PNG_COLOR_TYPE_PALETTE
                         (bit depths 1, 2, 4, 8)
                      PNG_COLOR_TYPE_RGB
                         (bit_depths 8, 16)
                      PNG_COLOR_TYPE_RGB_ALPHA
                         (bit_depths 8, 16)
                      PNG_COLOR_MASK_PALETTE
                      PNG_COLOR_MASK_COLOR
                      PNG_COLOR_MASK_ALPHA
     interlace_type - PNG_INTERLACE_NONE or
                      PNG_INTERLACE_ADAM7
     compression_type - (must be
                      PNG_COMPRESSION_TYPE_DEFAULT)
     filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                      or, if you are writing a PNG to
                      be embedded in a MNG datastream,
                      can also be
                      PNG_INTRAPIXEL_DIFFERENCING)
 If you call png_set_IHDR(), the call must appear before any of the
 other png_set_*() functions, because they might require access to some of
 the IHDR settings.  The remaining png_set_*() functions can be called
 in any order.
 If you wish, you can reset the compression_type, interlace_type, or
 filter_method later by calling png_set_IHDR() again; if you do this, the
 width, height, bit_depth, and color_type must be the same in each call.
     png_set_PLTE(png_ptr, info_ptr, palette,
        num_palette);
     palette        - the palette for the file
                      (array of png_color)
     num_palette    - number of entries in the palette
     png_set_gAMA(png_ptr, info_ptr, file_gamma);
     png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);
     file_gamma     - the gamma at which the image was
                      created (PNG_INFO_gAMA)
     int_file_gamma - 100,000 times the gamma at which
                      the image was created
     png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
                      green_x, green_y, blue_x, blue_y)
     png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
                      green_Y, green_Z, blue_X, blue_Y, blue_Z)
     png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
                      int_red_x, int_red_y, int_green_x, int_green_y,
                      int_blue_x, int_blue_y)
     png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
                      int_red_Z, int_green_X, int_green_Y, int_green_Z,
                      int_blue_X, int_blue_Y, int_blue_Z)
     {white,red,green,blue}_{x,y}
                      A color space encoding specified using the chromaticities
                      of the end points and the white point.
     {red,green,blue}_{X,Y,Z}
                      A color space encoding specified using the encoding end
                      points - the CIE tristimulus specification of the intended
                      color of the red, green and blue channels in the PNG RGB
                      data.  The white point is simply the sum of the three end
                      points.
     png_set_sRGB(png_ptr, info_ptr, srgb_intent);
     srgb_intent    - the rendering intent
                      (PNG_INFO_sRGB) The presence of
                      the sRGB chunk means that the pixel
                      data is in the sRGB color space.
                      This chunk also implies specific
                      values of gAMA and cHRM.  Rendering
                      intent is the CSS-1 property that
                      has been defined by the International
                      Color Consortium
                      (http://www.color.org).
                      It can be one of
                      PNG_sRGB_INTENT_SATURATION,
                      PNG_sRGB_INTENT_PERCEPTUAL,
                      PNG_sRGB_INTENT_ABSOLUTE, or
                      PNG_sRGB_INTENT_RELATIVE.
     png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
        srgb_intent);
     srgb_intent    - the rendering intent
                      (PNG_INFO_sRGB) The presence of the
                      sRGB chunk means that the pixel
                      data is in the sRGB color space.
                      This function also causes gAMA and
                      cHRM chunks with the specific values
                      that are consistent with sRGB to be
                      written.
     png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                        profile, proflen);
     name             - The profile name.
     compression_type - The compression type; always
                        PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                        You may give NULL to this argument to
                        ignore it.
     profile          - International Color Consortium color
                        profile data. May contain NULs.
     proflen          - length of profile data in bytes.
     png_set_sBIT(png_ptr, info_ptr, sig_bit);
     sig_bit        - the number of significant bits for
                      (PNG_INFO_sBIT) each of the gray, red,
                      green, and blue channels, whichever are
                      appropriate for the given color type
                      (png_color_16)
     png_set_tRNS(png_ptr, info_ptr, trans_alpha,
        num_trans, trans_color);
     trans_alpha    - array of alpha (transparency)
                      entries for palette (PNG_INFO_tRNS)
     num_trans      - number of transparent entries
                      (PNG_INFO_tRNS)
     trans_color    - graylevel or color sample values
                      (in order red, green, blue) of the
                      single transparent color for
                      non-paletted images (PNG_INFO_tRNS)
     png_set_hIST(png_ptr, info_ptr, hist);
     hist           - histogram of palette (array of
                      png_uint_16) (PNG_INFO_hIST)
     png_set_tIME(png_ptr, info_ptr, mod_time);
     mod_time       - time image was last modified
                      (PNG_VALID_tIME)
     png_set_bKGD(png_ptr, info_ptr, background);
     background     - background color (of type
                      png_color_16p) (PNG_VALID_bKGD)
     png_set_text(png_ptr, info_ptr, text_ptr, num_text);
     text_ptr       - array of png_text holding image
                      comments
     text_ptr[i].compression - type of compression used
                  on "text" PNG_TEXT_COMPRESSION_NONE
                            PNG_TEXT_COMPRESSION_zTXt
                            PNG_ITXT_COMPRESSION_NONE
                            PNG_ITXT_COMPRESSION_zTXt
     text_ptr[i].key   - keyword for comment.  Must contain
 -79 characters.
     text_ptr[i].text  - text comments for current
                          keyword.  Can be NULL or empty.
     text_ptr[i].text_length - length of text string,
                  after decompression, 0 for iTXt
     text_ptr[i].itxt_length - length of itxt string,
                  after decompression, 0 for tEXt/zTXt
     text_ptr[i].lang  - language of comment (NULL or
                          empty for unknown).
     text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                          or empty for unknown).
     Note that the itxt_length, lang, and lang_key
     members of the text_ptr structure only exist when the
     library is built with iTXt chunk support.  Prior to
     libpng-1.4.0 the library was built by default without
     iTXt support. Also note that when iTXt is supported,
     they contain NULL pointers when the "compression"
     field contains PNG_TEXT_COMPRESSION_NONE or
     PNG_TEXT_COMPRESSION_zTXt.
     num_text       - number of comments
     png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
        num_spalettes);
     palette_ptr    - array of png_sPLT_struct structures
                      to be added to the list of palettes
                      in the info structure.
     num_spalettes  - number of palette structures to be
                      added.
     png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
         unit_type);
     offset_x  - positive offset from the left
                      edge of the screen
     offset_y  - positive offset from the top
                      edge of the screen
     unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER
     png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
         unit_type);
     res_x       - pixels/unit physical resolution
                   in x direction
     res_y       - pixels/unit physical resolution
                   in y direction
     unit_type   - PNG_RESOLUTION_UNKNOWN,
                   PNG_RESOLUTION_METER
     png_set_sCAL(png_ptr, info_ptr, unit, width, height)
     unit        - physical scale units (an integer)
     width       - width of a pixel in physical scale units
     height      - height of a pixel in physical scale units
                   (width and height are doubles)
     png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)
     unit        - physical scale units (an integer)
     width       - width of a pixel in physical scale units
                   expressed as a string
     height      - height of a pixel in physical scale units
                  (width and height are strings like "2.54")
     png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
        num_unknowns)
     unknowns          - array of png_unknown_chunk
                         structures holding unknown chunks
     unknowns[i].name  - name of unknown chunk
     unknowns[i].data  - data of unknown chunk
     unknowns[i].size  - size of unknown chunk's data
     unknowns[i].location - position to write chunk in file
 : do not write chunk
                            PNG_HAVE_IHDR: before PLTE
                            PNG_HAVE_PLTE: before IDAT
                            PNG_AFTER_IDAT: after IDAT
 The "location" member is set automatically according to
 what part of the output file has already been written.
 You can change its value after calling png_set_unknown_chunks()
 as demonstrated in pngtest.c.  Within each of the "locations",
 the chunks are sequenced according to their position in the
 structure (that is, the value of "i", which is the order in which
 the chunk was either read from the input file or defined with
 png_set_unknown_chunks).
 A quick word about text and num_text.  text is an array of png_text
 structures.  num_text is the number of valid structures in the array.
 Each png_text structure holds a language code, a keyword, a text value,
 and a compression type.
 The compression types have the same valid numbers as the compression
 types of the image data.  Currently, the only valid number is zero.
 However, you can store text either compressed or uncompressed, unlike
 images, which always have to be compressed.  So if you don't want the
 text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
 Because tEXt and zTXt chunks don't have a language field, if you
 specify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt
 any language code or translated keyword will not be written out.
 Until text gets around a few hundred bytes, it is not worth compressing it.
 After the text has been written out to the file, the compression type
 is set to PNG_TEXT_COMPRESSION_NONE_WR or PNG_TEXT_COMPRESSION_zTXt_WR,
 so that it isn't written out again at the end (in case you are calling
 png_write_end() with the same struct).
 The keywords that are given in the PNG Specification are:
     Title            Short (one line) title or
                      caption for image
     Author           Name of image's creator
     Description      Description of image (possibly long)
     Copyright        Copyright notice
     Creation Time    Time of original image creation
                      (usually RFC 1123 format, see below)
     Software         Software used to create the image
     Disclaimer       Legal disclaimer
     Warning          Warning of nature of content
     Source           Device used to create the image
     Comment          Miscellaneous comment; conversion
                      from other image format
 The keyword-text pairs work like this.  Keywords should be short
 simple descriptions of what the comment is about.  Some typical
 keywords are found in the PNG specification, as is some recommendations
 on keywords.  You can repeat keywords in a file.  You can even write
 some text before the image and some after.  For example, you may want
 to put a description of the image before the image, but leave the
 disclaimer until after, so viewers working over modem connections
 don't have to wait for the disclaimer to go over the modem before
 they start seeing the image.  Finally, keywords should be full
 words, not abbreviations.  Keywords and text are in the ISO 8859-1
 (Latin-1) character set (a superset of regular ASCII) and can not
 contain NUL characters, and should not contain control or other
 unprintable characters.  To make the comments widely readable, stick
 with basic ASCII, and avoid machine specific character set extensions
 like the IBM-PC character set.  The keyword must be present, but
 you can leave off the text string on non-compressed pairs.
 Compressed pairs must have a text string, as only the text string
 is compressed anyway, so the compression would be meaningless.
 PNG supports modification time via the png_time structure.  Two
 conversion routines are provided, png_convert_from_time_t() for
 time_t and png_convert_from_struct_tm() for struct tm.  The
 time_t routine uses gmtime().  You don't have to use either of
 these, but if you wish to fill in the png_time structure directly,
 you should provide the time in universal time (GMT) if possible
 instead of your local time.  Note that the year number is the full
 year (e.g. 1998, rather than 98 - PNG is year 2000 compliant!), and
 that months start with 1.
 If you want to store the time of the original image creation, you should
 use a plain tEXt chunk with the "Creation Time" keyword.  This is
 necessary because the "creation time" of a PNG image is somewhat vague,
 depending on whether you mean the PNG file, the time the image was
 created in a non-PNG format, a still photo from which the image was
 scanned, or possibly the subject matter itself.  In order to facilitate
 machine-readable dates, it is recommended that the "Creation Time"
 tEXt chunk use RFC 1123 format dates (e.g. "22 May 1997 18:07:10 GMT"),
 although this isn't a requirement.  Unlike the tIME chunk, the
 "Creation Time" tEXt chunk is not expected to be automatically changed
 by the software.  To facilitate the use of RFC 1123 dates, a function
 png_convert_to_rfc1123_buffer(png_ptr, buffer, png_timep) is provided to
 convert from PNG time to an RFC 1123 format string.  The caller must provide
 a writeable buffer of at least 29 bytes.
 Writing unknown chunks
 You can use the png_set_unknown_chunks function to queue up private chunks
 for writing.  You give it a chunk name, location, raw data, and a size.  You
 also must use png_set_keep_unknown_chunks() to ensure that libpng will
 handle them.  That's all there is to it.  The chunks will be written by the
 next following png_write_info_before_PLTE, png_write_info, or png_write_end
 function, depending upon the specified location.  Any chunks previously
 read into the info structure's unknown-chunk list will also be written out
 in a sequence that satisfies the PNG specification's ordering rules.
 Here is an example of writing two private chunks, prVt and miNE:
     #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
     /* Set unknown chunk data */
     png_unknown_chunk unk_chunk[2];
     strcpy((char *) unk_chunk[0].name, "prVt";
     unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
     unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
     unk_chunk[0].location = PNG_HAVE_IHDR;
     strcpy((char *) unk_chunk[1].name, "miNE";
     unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
     unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
     unk_chunk[1].location = PNG_AFTER_IDAT;
     png_set_unknown_chunks(write_ptr, write_info_ptr,
         unk_chunk, 2);
     /* Needed because miNE is not safe-to-copy */
     png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
        (png_bytep) "miNE", 1);
     # if PNG_LIBPNG_VER < 10600
       /* Deal with unknown chunk location bug in 1.5.x and earlier */
       png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
       png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
     # endif
     # if PNG_LIBPNG_VER < 10500
       /* PNG_AFTER_IDAT writes two copies of the chunk prior to libpng-1.5.0,
        * one before IDAT and another after IDAT, so don't use it; only use
        * PNG_HAVE_IHDR location.  This call resets the location previously
        * set by assignment and png_set_unknown_chunk_location() for chunk 1.
        */
       png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
     # endif
     #endif
 The high-level write interface
 At this point there are two ways to proceed; through the high-level
 write interface, or through a sequence of low-level write operations.
 You can use the high-level interface if your image data is present
 in the info structure.  All defined output
 transformations are permitted, enabled by the following masks.
     PNG_TRANSFORM_IDENTITY      No transformation
     PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
     PNG_TRANSFORM_PACKSWAP      Change order of packed
                                 pixels to LSB first
     PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
     PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                 sBIT depth
     PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                 to BGRA
     PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                 to AG
     PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                 to transparency
     PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
     PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                       bytes (deprecated).
     PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                       filler bytes
     PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                       filler bytes
 If you have valid image data in the info structure (you can use
 png_set_rows() to put image data in the info structure), simply do this:
     png_write_png(png_ptr, info_ptr, png_transforms, NULL)
 where png_transforms is an integer containing the bitwise OR of some set of
 transformation flags.  This call is equivalent to png_write_info(),
 followed the set of transformations indicated by the transform mask,
 then png_write_image(), and finally png_write_end().
 (The final parameter of this call is not yet used.  Someday it might point
 to transformation parameters required by some future output transform.)
 You must use png_transforms and not call any png_set_transform() functions
 when you use png_write_png().
 The low-level write interface
 If you are going the low-level route instead, you are now ready to
 write all the file information up to the actual image data.  You do
 this with a call to png_write_info().
     png_write_info(png_ptr, info_ptr);
 Note that there is one transformation you may need to do before
 png_write_info().  In PNG files, the alpha channel in an image is the
 level of opacity.  If your data is supplied as a level of transparency,
 you can invert the alpha channel before you write it, so that 0 is
 fully transparent and 255 (in 8-bit or paletted images) or 65535
 (in 16-bit images) is fully opaque, with
     png_set_invert_alpha(png_ptr);
 This must appear before png_write_info() instead of later with the
 other transformations because in the case of paletted images the tRNS
 chunk data has to be inverted before the tRNS chunk is written.  If
 your image is not a paletted image, the tRNS data (which in such cases
 represents a single color to be rendered as transparent) won't need to
 be changed, and you can safely do this transformation after your
 png_write_info() call.
 If you need to write a private chunk that you want to appear before
 the PLTE chunk when PLTE is present, you can write the PNG info in
 two steps, and insert code to write your own chunk between them:
     png_write_info_before_PLTE(png_ptr, info_ptr);
     png_set_unknown_chunks(png_ptr, info_ptr, ...);
     png_write_info(png_ptr, info_ptr);
 After you've written the file information, you can set up the library
 to handle any special transformations of the image data.  The various
 ways to transform the data will be described in the order that they
 should occur.  This is important, as some of these change the color
 type and/or bit depth of the data, and some others only work on
 certain color types and bit depths.  Even though each transformation
 checks to see if it has data that it can do something with, you should
 make sure to only enable a transformation if it will be valid for the
 data.  For example, don't swap red and blue on grayscale data.
 PNG files store RGB pixels packed into 3 or 6 bytes.  This code tells
 the library to strip input data that has 4 or 8 bytes per pixel down
 to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
 bytes per pixel).
     png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);
 where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
 PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel
 is stored XRGB or RGBX.
 PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
 they can, resulting in, for example, 8 pixels per byte for 1 bit files.
 If the data is supplied at 1 pixel per byte, use this code, which will
 correctly pack the pixels into a single byte:
     png_set_packing(png_ptr);
 PNG files reduce possible bit depths to 1, 2, 4, 8, and 16.  If your
 data is of another bit depth, you can write an sBIT chunk into the
 file so that decoders can recover the original data if desired.
     /* Set the true bit depth of the image data */
     if (color_type & PNG_COLOR_MASK_COLOR)
     {
        sig_bit.red = true_bit_depth;
        sig_bit.green = true_bit_depth;
        sig_bit.blue = true_bit_depth;
     }
     else
     {
        sig_bit.gray = true_bit_depth;
     }
     if (color_type & PNG_COLOR_MASK_ALPHA)
     {
        sig_bit.alpha = true_bit_depth;
     }
     png_set_sBIT(png_ptr, info_ptr, &sig_bit);
 If the data is stored in the row buffer in a bit depth other than
 one supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
 this will scale the values to appear to be the correct bit depth as
 is required by PNG.
     png_set_shift(png_ptr, &sig_bit);
 PNG files store 16-bit pixels in network byte order (big-endian,
 ie. most significant bits first).  This code would be used if they are
 supplied the other way (little-endian, i.e. least significant bits
 first, the way PCs store them):
     if (bit_depth > 8)
        png_set_swap(png_ptr);
 If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
 need to change the order the pixels are packed into bytes, you can use:
     if (bit_depth < 8)
        png_set_packswap(png_ptr);
 PNG files store 3 color pixels in red, green, blue order.  This code
 would be used if they are supplied as blue, green, red:
     png_set_bgr(png_ptr);
 PNG files describe monochrome as black being zero and white being
 one. This code would be used if the pixels are supplied with this reversed
 (black being one and white being zero):
     png_set_invert_mono(png_ptr);
 Finally, you can write your own transformation function if none of
 the existing ones meets your needs.  This is done by setting a callback
 with
     png_set_write_user_transform_fn(png_ptr,
        write_transform_fn);
 You must supply the function
     void write_transform_fn(png_structp png_ptr, png_row_infop
        row_info, png_bytep data)
 See pngtest.c for a working example.  Your function will be called
 before any of the other transformations are processed.  If supported
 libpng also supplies an information routine that may be called from
 your callback:
    png_get_current_row_number(png_ptr);
    png_get_current_pass_number(png_ptr);
 This returns the current row passed to the transform.  With interlaced
 images the value returned is the row in the input sub-image image.  Use
 PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col, pass) to
 find the output pixel (x,y) given an interlaced sub-image pixel (row,col,pass).
 The discussion of interlace handling above contains more information on how to
 use these values.
 You can also set up a pointer to a user structure for use by your
 callback function.
     png_set_user_transform_info(png_ptr, user_ptr, 0, 0);
 The user_channels and user_depth parameters of this function are ignored
 when writing; you can set them to zero as shown.
 You can retrieve the pointer via the function png_get_user_transform_ptr().
 For example:
     voidp write_user_transform_ptr =
        png_get_user_transform_ptr(png_ptr);
 It is possible to have libpng flush any pending output, either manually,
 or automatically after a certain number of lines have been written.  To
 flush the output stream a single time call:
     png_write_flush(png_ptr);
 and to have libpng flush the output stream periodically after a certain
 number of scanlines have been written, call:
     png_set_flush(png_ptr, nrows);
 Note that the distance between rows is from the last time png_write_flush()
 was called, or the first row of the image if it has never been called.
 So if you write 50 lines, and then png_set_flush 25, it will flush the
 output on the next scanline, and every 25 lines thereafter, unless
 png_write_flush() is called before 25 more lines have been written.
 If nrows is too small (less than about 10 lines for a 640 pixel wide
 RGB image) the image compression may decrease noticeably (although this
 may be acceptable for real-time applications).  Infrequent flushing will
 only degrade the compression performance by a few percent over images
 that do not use flushing.
 Writing the image data
 That's it for the transformations.  Now you can write the image data.
 The simplest way to do this is in one function call.  If you have the
 whole image in memory, you can just call png_write_image() and libpng
 will write the image.  You will need to pass in an array of pointers to
 each row.  This function automatically handles interlacing, so you don't
 need to call png_set_interlace_handling() or call this function multiple
 times, or any of that other stuff necessary with png_write_rows().
     png_write_image(png_ptr, row_pointers);
 where row_pointers is:
     png_byte *row_pointers[height];
 You can point to void or char or whatever you use for pixels.
 If you don't want to write the whole image at once, you can
 use png_write_rows() instead.  If the file is not interlaced,
 this is simple:
     png_write_rows(png_ptr, row_pointers,
        number_of_rows);
 row_pointers is the same as in the png_write_image() call.
 If you are just writing one row at a time, you can do this with
 a single row_pointer instead of an array of row_pointers:
     png_bytep row_pointer = row;
     png_write_row(png_ptr, row_pointer);
 When the file is interlaced, things can get a good deal more complicated.
 The only currently (as of the PNG Specification version 1.2, dated July
 ) defined interlacing scheme for PNG files is the "Adam7" interlace
 scheme, that breaks down an image into seven smaller images of varying
 size.  libpng will build these images for you, or you can do them
 yourself.  If you want to build them yourself, see the PNG specification
 for details of which pixels to write when.
 If you don't want libpng to handle the interlacing details, just
 use png_set_interlace_handling() and call png_write_rows() the
 correct number of times to write all the sub-images
 (png_set_interlace_handling() returns the number of sub-images.)
 If you want libpng to build the sub-images, call this before you start
 writing any rows:
     number_of_passes = png_set_interlace_handling(png_ptr);
 This will return the number of passes needed.  Currently, this is seven,
 but may change if another interlace type is added.
 Then write the complete image number_of_passes times.
     png_write_rows(png_ptr, row_pointers, number_of_rows);
 Think carefully before you write an interlaced image.  Typically code that
 reads such images reads all the image data into memory, uncompressed, before
 doing any processing.  Only code that can display an image on the fly can
 take advantage of the interlacing and even then the image has to be exactly
 the correct size for the output device, because scaling an image requires
 adjacent pixels and these are not available until all the passes have been
 read.
 If you do write an interlaced image you will hardly ever need to handle
 the interlacing yourself.  Call png_set_interlace_handling() and use the
 approach described above.
 The only time it is conceivable that you will really need to write an
 interlaced image pass-by-pass is when you have read one pass by pass and
 made some pixel-by-pixel transformation to it, as described in the read
 code above.  In this case use the PNG_PASS_ROWS and PNG_PASS_COLS macros
 to determine the size of each sub-image in turn and simply write the rows
 you obtained from the read code.
 Finishing a sequential write
 After you are finished writing the image, you should finish writing
 the file.  If you are interested in writing comments or time, you should
 pass an appropriately filled png_info pointer.  If you are not interested,
 you can pass NULL.
     png_write_end(png_ptr, info_ptr);
 When you are done, you can free all memory used by libpng like this:
     png_destroy_write_struct(&png_ptr, &info_ptr);
 It is also possible to individually free the info_ptr members that
 point to libpng-allocated storage with the following function:
     png_free_data(png_ptr, info_ptr, mask, seq)
     mask  - identifies data to be freed, a mask
             containing the bitwise OR of one or
             more of
               PNG_FREE_PLTE, PNG_FREE_TRNS,
               PNG_FREE_HIST, PNG_FREE_ICCP,
               PNG_FREE_PCAL, PNG_FREE_ROWS,
               PNG_FREE_SCAL, PNG_FREE_SPLT,
               PNG_FREE_TEXT, PNG_FREE_UNKN,
             or simply PNG_FREE_ALL
     seq   - sequence number of item to be freed
             (-1 for all items)
 This function may be safely called when the relevant storage has
 already been freed, or has not yet been allocated, or was allocated
 by the user  and not by libpng,  and will in those cases do nothing.
 The "seq" parameter is ignored if only one item of the selected data
 type, such as PLTE, is allowed.  If "seq" is not -1, and multiple items
 are allowed for the data type identified in the mask, such as text or
 sPLT, only the n'th item in the structure is freed, where n is "seq".
 If you allocated data such as a palette that you passed in to libpng
 with png_set_*, you must not free it until just before the call to
 png_destroy_write_struct().
 The default behavior is only to free data that was allocated internally
 by libpng.  This can be changed, so that libpng will not free the data,
 or so that it will free data that was allocated by the user with png_malloc()
 or png_calloc() and passed in via a png_set_*() function, with
     png_data_freer(png_ptr, info_ptr, freer, mask)
     freer  - one of
                PNG_DESTROY_WILL_FREE_DATA
                PNG_SET_WILL_FREE_DATA
                PNG_USER_WILL_FREE_DATA
     mask   - which data elements are affected
              same choices as in png_free_data()
 For example, to transfer responsibility for some data from a read structure
 to a write structure, you could use
     png_data_freer(read_ptr, read_info_ptr,
        PNG_USER_WILL_FREE_DATA,
        PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
     png_data_freer(write_ptr, write_info_ptr,
        PNG_DESTROY_WILL_FREE_DATA,
        PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)
 thereby briefly reassigning responsibility for freeing to the user but
 immediately afterwards reassigning it once more to the write_destroy
 function.  Having done this, it would then be safe to destroy the read
 structure and continue to use the PLTE, tRNS, and hIST data in the write
 structure.
 This function only affects data that has already been allocated.
 You can call this function before calling after the png_set_*() functions
 to control whether the user or png_destroy_*() is supposed to free the data.
 When the user assumes responsibility for libpng-allocated data, the
 application must use
 png_free() to free it, and when the user transfers responsibility to libpng
 for data that the user has allocated, the user must have used png_malloc()
 or png_calloc() to allocate it.
 If you allocated text_ptr.text, text_ptr.lang, and text_ptr.translated_keyword
 separately, do not transfer responsibility for freeing text_ptr to libpng,
 because when libpng fills a png_text structure it combines these members with
 the key member, and png_free_data() will free only text_ptr.key.  Similarly,
 if you transfer responsibility for free'ing text_ptr from libpng to your
 application, your application must not separately free those members.
 For a more compact example of writing a PNG image, see the file example.c.
 V. Simplified API
 The simplified API, which became available in libpng-1.6.0, hides the details
 of both libpng and the PNG file format itself.
 It allows PNG files to be read into a very limited number of
 in-memory bitmap formats or to be written from the same formats.  If these
 formats do not accommodate your needs then you can, and should, use the more
 sophisticated APIs above - these support a wide variety of in-memory formats
 and a wide variety of sophisticated transformations to those formats as well
 as a wide variety of APIs to manipulate ancilliary information.
 To read a PNG file using the simplified API:
 ) Declare a 'png_image' structure (see below) on the
      stack and memset() it to all zero.
 ) Call the appropriate png_image_begin_read... function.
 ) Set the png_image 'format' member to the required
      format and allocate a buffer for the image.
 ) Call png_image_finish_read to read the image into
      your buffer.
 There are no restrictions on the format of the PNG input itself; all valid
 color types, bit depths, and interlace methods are acceptable, and the
 input image is transformed as necessary to the requested in-memory format
 during the png_image_finish_read() step.
 To write a PNG file using the simplified API:
 ) Declare a 'png_image' structure on the stack and memset()
      it to all zero.
 ) Initialize the members of the structure that describe the
      image, setting the 'format' member to the format of the
      image in memory.
 ) Call the appropriate png_image_write... function with a
      pointer to the image to write the PNG data.
 png_image is a structure that describes the in-memory format of an image
 when it is being read or define the in-memory format of an image that you
 need to write.  The "png_image" structure contains the following members:
    png_uint_32  version Set to PNG_IMAGE_VERSION
    png_uint_32  width   Image width in pixels (columns)
    png_uint_32  height  Image height in pixels (rows)
    png_uint_32  format  Image format as defined below
    png_uint_32  flags   A bit mask containing informational flags
    png_controlp opaque  Initialize to NULL, free with png_image_free
    png_uint_32  colormap_entries; Number of entries in the color-map
    png_uint_32  warning_or_error;
    char         message[64];
 In the event of an error or warning the following field warning_or_error
 field will be set to a non-zero value and the 'message' field will contain
 a '\0' terminated string with the libpng error or warning message.  If both
 warnings and an error were encountered, only the error is recorded.  If there
 are multiple warnings, only the first one is recorded.
 The upper 30 bits of this value are reserved; the low two bits contain
 a two bit code such that a value more than 1 indicates a failure in the API
 just called:
 - no warning or error
 - warning
 - error
 - error preceded by warning
 The pixels (samples) of the image have one to four channels whose components
 have original values in the range 0 to 1.0:
 : A single gray or luminance channel (G).
 : A gray/luminance channel and an alpha channel (GA).
 : Three red, green, blue color channels (RGB).
 : Three color channels and an alpha channel (RGBA).
 The channels are encoded in one of two ways:
   a) As a small integer, value 0..255, contained in a single byte.  For the
 alpha channel the original value is simply value/255.  For the color or
 luminance channels the value is encoded according to the sRGB specification
 and matches the 8-bit format expected by typical display devices.
 The color/gray channels are not scaled (pre-multiplied) by the alpha
 channel and are suitable for passing to color management software.
   b) As a value in the range 0..65535, contained in a 2-byte integer.  All
 channels can be converted to the original value by dividing by 65535; all
 channels are linear.  Color channels use the RGB encoding (RGB end-points) of
 the sRGB specification.  This encoding is identified by the
 PNG_FORMAT_FLAG_LINEAR flag below.
 When an alpha channel is present it is expected to denote pixel coverage
 of the color or luminance channels and is returned as an associated alpha
 channel: the color/gray channels are scaled (pre-multiplied) by the alpha
 value.
 When a color-mapped image is used as a result of calling
 png_image_read_colormap or png_image_write_colormap the channels are encoded
 in the color-map and the descriptions above apply to the color-map entries.
 The image data is encoded as small integers, value 0..255, that index the
 entries in the color-map.  One integer (one byte) is stored for each pixel.
 PNG_FORMAT_*
 The #defines to be used in png_image::format.  Each #define identifies a
 particular layout of channel data and, if present, alpha values.  There are
 separate defines for each of the two channel encodings.
 A format is built up using single bit flag values.  Not all combinations are
 valid: use the bit flag values below for testing a format returned by the
 read APIs, but set formats from the derived values.
 When reading or writing color-mapped images the format should be set to the
 format of the entries in the color-map then png_image_{read,write}_colormap
 called to read or write the color-map and set the format correctly for the
 image data.  Do not set the PNG_FORMAT_FLAG_COLORMAP bit directly!
 NOTE: libpng can be built with particular features disabled, if you see
 compiler errors because the definition of one of the following flags has been
 compiled out it is because libpng does not have the required support.  It is
 possible, however, for the libpng configuration to enable the format on just
 read or just write; in that case you may see an error at run time.  You can
 guard against this by checking for the definition of:
    PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED
    PNG_FORMAT_FLAG_ALPHA    0x01 format with an alpha channel
    PNG_FORMAT_FLAG_COLOR    0x02 color format: otherwise grayscale
    PNG_FORMAT_FLAG_LINEAR   0x04 png_uint_16 channels else png_byte
    PNG_FORMAT_FLAG_COLORMAP 0x08 libpng use only
    PNG_FORMAT_FLAG_BGR      0x10 BGR colors, else order is RGB
    PNG_FORMAT_FLAG_AFIRST   0x20 alpha channel comes first
 Supported formats are as follows.  Future versions of libpng may support more
 formats; for compatibility with older versions simply check if the format
 macro is defined using #ifdef.  These defines describe the in-memory layout
 of the components of the pixels of the image.
 First the single byte formats:
    PNG_FORMAT_GRAY 0
    PNG_FORMAT_GA   PNG_FORMAT_FLAG_ALPHA
    PNG_FORMAT_AG   (PNG_FORMAT_GA|PNG_FORMAT_FLAG_AFIRST)
    PNG_FORMAT_RGB  PNG_FORMAT_FLAG_COLOR
    PNG_FORMAT_BGR  (PNG_FORMAT_FLAG_COLOR|PNG_FORMAT_FLAG_BGR)
    PNG_FORMAT_RGBA (PNG_FORMAT_RGB|PNG_FORMAT_FLAG_ALPHA)
    PNG_FORMAT_ARGB (PNG_FORMAT_RGBA|PNG_FORMAT_FLAG_AFIRST)
    PNG_FORMAT_BGRA (PNG_FORMAT_BGR|PNG_FORMAT_FLAG_ALPHA)
    PNG_FORMAT_ABGR (PNG_FORMAT_BGRA|PNG_FORMAT_FLAG_AFIRST)
 Then the linear 2-byte formats.  When naming these "Y" is used to
 indicate a luminance (gray) channel.  The component order within the pixel
 is always the same - there is no provision for swapping the order of the
 components in the linear format.
    PNG_FORMAT_LINEAR_Y PNG_FORMAT_FLAG_LINEAR
    PNG_FORMAT_LINEAR_Y_ALPHA
       (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_ALPHA)
    PNG_FORMAT_LINEAR_RGB
       (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR)
    PNG_FORMAT_LINEAR_RGB_ALPHA
       (PNG_FORMAT_FLAG_LINEAR|PNG_FORMAT_FLAG_COLOR|
       PNG_FORMAT_FLAG_ALPHA)
 Color-mapped formats are obtained by calling png_image_{read,write}_colormap,
 as appropriate after setting png_image::format to the format of the color-map
 to be read or written.  Applications may check the value of
 PNG_FORMAT_FLAG_COLORMAP to see if they have called the colormap API.  The
 format of the color-map may be extracted using the following macro.
    PNG_FORMAT_OF_COLORMAP(fmt) ((fmt) & ~PNG_FORMAT_FLAG_COLORMAP)
 PNG_IMAGE macros
 These are convenience macros to derive information from a png_image
 structure.  The PNG_IMAGE_SAMPLE_ macros return values appropriate to the
 actual image sample values - either the entries in the color-map or the
 pixels in the image.  The PNG_IMAGE_PIXEL_ macros return corresponding values
 for the pixels and will always return 1 after a call to
 png_image_{read,write}_colormap.  The remaining macros return information
 about the rows in the image and the complete image.
 NOTE: All the macros that take a png_image::format parameter are compile time
 constants if the format parameter is, itself, a constant.  Therefore these
 macros can be used in array declarations and case labels where required.
 Similarly the macros are also pre-processor constants (sizeof is not used) so
 they can be used in #if tests.
 First the information about the samples.
   PNG_IMAGE_SAMPLE_CHANNELS(fmt)
     Returns the total number of channels in a given format: 1..4
   PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
     Returns the size in bytes of a single component of a pixel or color-map
     entry (as appropriate) in the image.
   PNG_IMAGE_SAMPLE_SIZE(fmt)
     This is the size of the sample data for one sample.  If the image is
     color-mapped it is the size of one color-map entry (and image pixels are
     one byte in size), otherwise it is the size of one image pixel.
   PNG_IMAGE_COLORMAP_SIZE(fmt)
    The size of the color-map required by the format; this is the size of the
    color-map buffer passed to the png_image_{read,write}_colormap APIs, it is
    a fixed number determined by the format so can easily be allocated on the
    stack if necessary.
 #define PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)\
    (PNG_IMAGE_SAMPLE_CHANNELS(fmt) * 256)
    /* The maximum size of the color-map required by the format expressed in a
     * count of components.  This can be used to compile-time allocate a
     * color-map:
     *
     * png_uint_16 colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];
     *
     * png_byte colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];
     *
     * Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use the
     * information from one of the png_image_begin_read_ APIs and dynamically
     * allocate the required memory.
     */
 Corresponding information about the pixels
   PNG_IMAGE_PIXEL_(test,fmt)
   PNG_IMAGE_PIXEL_CHANNELS(fmt)
    The number of separate channels (components) in a pixel; 1 for a
    color-mapped image.
   PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)\
    The size, in bytes, of each component in a pixel; 1 for a color-mapped
    image.
   PNG_IMAGE_PIXEL_SIZE(fmt)
    The size, in bytes, of a complete pixel; 1 for a color-mapped image.
 Information about the whole row, or whole image
   PNG_IMAGE_ROW_STRIDE(image)
    Returns the total number of components in a single row of the image; this
    is the minimum 'row stride', the minimum count of components between each
    row.  For a color-mapped image this is the minimum number of bytes in a
    row.
   PNG_IMAGE_BUFFER_SIZE(image, row_stride)
     Returns the size, in bytes, of an image buffer given a png_image and a row
     stride - the number of components to leave space for in each row.
   PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
     This indicates the the RGB values of the in-memory bitmap do not
     correspond to the red, green and blue end-points defined by sRGB.
   PNG_IMAGE_FLAG_COLORMAP == 0x02
     The PNG is color-mapped.  If this flag is set png_image_read_colormap
     can be used without further loss of image information.  If it is not set
     png_image_read_colormap will cause significant loss if the image has any
 READ APIs
    The png_image passed to the read APIs must have been initialized by setting
    the png_controlp field 'opaque' to NULL (or, better, memset the whole thing.)
    int png_image_begin_read_from_file( png_imagep image,
      const char *file_name)
      The named file is opened for read and the image header
      is filled in from the PNG header in the file.
    int png_image_begin_read_from_stdio (png_imagep image,
      FILE* file)
       The PNG header is read from the stdio FILE object.
    int png_image_begin_read_from_memory(png_imagep image,
       png_const_voidp memory, png_size_t size)
       The PNG header is read from the given memory buffer.
    int png_image_finish_read(png_imagep image,
       png_colorp background, void *buffer,
       png_int_32 row_stride, void *colormap));
       Finish reading the image into the supplied buffer and
       clean up the png_image structure.
       row_stride is the step, in png_byte or png_uint_16 units
       as appropriate, between adjacent rows.  A positive stride
       indicates that the top-most row is first in the buffer -
       the normal top-down arrangement.  A negative stride
       indicates that the bottom-most row is first in the buffer.
       background need only be supplied if an alpha channel must
       be removed from a png_byte format and the removal is to be
       done by compositing on a solid color; otherwise it may be
       NULL and any composition will be done directly onto the
       buffer.  The value is an sRGB color to use for the
       background, for grayscale output the green channel is used.
       For linear output removing the alpha channel is always done
       by compositing on black.
    void png_image_free(png_imagep image)
       Free any data allocated by libpng in image->opaque,
       setting the pointer to NULL.  May be called at any time
       after the structure is initialized.
 When the simplified API needs to convert between sRGB and linear colorspaces,
 the actual sRGB transfer curve defined in the sRGB specification (see the
 article at http://en.wikipedia.org/wiki/SRGB) is used, not the gamma=1/2.2
 approximation used elsewhere in libpng.
 WRITE APIS
 For write you must initialize a png_image structure to describe the image to
 be written:
    version: must be set to PNG_IMAGE_VERSION
    opaque: must be initialized to NULL
    width: image width in pixels
    height: image height in rows
    format: the format of the data you wish to write
    flags: set to 0 unless one of the defined flags applies; set
       PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
       where the RGB values do not correspond to the colors in sRGB.
    colormap_entries: set to the number of entries in the color-map (0 to 256)
    int png_image_write_to_file, (png_imagep image,
       const char *file, int convert_to_8bit, const void *buffer,
       png_int_32 row_stride, const void *colormap));
       Write the image to the named file.
    int png_image_write_to_stdio(png_imagep image, FILE *file,
       int convert_to_8_bit, const void *buffer,
       png_int_32 row_stride, const void *colormap)
       Write the image to the given (FILE*).
 With all write APIs if image is in one of the linear formats with
 (png_uint_16) data then setting convert_to_8_bit will cause the output to be
 a (png_byte) PNG gamma encoded according to the sRGB specification, otherwise
 a 16-bit linear encoded PNG file is written.
 With all APIs row_stride is handled as in the read APIs - it is the spacing
 from one row to the next in component sized units (float) and if negative
 indicates a bottom-up row layout in the buffer.
 Note that the write API does not support interlacing, sub-8-bit pixels,
 and indexed (paletted) images.
 VI. Modifying/Customizing libpng
 There are two issues here.  The first is changing how libpng does
 standard things like memory allocation, input/output, and error handling.
 The second deals with more complicated things like adding new chunks,
 adding new transformations, and generally changing how libpng works.
 Both of those are compile-time issues; that is, they are generally
 determined at the time the code is written, and there is rarely a need
 to provide the user with a means of changing them.
 Memory allocation, input/output, and error handling
 All of the memory allocation, input/output, and error handling in libpng
 goes through callbacks that are user-settable.  The default routines are
 in pngmem.c, pngrio.c, pngwio.c, and pngerror.c, respectively.  To change
 these functions, call the appropriate png_set_*_fn() function.
 Memory allocation is done through the functions png_malloc(), png_calloc(),
 and png_free().  The png_malloc() and png_free() functions currently just
 call the standard C functions and png_calloc() calls png_malloc() and then
 clears the newly allocated memory to zero; note that png_calloc(png_ptr, size)
 is not the same as the calloc(number, size) function provided by stdlib.h.
 There is limited support for certain systems with segmented memory
 architectures and the types of pointers declared by png.h match this; you
 will have to use appropriate pointers in your application.  Since it is
 unlikely that the method of handling memory allocation on a platform
 will change between applications, these functions must be modified in
 the library at compile time.  If you prefer to use a different method
 of allocating and freeing data, you can use png_create_read_struct_2() or
 png_create_write_struct_2() to register your own functions as described
 above.  These functions also provide a void pointer that can be retrieved
 via
     mem_ptr=png_get_mem_ptr(png_ptr);
 Your replacement memory functions must have prototypes as follows:
     png_voidp malloc_fn(png_structp png_ptr,
        png_alloc_size_t size);
     void free_fn(png_structp png_ptr, png_voidp ptr);
 Your malloc_fn() must return NULL in case of failure.  The png_malloc()
 function will normally call png_error() if it receives a NULL from the
 system memory allocator or from your replacement malloc_fn().
 Your free_fn() will never be called with a NULL ptr, since libpng's
 png_free() checks for NULL before calling free_fn().
 Input/Output in libpng is done through png_read() and png_write(),
 which currently just call fread() and fwrite().  The FILE * is stored in
 png_struct and is initialized via png_init_io().  If you wish to change
 the method of I/O, the library supplies callbacks that you can set
 through the function png_set_read_fn() and png_set_write_fn() at run
 time, instead of calling the png_init_io() function.  These functions
 also provide a void pointer that can be retrieved via the function
 png_get_io_ptr().  For example:
     png_set_read_fn(png_structp read_ptr,
         voidp read_io_ptr, png_rw_ptr read_data_fn)
     png_set_write_fn(png_structp write_ptr,
         voidp write_io_ptr, png_rw_ptr write_data_fn,
         png_flush_ptr output_flush_fn);
     voidp read_io_ptr = png_get_io_ptr(read_ptr);
     voidp write_io_ptr = png_get_io_ptr(write_ptr);
 The replacement I/O functions must have prototypes as follows:
     void user_read_data(png_structp png_ptr,
         png_bytep data, png_size_t length);
     void user_write_data(png_structp png_ptr,
         png_bytep data, png_size_t length);
     void user_flush_data(png_structp png_ptr);
 The user_read_data() function is responsible for detecting and
 handling end-of-data errors.
 Supplying NULL for the read, write, or flush functions sets them back
 to using the default C stream functions, which expect the io_ptr to
 point to a standard *FILE structure.  It is probably a mistake
 to use NULL for one of write_data_fn and output_flush_fn but not both
 of them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
 It is an error to read from a write stream, and vice versa.
 Error handling in libpng is done through png_error() and png_warning().
 Errors handled through png_error() are fatal, meaning that png_error()
 should never return to its caller.  Currently, this is handled via
 setjmp() and longjmp() (unless you have compiled libpng with
 PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()),
 but you could change this to do things like exit() if you should wish,
 as long as your function does not return.
 On non-fatal errors, png_warning() is called
 to print a warning message, and then control returns to the calling code.
 By default png_error() and png_warning() print a message on stderr via
 fprintf() unless the library is compiled with PNG_NO_CONSOLE_IO defined
 (because you don't want the messages) or PNG_NO_STDIO defined (because
 fprintf() isn't available).  If you wish to change the behavior of the error
 functions, you will need to set up your own message callbacks.  These
 functions are normally supplied at the time that the png_struct is created.
 It is also possible to redirect errors and warnings to your own replacement
 functions after png_create_*_struct() has been called by calling:
     png_set_error_fn(png_structp png_ptr,
         png_voidp error_ptr, png_error_ptr error_fn,
         png_error_ptr warning_fn);
     png_voidp error_ptr = png_get_error_ptr(png_ptr);
 If NULL is supplied for either error_fn or warning_fn, then the libpng
 default function will be used, calling fprintf() and/or longjmp() if a
 problem is encountered.  The replacement error functions should have
 parameters as follows:
     void user_error_fn(png_structp png_ptr,
         png_const_charp error_msg);
     void user_warning_fn(png_structp png_ptr,
         png_const_charp warning_msg);
 The motivation behind using setjmp() and longjmp() is the C++ throw and
 catch exception handling methods.  This makes the code much easier to write,
 as there is no need to check every return code of every function call.
 However, there are some uncertainties about the status of local variables
 after a longjmp, so the user may want to be careful about doing anything
 after setjmp returns non-zero besides returning itself.  Consult your
 compiler documentation for more details.  For an alternative approach, you
 may wish to use the "cexcept" facility (see http://cexcept.sourceforge.net),
 which is illustrated in pngvalid.c and in contrib/visupng.
 Beginning in libpng-1.4.0, the png_set_benign_errors() API became available.
 You can use this to handle certain errors (normally handled as errors)
 as warnings.
     png_set_benign_errors (png_ptr, int allowed);
     allowed: 0: treat png_benign_error() as an error.
 : treat png_benign_error() as a warning.
 As of libpng-1.6.0, the default condition is to treat benign errors as
 warnings while reading and as errors while writing.
 Custom chunks
 If you need to read or write custom chunks, you may need to get deeper
 into the libpng code.  The library now has mechanisms for storing
 and writing chunks of unknown type; you can even declare callbacks
 for custom chunks.  However, this may not be good enough if the
 library code itself needs to know about interactions between your
 chunk and existing `intrinsic' chunks.
 If you need to write a new intrinsic chunk, first read the PNG
 specification. Acquire a first level of understanding of how it works.
 Pay particular attention to the sections that describe chunk names,
 and look at how other chunks were designed, so you can do things
 similarly.  Second, check out the sections of libpng that read and
 write chunks.  Try to find a chunk that is similar to yours and use
 it as a template.  More details can be found in the comments inside
 the code.  It is best to handle private or unknown chunks in a generic method,
 via callback functions, instead of by modifying libpng functions. This
 is illustrated in pngtest.c, which uses a callback function to handle a
 private "vpAg" chunk and the new "sTER" chunk, which are both unknown to
 libpng.
 If you wish to write your own transformation for the data, look through
 the part of the code that does the transformations, and check out some of
 the simpler ones to get an idea of how they work.  Try to find a similar
 transformation to the one you want to add and copy off of it.  More details
 can be found in the comments inside the code itself.
 Configuring for 16-bit platforms
 You will want to look into zconf.h to tell zlib (and thus libpng) that
 it cannot allocate more then 64K at a time.  Even if you can, the memory
 won't be accessible.  So limit zlib and libpng to 64K by defining MAXSEG_64K.
 Configuring for DOS
 For DOS users who only have access to the lower 640K, you will
 have to limit zlib's memory usage via a png_set_compression_mem_level()
 call.  See zlib.h or zconf.h in the zlib library for more information.
 Configuring for Medium Model
 Libpng's support for medium model has been tested on most of the popular
 compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
 defined, and FAR gets defined to far in pngconf.h, and you should be
 all set.  Everything in the library (except for zlib's structure) is
 expecting far data.  You must use the typedefs with the p or pp on
 the end for pointers (or at least look at them and be careful).  Make
 note that the rows of data are defined as png_bytepp, which is
 an "unsigned char far * far *".
 Configuring for gui/windowing platforms:
 You will need to write new error and warning functions that use the GUI
 interface, as described previously, and set them to be the error and
 warning functions at the time that png_create_*_struct() is called,
 in order to have them available during the structure initialization.
 They can be changed later via png_set_error_fn().  On some compilers,
 you may also have to change the memory allocators (png_malloc, etc.).
 Configuring for compiler xxx:
 All includes for libpng are in pngconf.h.  If you need to add, change
 or delete an include, this is the place to do it.
 The includes that are not needed outside libpng are placed in pngpriv.h,
 which is only used by the routines inside libpng itself.
 The files in libpng proper only include pngpriv.h and png.h, which
 in turn includes pngconf.h and, as of libpng-1.5.0, pnglibconf.h.
 As of libpng-1.5.0, pngpriv.h also includes three other private header
 files, pngstruct.h, pnginfo.h, and pngdebug.h, which contain material
 that previously appeared in the public headers.
 Configuring zlib:
 There are special functions to configure the compression.  Perhaps the
 most useful one changes the compression level, which currently uses
 input compression values in the range 0 - 9.  The library normally
 uses the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests
 have shown that for a large majority of images, compression values in
 the range 3-6 compress nearly as well as higher levels, and do so much
 faster.  For online applications it may be desirable to have maximum speed
 (Z_BEST_SPEED = 1).  With versions of zlib after v0.99, you can also
 specify no compression (Z_NO_COMPRESSION = 0), but this would create
 files larger than just storing the raw bitmap.  You can specify the
 compression level by calling:
     #include zlib.h
     png_set_compression_level(png_ptr, level);
 Another useful one is to reduce the memory level used by the library.
 The memory level defaults to 8, but it can be lowered if you are
 short on memory (running DOS, for example, where you only have 640K).
 Note that the memory level does have an effect on compression; among
 other things, lower levels will result in sections of incompressible
 data being emitted in smaller stored blocks, with a correspondingly
 larger relative overhead of up to 15% in the worst case.
     #include zlib.h
     png_set_compression_mem_level(png_ptr, level);
 The other functions are for configuring zlib.  They are not recommended
 for normal use and may result in writing an invalid PNG file.  See
 zlib.h for more information on what these mean.
     #include zlib.h
     png_set_compression_strategy(png_ptr,
         strategy);
     png_set_compression_window_bits(png_ptr,
         window_bits);
     png_set_compression_method(png_ptr, method);
     png_set_compression_buffer_size(png_ptr, size);
 As of libpng version 1.5.4, additional APIs became
 available to set these separately for non-IDAT
 compressed chunks such as zTXt, iTXt, and iCCP:
     #include zlib.h
     #if PNG_LIBPNG_VER >= 10504
     png_set_text_compression_level(png_ptr, level);
     png_set_text_compression_mem_level(png_ptr, level);
     png_set_text_compression_strategy(png_ptr,
         strategy);
     png_set_text_compression_window_bits(png_ptr,
         window_bits);
     png_set_text_compression_method(png_ptr, method);
     #endif
 Controlling row filtering
 If you want to control whether libpng uses filtering or not, which
 filters are used, and how it goes about picking row filters, you
 can call one of these functions.  The selection and configuration
 of row filters can have a significant impact on the size and
 encoding speed and a somewhat lesser impact on the decoding speed
 of an image.  Filtering is enabled by default for RGB and grayscale
 images (with and without alpha), but not for paletted images nor
 for any images with bit depths less than 8 bits/pixel.
 The 'method' parameter sets the main filtering method, which is
 currently only '0' in the PNG 1.2 specification.  The 'filters'
 parameter sets which filter(s), if any, should be used for each
 scanline.  Possible values are PNG_ALL_FILTERS and PNG_NO_FILTERS
 to turn filtering on and off, respectively.
 Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
 PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
 ORed together with '|' to specify one or more filters to use.
 These filters are described in more detail in the PNG specification.
 If you intend to change the filter type during the course of writing
 the image, you should start with flags set for all of the filters
 you intend to use so that libpng can initialize its internal
 structures appropriately for all of the filter types.  (Note that this
 means the first row must always be adaptively filtered, because libpng
 currently does not allocate the filter buffers until png_write_row()
 is called for the first time.)
     filters = PNG_FILTER_NONE | PNG_FILTER_SUB
               PNG_FILTER_UP | PNG_FILTER_AVG |
               PNG_FILTER_PAETH | PNG_ALL_FILTERS;
     png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
        filters);
               The second parameter can also be
               PNG_INTRAPIXEL_DIFFERENCING if you are
               writing a PNG to be embedded in a MNG
               datastream.  This parameter must be the
               same as the value of filter_method used
               in png_set_IHDR().
 It is also possible to influence how libpng chooses from among the
 available filters.  This is done in one or both of two ways - by
 telling it how important it is to keep the same filter for successive
 rows, and by telling it the relative computational costs of the filters.
     double weights[3] = {1.5, 1.3, 1.1},
        costs[PNG_FILTER_VALUE_LAST] =
        {1.0, 1.3, 1.3, 1.5, 1.7};
     png_set_filter_heuristics(png_ptr,
        PNG_FILTER_HEURISTIC_WEIGHTED, 3,
        weights, costs);
 The weights are multiplying factors that indicate to libpng that the
 row filter should be the same for successive rows unless another row filter
 is that many times better than the previous filter.  In the above example,
 if the previous 3 filters were SUB, SUB, NONE, the SUB filter could have a
 "sum of absolute differences" 1.5 x 1.3 times higher than other filters
 and still be chosen, while the NONE filter could have a sum 1.1 times
 higher than other filters and still be chosen.  Unspecified weights are
 taken to be 1.0, and the specified weights should probably be declining
 like those above in order to emphasize recent filters over older filters.
 The filter costs specify for each filter type a relative decoding cost
 to be considered when selecting row filters.  This means that filters
 with higher costs are less likely to be chosen over filters with lower
 costs, unless their "sum of absolute differences" is that much smaller.
 The costs do not necessarily reflect the exact computational speeds of
 the various filters, since this would unduly influence the final image
 size.
 Note that the numbers above were invented purely for this example and
 are given only to help explain the function usage.  Little testing has
 been done to find optimum values for either the costs or the weights.
 Removing unwanted object code
 There are a bunch of #define's in pngconf.h that control what parts of
 libpng are compiled.  All the defines end in _SUPPORTED.  If you are
 never going to use a capability, you can change the #define to #undef
 before recompiling libpng and save yourself code and data space, or
 you can turn off individual capabilities with defines that begin with
 PNG_NO_.
 In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead.
 You can also turn all of the transforms and ancillary chunk capabilities
 off en masse with compiler directives that define
 PNG_NO_READ[or WRITE]_TRANSFORMS, or PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS,
 or all four,
 along with directives to turn on any of the capabilities that you do
 want.  The PNG_NO_READ[or WRITE]_TRANSFORMS directives disable the extra
 transformations but still leave the library fully capable of reading
 and writing PNG files with all known public chunks. Use of the
 PNG_NO_READ[or WRITE]_ANCILLARY_CHUNKS directive produces a library
 that is incapable of reading or writing ancillary chunks.  If you are
 not using the progressive reading capability, you can turn that off
 with PNG_NO_PROGRESSIVE_READ (don't confuse this with the INTERLACING
 capability, which you'll still have).
 All the reading and writing specific code are in separate files, so the
 linker should only grab the files it needs.  However, if you want to
 make sure, or if you are building a stand alone library, all the
 reading files start with "pngr" and all the writing files start with "pngw".
 The files that don't match either (like png.c, pngtrans.c, etc.)
 are used for both reading and writing, and always need to be included.
 The progressive reader is in pngpread.c
 If you are creating or distributing a dynamically linked library (a .so
 or DLL file), you should not remove or disable any parts of the library,
 as this will cause applications linked with different versions of the
 library to fail if they call functions not available in your library.
 The size of the library itself should not be an issue, because only
 those sections that are actually used will be loaded into memory.
 Requesting debug printout
 The macro definition PNG_DEBUG can be used to request debugging
 printout.  Set it to an integer value in the range 0 to 3.  Higher
 numbers result in increasing amounts of debugging information.  The
 information is printed to the "stderr" file, unless another file
 name is specified in the PNG_DEBUG_FILE macro definition.
 When PNG_DEBUG > 0, the following functions (macros) become available:
    png_debug(level, message)
    png_debug1(level, message, p1)
    png_debug2(level, message, p1, p2)
 in which "level" is compared to PNG_DEBUG to decide whether to print
 the message, "message" is the formatted string to be printed,
 and p1 and p2 are parameters that are to be embedded in the string
 according to printf-style formatting directives.  For example,
    png_debug1(2, "foo=%d", foo);
 is expanded to
    if (PNG_DEBUG > 2)
       fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);
 When PNG_DEBUG is defined but is zero, the macros aren't defined, but you
 can still use PNG_DEBUG to control your own debugging:
    #ifdef PNG_DEBUG
        fprintf(stderr, ...
    #endif
 When PNG_DEBUG = 1, the macros are defined, but only png_debug statements
 having level = 0 will be printed.  There aren't any such statements in
 this version of libpng, but if you insert some they will be printed.
 Prepending a prefix to exported symbols
 Starting with libpng-1.6.0, you can configure libpng (when using the
 "configure" script) to prefix all exported symbols by means of the
 configuration option "--with-libpng-prefix=FOO_", where FOO_ can be any
 string beginning with a letter and containing only uppercase
 and lowercase letters, digits, and the underscore (i.e., a C language
 identifier).  This creates a set of macros in pnglibconf.h, so this is
 transparent to applications; their function calls get transformed by
 the macros to use the modified names.
 VII.  MNG support
 The MNG specification (available at http://www.libpng.org/pub/mng) allows
 certain extensions to PNG for PNG images that are embedded in MNG datastreams.
 Libpng can support some of these extensions.  To enable them, use the
 png_permit_mng_features() function:
    feature_set = png_permit_mng_features(png_ptr, mask)
    mask is a png_uint_32 containing the bitwise OR of the
         features you want to enable.  These include
         PNG_FLAG_MNG_EMPTY_PLTE
         PNG_FLAG_MNG_FILTER_64
         PNG_ALL_MNG_FEATURES
    feature_set is a png_uint_32 that is the bitwise AND of
       your mask with the set of MNG features that is
       supported by the version of libpng that you are using.
 It is an error to use this function when reading or writing a standalone
 PNG file with the PNG 8-byte signature.  The PNG datastream must be wrapped
 in a MNG datastream.  As a minimum, it must have the MNG 8-byte signature
 and the MHDR and MEND chunks.  Libpng does not provide support for these
 or any other MNG chunks; your application must provide its own support for
 them.  You may wish to consider using libmng (available at
 http://www.libmng.com) instead.
 VIII.  Changes to Libpng from version 0.88
 It should be noted that versions of libpng later than 0.96 are not
 distributed by the original libpng author, Guy Schalnat, nor by
 Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
 distributed versions 0.89 through 0.96, but rather by another member
 of the original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas are
 still alive and well, but they have moved on to other things.
 The old libpng functions png_read_init(), png_write_init(),
 png_info_init(), png_read_destroy(), and png_write_destroy() have been
 moved to PNG_INTERNAL in version 0.95 to discourage their use.  These
 functions will be removed from libpng version 1.4.0.
 The preferred method of creating and initializing the libpng structures is
 via the png_create_read_struct(), png_create_write_struct(), and
 png_create_info_struct() because they isolate the size of the structures
 from the application, allow version error checking, and also allow the
 use of custom error handling routines during the initialization, which
 the old functions do not.  The functions png_read_destroy() and
 png_write_destroy() do not actually free the memory that libpng
 allocated for these structs, but just reset the data structures, so they
 can be used instead of png_destroy_read_struct() and
 png_destroy_write_struct() if you feel there is too much system overhead
 allocating and freeing the png_struct for each image read.
 Setting the error callbacks via png_set_message_fn() before
 png_read_init() as was suggested in libpng-0.88 is no longer supported
 because this caused applications that do not use custom error functions
 to fail if the png_ptr was not initialized to zero.  It is still possible
 to set the error callbacks AFTER png_read_init(), or to change them with
 png_set_error_fn(), which is essentially the same function, but with a new
 name to force compilation errors with applications that try to use the old
 method.
 Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at libpng-1.0.6;
 however, iTXt support was not enabled by default.
 Starting with version 1.0.7, you can find out which version of the library
 you are using at run-time:
    png_uint_32 libpng_vn = png_access_version_number();
 The number libpng_vn is constructed from the major version, minor
 version with leading zero, and release number with leading zero,
 (e.g., libpng_vn for version 1.0.7 is 10007).
 Note that this function does not take a png_ptr, so you can call it
 before you've created one.
 You can also check which version of png.h you used when compiling your
 application:
    png_uint_32 application_vn = PNG_LIBPNG_VER;
 IX.  Changes to Libpng from version 1.0.x to 1.2.x
 Support for user memory management was enabled by default.  To
 accomplish this, the functions png_create_read_struct_2(),
 png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
 png_malloc_default(), and png_free_default() were added.
 Support for the iTXt chunk has been enabled by default as of
 version 1.2.41.
 Support for certain MNG features was enabled.
 Support for numbered error messages was added.  However, we never got
 around to actually numbering the error messages.  The function
 png_set_strip_error_numbers() was added (Note: the prototype for this
 function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
 builds of libpng-1.2.15.  It was restored in libpng-1.2.36).
 The png_malloc_warn() function was added at libpng-1.2.3.  This issues
 a png_warning and returns NULL instead of aborting when it fails to
 acquire the requested memory allocation.
 Support for setting user limits on image width and height was enabled
 by default.  The functions png_set_user_limits(), png_get_user_width_max(),
 and png_get_user_height_max() were added at libpng-1.2.6.
 The png_set_add_alpha() function was added at libpng-1.2.7.
 The function png_set_expand_gray_1_2_4_to_8() was added at libpng-1.2.9.
 Unlike png_set_gray_1_2_4_to_8(), the new function does not expand the
 tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function is
 deprecated.
 A number of macro definitions in support of runtime selection of
 assembler code features (especially Intel MMX code support) were
 added at libpng-1.2.0:
     PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
     PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
     PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
     PNG_ASM_FLAG_MMX_READ_INTERLACE
     PNG_ASM_FLAG_MMX_READ_FILTER_SUB
     PNG_ASM_FLAG_MMX_READ_FILTER_UP
     PNG_ASM_FLAG_MMX_READ_FILTER_AVG
     PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
     PNG_ASM_FLAGS_INITIALIZED
     PNG_MMX_READ_FLAGS
     PNG_MMX_FLAGS
     PNG_MMX_WRITE_FLAGS
     PNG_MMX_FLAGS
 We added the following functions in support of runtime
 selection of assembler code features:
     png_get_mmx_flagmask()
     png_set_mmx_thresholds()
     png_get_asm_flags()
     png_get_mmx_bitdepth_threshold()
     png_get_mmx_rowbytes_threshold()
     png_set_asm_flags()
 We replaced all of these functions with simple stubs in libpng-1.2.20,
 when the Intel assembler code was removed due to a licensing issue.
 These macros are deprecated:
     PNG_READ_TRANSFORMS_NOT_SUPPORTED
     PNG_PROGRESSIVE_READ_NOT_SUPPORTED
     PNG_NO_SEQUENTIAL_READ_SUPPORTED
     PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
     PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
     PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED
 They have been replaced, respectively, by:
     PNG_NO_READ_TRANSFORMS
     PNG_NO_PROGRESSIVE_READ
     PNG_NO_SEQUENTIAL_READ
     PNG_NO_WRITE_TRANSFORMS
     PNG_NO_READ_ANCILLARY_CHUNKS
     PNG_NO_WRITE_ANCILLARY_CHUNKS
 PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been
 deprecated since libpng-1.0.16 and libpng-1.2.6.
 The function
     png_check_sig(sig, num)
 was replaced with
     !png_sig_cmp(sig, 0, num)
 It has been deprecated since libpng-0.90.
 The function
     png_set_gray_1_2_4_to_8()
 which also expands tRNS to alpha was replaced with
     png_set_expand_gray_1_2_4_to_8()
 which does not. It has been deprecated since libpng-1.0.18 and 1.2.9.
 X.  Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
 Private libpng prototypes and macro definitions were moved from
 png.h and pngconf.h into a new pngpriv.h header file.
 Functions png_set_benign_errors(), png_benign_error(), and
 png_chunk_benign_error() were added.
 Support for setting the maximum amount of memory that the application
 will allocate for reading chunks was added, as a security measure.
 The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
 were added to the library.
 We implemented support for I/O states by adding png_ptr member io_state
 and functions png_get_io_chunk_name() and png_get_io_state() in pngget.c
 We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level
 input transforms.
 Checking for and reporting of errors in the IHDR chunk is more thorough.
 Support for global arrays was removed, to improve thread safety.
 Some obsolete/deprecated macros and functions have been removed.
 Typecasted NULL definitions such as
    #define png_voidp_NULL            (png_voidp)NULL
 were eliminated.  If you used these in your application, just use
 NULL instead.
 The png_struct and info_struct members "trans" and "trans_values" were
 changed to "trans_alpha" and "trans_color", respectively.
 The obsolete, unused pnggccrd.c and pngvcrd.c files and related makefiles
 were removed.
 The PNG_1_0_X and PNG_1_2_X macros were eliminated.
 The PNG_LEGACY_SUPPORTED macro was eliminated.
 Many WIN32_WCE #ifdefs were removed.
 The functions png_read_init(info_ptr), png_write_init(info_ptr),
 png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
 have been removed.  They have been deprecated since libpng-0.95.
 The png_permit_empty_plte() was removed. It has been deprecated
 since libpng-1.0.9.  Use png_permit_mng_features() instead.
 We removed the obsolete stub functions png_get_mmx_flagmask(),
 png_set_mmx_thresholds(), png_get_asm_flags(),
 png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
 png_set_asm_flags(), and png_mmx_supported()
 We removed the obsolete png_check_sig(), png_memcpy_check(), and
 png_memset_check() functions.  Instead use !png_sig_cmp(), memcpy(),
 and memset(), respectively.
 The function png_set_gray_1_2_4_to_8() was removed. It has been
 deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
 png_set_expand_gray_1_2_4_to_8() because the former function also
 expanded any tRNS chunk to an alpha channel.
 Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32
 were added and are used by default instead of the corresponding
 functions. Unfortunately,
 from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
 function) incorrectly returned a value of type png_uint_32.
 We changed the prototype for png_malloc() from
     png_malloc(png_structp png_ptr, png_uint_32 size)
 to
     png_malloc(png_structp png_ptr, png_alloc_size_t size)
 This also applies to the prototype for the user replacement malloc_fn().
 The png_calloc() function was added and is used in place of
 of "png_malloc(); memset();" except in the case in png_read_png()
 where the array consists of pointers; in this case a "for" loop is used
 after the png_malloc() to set the pointers to NULL, to give robust.
 behavior in case the application runs out of memory part-way through
 the process.
 We changed the prototypes of png_get_compression_buffer_size() and
 png_set_compression_buffer_size() to work with png_size_t instead of
 png_uint_32.
 Support for numbered error messages was removed by default, since we
 never got around to actually numbering the error messages. The function
 png_set_strip_error_numbers() was removed from the library by default.
 The png_zalloc() and png_zfree() functions are no longer exported.
 The png_zalloc() function no longer zeroes out the memory that it
 allocates.  Applications that called png_zalloc(png_ptr, number, size)
 can call png_calloc(png_ptr, number*size) instead, and can call
 png_free() instead of png_zfree().
 Support for dithering was disabled by default in libpng-1.4.0, because
 it has not been well tested and doesn't actually "dither".
 The code was not
 removed, however, and could be enabled by building libpng with
 PNG_READ_DITHER_SUPPORTED defined.  In libpng-1.4.2, this support
 was re-enabled, but the function was renamed png_set_quantize() to
 reflect more accurately what it actually does.  At the same time,
 the PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
 PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED
 was renamed to PNG_READ_QUANTIZE_SUPPORTED.
 We removed the trailing '.' from the warning and error messages.
 XI.  Changes to Libpng from version 1.4.x to 1.5.x
 From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
 function) incorrectly returned a value of type png_uint_32.
 The incorrect macro was removed from libpng-1.4.5.
 Checking for invalid palette index on write was added at libpng
 .5.10.  If a pixel contains an invalid (out-of-range) index libpng issues
 a benign error.  This is enabled by default because this condition is an
 error according to the PNG specification, Clause 11.3.2, but the error can
 be ignored in each png_ptr with
    png_set_check_for_invalid_index(png_ptr, allowed);
       allowed  - one of
 : disable benign error (accept the
                     invalid data without warning).
 : enable benign error (treat the
                     invalid data as an error or a
                     warning).
 If the error is ignored, or if png_benign_error() treats it as a warning,
 any invalid pixels are decoded as opaque black by the decoder and written
 as-is by the encoder.
 Retrieving the maximum palette index found was added at libpng-1.5.15.
 This statement must appear after png_read_png() or png_read_image() while
 reading, and after png_write_png() or png_write_image() while writing.
    int max_palette = png_get_palette_max(png_ptr, info_ptr);
 This will return the maximum palette index found in the image, or "-1" if
 the palette was not checked, or "0" if no palette was found.  Note that this
 does not account for any palette index used by ancillary chunks such as the
 bKGD chunk; you must check those separately to determine the maximum
 palette index actually used.
 A. Changes that affect users of libpng
 There are no substantial API changes between the non-deprecated parts of
 the 1.4.5 API and the 1.5.0 API; however, the ability to directly access
 members of the main libpng control structures, png_struct and png_info,
 deprecated in earlier versions of libpng, has been completely removed from
 libpng 1.5.
 We no longer include zlib.h in png.h.  The include statement has been moved
 to pngstruct.h, where it is not accessible by applications. Applications that
 need access to information in zlib.h will need to add the '#include "zlib.h"'
 directive.  It does not matter whether this is placed prior to or after
 the '"#include png.h"' directive.
 The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer used
 and were removed.
 We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
 macros into a private header file (pngpriv.h) that is not accessible to
 applications.
 In png_get_iCCP, the type of "profile" was changed from png_charpp
 to png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.
 There are changes of form in png.h, including new and changed macros to
 declare parts of the API.  Some API functions with arguments that are
 pointers to data not modified within the function have been corrected to
 declare these arguments with PNG_CONST.
 Much of the internal use of C macros to control the library build has also
 changed and some of this is visible in the exported header files, in
 particular the use of macros to control data and API elements visible
 during application compilation may require significant revision to
 application code.  (It is extremely rare for an application to do this.)
 Any program that compiled against libpng 1.4 and did not use deprecated
 features or access internal library structures should compile and work
 against libpng 1.5, except for the change in the prototype for
 png_get_iCCP() and png_set_iCCP() API functions mentioned above.
 libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing of
 interlaced images.  The macros return the number of rows and columns in
 each pass and information that can be used to de-interlace and (if
 absolutely necessary) interlace an image.
 libpng 1.5.0 adds an API png_longjmp(png_ptr, value).  This API calls
 the application-provided png_longjmp_ptr on the internal, but application
 initialized, longjmp buffer.  It is provided as a convenience to avoid
 the need to use the png_jmpbuf macro, which had the unnecessary side
 effect of resetting the internal png_longjmp_ptr value.
 libpng 1.5.0 includes a complete fixed point API.  By default this is
 present along with the corresponding floating point API.  In general the
 fixed point API is faster and smaller than the floating point one because
 the PNG file format used fixed point, not floating point.  This applies
 even if the library uses floating point in internal calculations.  A new
 macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether the library
 uses floating point arithmetic (the default) or fixed point arithmetic
 internally for performance critical calculations such as gamma correction.
 In some cases, the gamma calculations may produce slightly different
 results.  This has changed the results in png_rgb_to_gray and in alpha
 composition (png_set_background for example). This applies even if the
 original image was already linear (gamma == 1.0) and, therefore, it is
 not necessary to linearize the image.  This is because libpng has *not*
 been changed to optimize that case correctly, yet.
 Fixed point support for the sCAL chunk comes with an important caveat;
 the sCAL specification uses a decimal encoding of floating point values
 and the accuracy of PNG fixed point values is insufficient for
 representation of these values. Consequently a "string" API
 (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of reading
 arbitrary sCAL chunks in the absence of either the floating point API or
 internal floating point calculations.  Starting with libpng-1.5.0, both
 of these functions are present when PNG_sCAL_SUPPORTED is defined.  Prior
 to libpng-1.5.0, their presence also depended upon PNG_FIXED_POINT_SUPPORTED
 being defined and PNG_FLOATING_POINT_SUPPORTED not being defined.
 Applications no longer need to include the optional distribution header
 file pngusr.h or define the corresponding macros during application
 build in order to see the correct variant of the libpng API.  From 1.5.0
 application code can check for the corresponding _SUPPORTED macro:
 #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
    /* code that uses the inch conversion APIs. */
 #endif
 This macro will only be defined if the inch conversion functions have been
 compiled into libpng.  The full set of macros, and whether or not support
 has been compiled in, are available in the header file pnglibconf.h.
 This header file is specific to the libpng build.  Notice that prior to
 .5.0 the _SUPPORTED macros would always have the default definition unless
 reset by pngusr.h or by explicit settings on the compiler command line.
 These settings may produce compiler warnings or errors in 1.5.0 because
 of macro redefinition.
 Applications can now choose whether to use these macros or to call the
 corresponding function by defining PNG_USE_READ_MACROS or
 PNG_NO_USE_READ_MACROS before including png.h.  Notice that this is
 only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to 1.5.0
 will lead to a link failure.
 Prior to libpng-1.5.4, the zlib compressor used the same set of parameters
 when compressing the IDAT data and textual data such as zTXt and iCCP.
 In libpng-1.5.4 we reinitialized the zlib stream for each type of data.
 We added five png_set_text_*() functions for setting the parameters to
 use with textual data.
 Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
 option was off by default, and slightly inaccurate scaling occurred.
 This option can no longer be turned off, and the choice of accurate
 or inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8()
 API for accurate scaling or the old png_set_strip_16_to_8() API for simple
 chopping.  In libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
 macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8
 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two
 png_set_*_16_to_8() functions separately.
 Prior to libpng-1.5.4, the png_set_user_limits() function could only be
 used to reduce the width and height limits from the value of
 PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
 that it could be used to override them.  Now this function will reduce or
 increase the limits.
 Starting in libpng-1.5.10, the user limits can be set en masse with the
 configuration option PNG_SAFE_LIMITS_SUPPORTED.  If this option is enabled,
 a set of "safe" limits is applied in pngpriv.h.  These can be overridden by
 application calls to png_set_user_limits(), png_set_user_chunk_cache_max(),
 and/or png_set_user_malloc_max() that increase or decrease the limits.  Also,
 in libpng-1.5.10 the default width and height limits were increased
 from 1,000,000 to 0x7ffffff (i.e., made unlimited).  Therefore, the
 limits are now
                                default      safe
    png_user_width_max        0x7fffffff    1,000,000
    png_user_height_max       0x7fffffff    1,000,000
    png_user_chunk_cache_max  0 (unlimited)   128
    png_user_chunk_malloc_max 0 (unlimited) 8,000,000
 The png_set_option() function (and the "options" member of the png struct) was
 added to libpng-1.5.15.
 B. Changes to the build and configuration of libpng
 Details of internal changes to the library code can be found in the CHANGES
 file and in the GIT repository logs.  These will be of no concern to the vast
 majority of library users or builders; however, the few who configure libpng
 to a non-default feature set may need to change how this is done.
 There should be no need for library builders to alter build scripts if
 these use the distributed build support - configure or the makefiles -
 however, users of the makefiles may care to update their build scripts
 to build pnglibconf.h where the corresponding makefile does not do so.
 Building libpng with a non-default configuration has changed completely.
 The old method using pngusr.h should still work correctly even though the
 way pngusr.h is used in the build has been changed; however, library
 builders will probably want to examine the changes to take advantage of
 new capabilities and to simplify their build system.
 B.1 Specific changes to library configuration capabilities
 The library now supports a complete fixed point implementation and can
 thus be used on systems that have no floating point support or very
 limited or slow support.  Previously gamma correction, an essential part
 of complete PNG support, required reasonably fast floating point.
 As part of this the choice of internal implementation has been made
 independent of the choice of fixed versus floating point APIs and all the
 missing fixed point APIs have been implemented.
 The exact mechanism used to control attributes of API functions has
 changed.  A single set of operating system independent macro definitions
 is used and operating system specific directives are defined in
 pnglibconf.h
 As part of this the mechanism used to choose procedure call standards on
 those systems that allow a choice has been changed.  At present this only
 affects certain Microsoft (DOS, Windows) and IBM (OS/2) operating systems
 running on Intel processors.  As before, PNGAPI is defined where required
 to control the exported API functions; however, two new macros, PNGCBAPI
 and PNGCAPI, are used instead for callback functions (PNGCBAPI) and
 (PNGCAPI) for functions that must match a C library prototype (currently
 only png_longjmp_ptr, which must match the C longjmp function.)  The new
 approach is documented in pngconf.h
 Despite these changes, libpng 1.5.0 only supports the native C function
 calling standard on those platforms tested so far (__cdecl on Microsoft
 Windows).  This is because the support requirements for alternative
 calling conventions seem to no longer exist.  Developers who find it
 necessary to set PNG_API_RULE to 1 should advise the mailing list
 (png-mng-implement) of this and library builders who use Openwatcom and
 therefore set PNG_API_RULE to 2 should also contact the mailing list.
 A new test program, pngvalid, is provided in addition to pngtest.
 pngvalid validates the arithmetic accuracy of the gamma correction
 calculations and includes a number of validations of the file format.
 A subset of the full range of tests is run when "make check" is done
 (in the 'configure' build.)  pngvalid also allows total allocated memory
 usage to be evaluated and performs additional memory overwrite validation.
 Many changes to individual feature macros have been made. The following
 are the changes most likely to be noticed by library builders who
 configure libpng:
 ) All feature macros now have consistent naming:
 #define PNG_NO_feature turns the feature off
 #define PNG_feature_SUPPORTED turns the feature on
 pnglibconf.h contains one line for each feature macro which is either:
 #define PNG_feature_SUPPORTED
 if the feature is supported or:
 /*#undef PNG_feature_SUPPORTED*/
 if it is not.  Library code consistently checks for the 'SUPPORTED' macro.
 It does not, and libpng applications should not, check for the 'NO' macro
 which will not normally be defined even if the feature is not supported.
 The 'NO' macros are only used internally for setting or not setting the
 corresponding 'SUPPORTED' macros.
 Compatibility with the old names is provided as follows:
 PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED
 And the following definitions disable the corresponding feature:
 PNG_SETJMP_NOT_SUPPORTED disables SETJMP
 PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS
 PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV
 PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS
 PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS
 PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables WRITE_ANCILLARY_CHUNKS
 Library builders should remove use of the above, inconsistent, names.
 ) Warning and error message formatting was previously conditional on
 the STDIO feature. The library has been changed to use the
 CONSOLE_IO feature instead. This means that if CONSOLE_IO is disabled
 the library no longer uses the printf(3) functions, even though the
 default read/write implementations use (FILE) style stdio.h functions.
 ) Three feature macros now control the fixed/floating point decisions:
 PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs
 PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in
 practice these are normally required internally anyway (because the PNG
 file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
 merely stops the function from being exported.
 PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
 point implementation or the fixed point one.  Typically the fixed point
 implementation is larger and slower than the floating point implementation
 on a system that supports floating point; however, it may be faster on a
 system which lacks floating point hardware and therefore uses a software
 emulation.
 ) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.  This allows the
 functions to read and write ints to be disabled independently of
 PNG_USE_READ_MACROS, which allows libpng to be built with the functions
 even though the default is to use the macros - this allows applications
 to choose at app buildtime whether or not to use macros (previously
 impossible because the functions weren't in the default build.)
 B.2 Changes to the configuration mechanism
 Prior to libpng-1.5.0 library builders who needed to configure libpng
 had either to modify the exported pngconf.h header file to add system
 specific configuration or had to write feature selection macros into
 pngusr.h and cause this to be included into pngconf.h by defining
 PNG_USER_CONFIG. The latter mechanism had the disadvantage that an
 application built without PNG_USER_CONFIG defined would see the
 unmodified, default, libpng API and thus would probably fail to link.
 These mechanisms still work in the configure build and in any makefile
 build that builds pnglibconf.h, although the feature selection macros
 have changed somewhat as described above.  In 1.5.0, however, pngusr.h is
 processed only once, when the exported header file pnglibconf.h is built.
 pngconf.h no longer includes pngusr.h, therefore pngusr.h is ignored after the
 build of pnglibconf.h and it is never included in an application build.
 The rarely used alternative of adding a list of feature macros to the
 CFLAGS setting in the build also still works; however, the macros will be
 copied to pnglibconf.h and this may produce macro redefinition warnings
 when the individual C files are compiled.
 All configuration now only works if pnglibconf.h is built from
 scripts/pnglibconf.dfa.  This requires the program awk.  Brian Kernighan
 (the original author of awk) maintains C source code of that awk and this
 and all known later implementations (often called by subtly different
 names - nawk and gawk for example) are adequate to build pnglibconf.h.
 The Sun Microsystems (now Oracle) program 'awk' is an earlier version
 and does not work; this may also apply to other systems that have a
 functioning awk called 'nawk'.
 Configuration options are now documented in scripts/pnglibconf.dfa.  This
 file also includes dependency information that ensures a configuration is
 consistent; that is, if a feature is switched off dependent features are
 also removed.  As a recommended alternative to using feature macros in
 pngusr.h a system builder may also define equivalent options in pngusr.dfa
 (or, indeed, any file) and add that to the configuration by setting
 DFA_XTRA to the file name.  The makefiles in contrib/pngminim illustrate
 how to do this, and a case where pngusr.h is still required.
 XII.  Changes to Libpng from version 1.5.x to 1.6.x
 A "simplified API" has been added (see documentation in png.h and a simple
 example in contrib/examples/pngtopng.c).  The new publicly visible API
 includes the following:
    macros:
      PNG_FORMAT_*
      PNG_IMAGE_*
    structures:
      png_control
      png_image
    read functions
      png_image_begin_read_from_file()
      png_image_begin_read_from_stdio()
      png_image_begin_read_from_memory()
      png_image_finish_read()
      png_image_free()
    write functions
      png_image_write_to_file()
      png_image_write_to_stdio()
 Starting with libpng-1.6.0, you can configure libpng to prefix all exported
 symbols, using the PNG_PREFIX macro.
 We no longer include string.h in png.h.  The include statement has been moved
 to pngpriv.h, where it is not accessible by applications.  Applications that
 need access to information in string.h must add an '#include <string.h>'
 directive.  It does not matter whether this is placed prior to or after
 the '#include "png.h"' directive.
 The following API are now DEPRECATED:
    png_info_init_3()
    png_convert_to_rfc1123() which has been replaced
      with png_convert_to_rfc1123_buffer()
    png_data_freer()
    png_malloc_default()
    png_free_default()
    png_reset_zstream()
 The following have been removed:
    png_get_io_chunk_name(), which has been replaced
      with png_get_io_chunk_type().  The new
      function returns a 32-bit integer instead of
      a string.
    The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
      png_memset() macros are no longer used in the libpng sources and
      have been removed.  These had already been made invisible to applications
      (i.e., defined in the private pngpriv.h header file) since libpng-1.5.0.
 The signatures of many exported functions were changed, such that
    png_structp became png_structrp or png_const_structrp
    png_infop became png_inforp or png_const_inforp
 where "rp" indicates a "restricted pointer".
 Error detection in some chunks has improved; in particular the iCCP chunk
 reader now does pretty complete validation of the basic format.  Some bad
 profiles that were previously accepted are now accepted with a warning or
 rejected, depending upon the png_set_benign_errors() setting, in particular the
 very old broken Microsoft/HP 3144-byte sRGB profile.  The PNG spec requirement
 that only grayscale profiles may appear in images with color type 0 or 4 and
 that even if the image only contains gray pixels, only RGB profiles may appear
 in images with color type 2, 3, or 6, is now enforced.  The sRGB chunk
 is allowed to appear in images with any color type.
 Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk contained
 an empty language field or an empty translated keyword.  Both of these
 are allowed by the PNG specification, so these warnings are no longer issued.
 The library now issues an error if the application attempts to set a
 transform after it calls png_read_update_info() or if it attempts to call
 both png_read_update_info() and png_start_read_image() or to call either
 of them more than once.
 The default condition for benign_errors is now to treat benign errors as
 warnings while reading and as errors while writing.
 The library now issues a warning if both background processing and RGB to
 gray are used when gamma correction happens. As with previous versions of
 the library the results are numerically very incorrect in this case.
 There are some minor arithmetic changes in some transforms such as
 png_set_background(), that might be detected by certain regression tests.
 Unknown chunk handling has been improved internally, without any API change.
 This adds more correct option control of the unknown handling, corrects
 a pre-existing bug where the per-chunk 'keep' setting is ignored, and makes
 it possible to skip IDAT chunks in the sequential reader.
 The machine-generated configure files are no longer included in branches
 libpng16 and later of the GIT repository.  They continue to be included
 in the tarball releases, however.
 Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the IDAT
 stream to set the size of the sliding window for reading instead of using the
 default 32-kbyte sliding window size.  It was discovered that there are
 hundreds of PNG files in the wild that have incorrect CMF bytes that caused
 libpng to issue a "too far back" error and reject the file.  Libpng-1.6.3 and
 later calculate their own safe CMF from the image dimensions, provide a way
 to revert to the libpng-1.5.x behavior (ignoring the CMF bytes and using a
 -kbyte sliding window), by using
     png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
         PNG_OPTION_ON);
 and provide a tool (contrib/tools/pngfix) for optimizing the CMF bytes
 correctly.
 Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the wrong
 length, which resulted in PNG files that cannot be read beyond the bad iTXt
 chunk.  This error was fixed in libpng-1.6.3, and a tool (called
 contrib/tools/png-fix-itxt) has been added to the libpng distribution.
 XIII.  Detecting libpng
 The png_get_io_ptr() function has been present since libpng-0.88, has never
 changed, and is unaffected by conditional compilation macros.  It is the
 best choice for use in configure scripts for detecting the presence of any
 libpng version since 0.88.  In an autoconf "configure.in" you could use
     AC_CHECK_LIB(png, png_get_io_ptr, ...
 XV. Source code repository
 Since about February 2009, version 1.2.34, libpng has been under "git" source
 control.  The git repository was built from old libpng-x.y.z.tar.gz files
 going back to version 0.70.  You can access the git repository (read only)
 at
     git://git.code.sf.net/p/libpng/code
 or you can browse it with a web browser by selecting the "code" button at
     https://sourceforge.net/projects/libpng
 Patches can be sent to glennrp at users.sourceforge.net or to
 png-mng-implement at lists.sourceforge.net or you can upload them to
 the libpng bug tracker at
     http://libpng.sourceforge.net
 We also accept patches built from the tar or zip distributions, and
 simple verbal discriptions of bug fixes, reported either to the
 SourceForge bug tracker, to the png-mng-implement at lists.sf.net
 mailing list, or directly to glennrp.
 XV. Coding style
 Our coding style is similar to the "Allman" style, with curly
 braces on separate lines:
     if (condition)
     {
        action;
     }
     else if (another condition)
     {
        another action;
     }
 The braces can be omitted from simple one-line actions:
     if (condition)
        return (0);
 We use 3-space indentation, except for continued statements which
 are usually indented the same as the first line of the statement
 plus four more spaces.
 For macro definitions we use 2-space indentation, always leaving the "#"
 in the first column.
     #ifndef PNG_NO_FEATURE
     #  ifndef PNG_FEATURE_SUPPORTED
     #    define PNG_FEATURE_SUPPORTED
     #  endif
     #endif
 Comments appear with the leading "/*" at the same indentation as
 the statement that follows the comment:
     /* Single-line comment */
     statement;
     /* This is a multiple-line
      * comment.
      */
     statement;
 Very short comments can be placed after the end of the statement
 to which they pertain:
     statement;    /* comment */
 We don't use C++ style ("//") comments. We have, however,
 used them in the past in some now-abandoned MMX assembler
 code.
 Functions and their curly braces are not indented, and
 exported functions are marked with PNGAPI:
  /* This is a public function that is visible to
   * application programmers. It does thus-and-so.
   */
  void PNGAPI
  png_exported_function(png_ptr, png_info, foo)
  {
     body;
  }
 The prototypes for all exported functions appear in png.h,
 above the comment that says
     /* Maintainer: Put new public prototypes here ... */
 We mark all non-exported functions with "/* PRIVATE */"":
  void /* PRIVATE */
  png_non_exported_function(png_ptr, png_info, foo)
  {
     body;
  }
 The prototypes for non-exported functions (except for those in
 pngtest) appear in
 pngpriv.h
 above the comment that says
   /* Maintainer: Put new private prototypes here ^ */
 We put a space after the "sizeof" operator and we omit the
 optional parentheses around its argument when the argument
 is an expression, not a type name, and we always enclose the
 sizeof operator, with its argument, in parentheses:
   (sizeof (png_uint_32))
   (sizeof array)
 Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as
 though it were a function.
 To avoid polluting the global namespace, the names of all exported
 functions and variables begin with "png_", and all publicly visible C
 preprocessor macros begin with "PNG".  We request that applications that
 use libpng *not* begin any of their own symbols with either of these strings.
 We put a space after each comma and after each semicolon
 in "for" statements, and we put spaces before and after each
 C binary operator and after "for" or "while", and before
 "?".  We don't put a space between a typecast and the expression
 being cast, nor do we put one between a function name and the
 left parenthesis that follows it:
     for (i = 2; i > 0; --i)
        y[i] = a(x) + (int)b;
 We prefer #ifdef and #ifndef to #if defined() and #if !defined()
 when there is only one macro being tested.  We always use parentheses
 with "defined".
 We prefer to express integers that are used as bit masks in hex format,
 with an even number of lower-case hex digits (e.g., 0x00, 0xff, 0x0100).
 We prefer to use underscores in variable names rather than camelCase, except
 for a few type names that we inherit from zlib.h.
 We do not use the TAB character for indentation in the C sources.
 Lines do not exceed 80 characters.
 Other rules can be inferred by inspecting the libpng source.
 XVI. Y2K Compliance in libpng
 February 6, 2014
 Since the PNG Development group is an ad-hoc body, we can't make
 an official declaration.
 This is your unofficial assurance that libpng from version 0.71 and
 upward through 1.6.9 are Y2K compliant.  It is my belief that earlier
 versions were also Y2K compliant.
 Libpng only has two year fields.  One is a 2-byte unsigned integer
 that will hold years up to 65535.  The other, which is deprecated,
 holds the date in text format, and will hold years up to 9999.
 The integer is
     "png_uint_16 year" in png_time_struct.
 The string is
     "char time_buffer[29]" in png_struct.  This is no longer used
 in libpng-1.6.x and will be removed from libpng-1.7.0.
 There are seven time-related functions:
     png_convert_to_rfc_1123() in png.c
       (formerly png_convert_to_rfc_1152() in error)
     png_convert_from_struct_tm() in pngwrite.c, called
       in pngwrite.c
     png_convert_from_time_t() in pngwrite.c
     png_get_tIME() in pngget.c
     png_handle_tIME() in pngrutil.c, called in pngread.c
     png_set_tIME() in pngset.c
     png_write_tIME() in pngwutil.c, called in pngwrite.c
 All appear to handle dates properly in a Y2K environment.  The
 png_convert_from_time_t() function calls gmtime() to convert from system
 clock time, which returns (year - 1900), which we properly convert to
 the full 4-digit year.  There is a possibility that applications using
 libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
 function, or that they are incorrectly passing only a 2-digit year
 instead of "year - 1900" into the png_convert_from_struct_tm() function,
 but this is not under our control.  The libpng documentation has always
 stated that it works with 4-digit years, and the APIs have been
 documented as such.
 The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
 integer to hold the year, and can hold years as large as 65535.
 zlib, upon which libpng depends, is also Y2K compliant.  It contains
 no date-related code.
    Glenn Randers-Pehrson
    libpng maintainer
    PNG Development Group

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