The Tor Browser / file revision

 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

                  1: ignore; do not keep

                  2: keep only if safe-to-copy

                  3: 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[]=

       {

         104,  73,  83,  84, (png_byte) '\0',   /* hIST */

         105,  84,  88, 116, (png_byte) '\0',   /* iTXt */

         112,  67,  65,  76, (png_byte) '\0',   /* pCAL */

         115,  67,  65,  76, (png_byte) '\0',   /* sCAL */

         115,  80,  76,  84, (png_byte) '\0',   /* sPLT */

         116,  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:

 8-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.

 16-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

                                 8-bit accurately

     PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to

                                 8-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:

 1) 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.

 2) 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.

 3) The number of significant bits in each component value.  Libpng uses this to

 optimize gamma handling by reducing the internal lookup table sizes.

 4) 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

                      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_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),

                      4 (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

                          1-79 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,

        72 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

 2 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.

 16-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

 8 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

 1.5.4).

 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

    01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -

    31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q

     0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB

    0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt

    0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt

     2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B

    2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt

    2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt

     3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q

    3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt

    3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt

    4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA

    4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G

    6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA

    6O    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

 1 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

        64K.  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

        1 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

 1.5.10.  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