The Tor Browser / file revision

 *******************************************************************************

 **     Background

 *******************************************************************************

 libjpeg-turbo is a JPEG image codec that uses SIMD instructions (MMX, SSE2,

 NEON) to accelerate baseline JPEG compression and decompression on x86, x86-64,

 and ARM systems.  On such systems, libjpeg-turbo is generally 2-4x as fast as

 libjpeg, all else being equal.  On other types of systems, libjpeg-turbo can

 still outperform libjpeg by a significant amount, by virtue of its

 highly-optimized Huffman coding routines.  In many cases, the performance of

 libjpeg-turbo rivals that of proprietary high-speed JPEG codecs.

 libjpeg-turbo implements both the traditional libjpeg API as well as the less

 powerful but more straightforward TurboJPEG API.  libjpeg-turbo also features

 colorspace extensions that allow it to compress from/decompress to 32-bit and

 big-endian pixel buffers (RGBX, XBGR, etc.), as well as a full-featured Java

 interface.

 libjpeg-turbo was originally based on libjpeg/SIMD, an MMX-accelerated

 derivative of libjpeg v6b developed by Miyasaka Masaru.  The TigerVNC and

 VirtualGL projects made numerous enhancements to the codec in 2009, and in

 early 2010, libjpeg-turbo spun off into an independent project, with the goal

 of making high-speed JPEG compression/decompression technology available to a

 broader range of users and developers.

 *******************************************************************************

 **     License

 *******************************************************************************

 Most of libjpeg-turbo inherits the non-restrictive, BSD-style license used by

 libjpeg (see README.)  The TurboJPEG wrapper (both C and Java versions) and

 associated test programs bear a similar license, which is reproduced below:

 Redistribution and use in source and binary forms, with or without

 modification, are permitted provided that the following conditions are met:

 - Redistributions of source code must retain the above copyright notice,

   this list of conditions and the following disclaimer.

 - Redistributions in binary form must reproduce the above copyright notice,

   this list of conditions and the following disclaimer in the documentation

   and/or other materials provided with the distribution.

 - Neither the name of the libjpeg-turbo Project nor the names of its

   contributors may be used to endorse or promote products derived from this

   software without specific prior written permission.

 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS",

 AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE

 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

 INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

 CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

 POSSIBILITY OF SUCH DAMAGE.

 *******************************************************************************

 **     Using libjpeg-turbo

 *******************************************************************************

 libjpeg-turbo includes two APIs that can be used to compress and decompress

 JPEG images:

   TurboJPEG API:  This API provides an easy-to-use interface for compressing

   and decompressing JPEG images in memory.  It also provides some functionality

   that would not be straightforward to achieve using the underlying libjpeg

   API, such as generating planar YUV images and performing multiple

   simultaneous lossless transforms on an image.  The Java interface for

   libjpeg-turbo is written on top of the TurboJPEG API.

   libjpeg API:  This is the de facto industry-standard API for compressing and

   decompressing JPEG images.  It is more difficult to use than the TurboJPEG

   API but also more powerful.  The libjpeg API implementation in libjpeg-turbo

   is both API/ABI-compatible and mathematically compatible with libjpeg v6b.

   It can also optionally be configured to be API/ABI-compatible with libjpeg v7

   and v8 (see below.)

 There is no significant performance advantage to either API when both are used

 to perform similar operations.

 ======================

 Installation Directory

 ======================

 This document assumes that libjpeg-turbo will be installed in the default

 directory (/opt/libjpeg-turbo on Un*x and Mac systems and

 c:\libjpeg-turbo[-gcc][64] on Windows systems.  If your installation of

 libjpeg-turbo resides in a different directory, then adjust the instructions

 accordingly.

 =============================

 Replacing libjpeg at Run Time

 =============================

 Un*x

 ----

 If a Un*x application is dynamically linked with libjpeg, then you can replace

 libjpeg with libjpeg-turbo at run time by manipulating LD_LIBRARY_PATH.

 For instance:

   [Using libjpeg]

   > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg

   real  0m0.392s

   user  0m0.074s

   sys   0m0.020s

   [Using libjpeg-turbo]

   > export LD_LIBRARY_PATH=/opt/libjpeg-turbo/{lib}:$LD_LIBRARY_PATH

   > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg

   real  0m0.109s

   user  0m0.029s

   sys   0m0.010s

 ({lib} = lib32 or lib64, depending on whether you wish to use the 32-bit or the

 64-bit version of libjpeg-turbo.)

 System administrators can also replace the libjpeg symlinks in /usr/lib* with

 links to the libjpeg-turbo dynamic library located in /opt/libjpeg-turbo/{lib}.

 This will effectively accelerate every application that uses the libjpeg

 dynamic library on the system.

 Windows

 -------

 If a Windows application is dynamically linked with libjpeg, then you can

 replace libjpeg with libjpeg-turbo at run time by backing up the application's

 copy of jpeg62.dll, jpeg7.dll, or jpeg8.dll (assuming the application has its

 own local copy of this library) and copying the corresponding DLL from

 libjpeg-turbo into the application's install directory.  The official

 libjpeg-turbo binary packages only provide jpeg62.dll.  If the application uses

 jpeg7.dll or jpeg8.dll instead, then it will be necessary to build

 libjpeg-turbo from source (see "libjpeg v7 and v8 API/ABI Emulation" below.)

 The following information is specific to the official libjpeg-turbo binary

 packages for Visual C++:

 -- jpeg62.dll requires the Visual C++ 2008 C run-time DLL (msvcr90.dll).

 msvcr90.dll ships with more recent versions of Windows, but users of older

 Windows releases can obtain it from the Visual C++ 2008 Redistributable

 Package, which is available as a free download from Microsoft's web site.

 -- Features of the libjpeg API that require passing a C run-time structure,

 such as a file handle, from an application to the library will probably not

 work with jpeg62.dll, unless the application is also built to use the Visual

 C++ 2008 C run-time DLL.  In particular, this affects jpeg_stdio_dest() and

 jpeg_stdio_src().

 Mac

 ---

 Mac applications typically embed their own copies of the libjpeg dylib inside

 the (hidden) application bundle, so it is not possible to globally replace

 libjpeg on OS X systems.  Replacing the application's version of the libjpeg

 dylib would generally involve copying libjpeg.*.dylib from libjpeg-turbo into

 the appropriate place in the application bundle and using install_name_tool to

 repoint the libjpeg-turbo dylib to its new directory.  This requires an

 advanced knowledge of OS X and would not survive an upgrade or a re-install of

 the application.  Thus, it is not recommended for most users.

 ========================================

 Using libjpeg-turbo in Your Own Programs

 ========================================

 For the most part, libjpeg-turbo should work identically to libjpeg, so in

 most cases, an application can be built against libjpeg and then run against

 libjpeg-turbo.  On Un*x systems and Cygwin, you can build against libjpeg-turbo

 instead of libjpeg by setting

   CPATH=/opt/libjpeg-turbo/include

   and

   LIBRARY_PATH=/opt/libjpeg-turbo/{lib}

 ({lib} = lib32 or lib64, depending on whether you are building a 32-bit or a

 64-bit application.)

 If using MinGW, then set

   CPATH=/c/libjpeg-turbo-gcc[64]/include

   and

   LIBRARY_PATH=/c/libjpeg-turbo-gcc[64]/lib

 Building against libjpeg-turbo is useful, for instance, if you want to build an

 application that leverages the libjpeg-turbo colorspace extensions (see below.)

 On Un*x systems, you would still need to manipulate LD_LIBRARY_PATH or create

 appropriate symlinks to use libjpeg-turbo at run time.  On such systems, you

 can pass -R /opt/libjpeg-turbo/{lib} to the linker to force the use of

 libjpeg-turbo at run time rather than libjpeg (also useful if you want to

 leverage the colorspace extensions), or you can link against the libjpeg-turbo

 static library.

 To force a Un*x or MinGW application to link against the static version of

 libjpeg-turbo, you can use the following linker options:

   -Wl,-Bstatic -ljpeg -Wl,-Bdynamic

 On OS X, simply add /opt/libjpeg-turbo/lib/libjpeg.a to the linker command

 line.

 To build Visual C++ applications using libjpeg-turbo, add

 c:\libjpeg-turbo[64]\include to the system or user INCLUDE environment

 variable and c:\libjpeg-turbo[64]\lib to the system or user LIB environment

 variable, and then link against either jpeg.lib (to use the DLL version of

 libjpeg-turbo) or jpeg-static.lib (to use the static version of libjpeg-turbo.)

 =====================

 Colorspace Extensions

 =====================

 libjpeg-turbo includes extensions that allow JPEG images to be compressed

 directly from (and decompressed directly to) buffers that use BGR, BGRX,

 RGBX, XBGR, and XRGB pixel ordering.  This is implemented with ten new

 colorspace constants:

   JCS_EXT_RGB   /* red/green/blue */

   JCS_EXT_RGBX  /* red/green/blue/x */

   JCS_EXT_BGR   /* blue/green/red */

   JCS_EXT_BGRX  /* blue/green/red/x */

   JCS_EXT_XBGR  /* x/blue/green/red */

   JCS_EXT_XRGB  /* x/red/green/blue */

   JCS_EXT_RGBA  /* red/green/blue/alpha */

   JCS_EXT_BGRA  /* blue/green/red/alpha */

   JCS_EXT_ABGR  /* alpha/blue/green/red */

   JCS_EXT_ARGB  /* alpha/red/green/blue */

 Setting cinfo.in_color_space (compression) or cinfo.out_color_space

 (decompression) to one of these values will cause libjpeg-turbo to read the

 red, green, and blue values from (or write them to) the appropriate position in

 the pixel when compressing from/decompressing to an RGB buffer.

 Your application can check for the existence of these extensions at compile

 time with:

   #ifdef JCS_EXTENSIONS

 At run time, attempting to use these extensions with a libjpeg implementation

 that does not support them will result in a "Bogus input colorspace" error.

 Applications can trap this error in order to test whether run-time support is

 available for the colorspace extensions.

 When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the

 X byte is undefined, and in order to ensure the best performance, libjpeg-turbo

 can set that byte to whatever value it wishes.  If an application expects the X

 byte to be used as an alpha channel, then it should specify JCS_EXT_RGBA,

 JCS_EXT_BGRA, JCS_EXT_ABGR, or JCS_EXT_ARGB.  When these colorspace constants

 are used, the X byte is guaranteed to be 0xFF, which is interpreted as opaque.

 Your application can check for the existence of the alpha channel colorspace

 extensions at compile time with:

   #ifdef JCS_ALPHA_EXTENSIONS

 jcstest.c, located in the libjpeg-turbo source tree, demonstrates how to check

 for the existence of the colorspace extensions at compile time and run time.

 ===================================

 libjpeg v7 and v8 API/ABI Emulation

 ===================================

 With libjpeg v7 and v8, new features were added that necessitated extending the

 compression and decompression structures.  Unfortunately, due to the exposed

 nature of those structures, extending them also necessitated breaking backward

 ABI compatibility with previous libjpeg releases.  Thus, programs that were

 built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is

 based on the libjpeg v6b code base.  Although libjpeg v7 and v8 are still not

 as widely used as v6b, enough programs (including a few Linux distros) made

 the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs

 in libjpeg-turbo.  It should be noted, however, that this feature was added

 primarily so that applications that had already been compiled to use libjpeg

 v7+ could take advantage of accelerated baseline JPEG encoding/decoding

 without recompiling.  libjpeg-turbo does not claim to support all of the

 libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all

 cases (see below.)

 By passing an argument of --with-jpeg7 or --with-jpeg8 to configure, or an

 argument of -DWITH_JPEG7=1 or -DWITH_JPEG8=1 to cmake, you can build a version

 of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that programs

 that are built against libjpeg v7 or v8 can be run with libjpeg-turbo.  The

 following section describes which libjpeg v7+ features are supported and which

 aren't.

 Support for libjpeg v7 and v8 Features:

 ---------------------------------------

 Fully supported:

 -- libjpeg: IDCT scaling extensions in decompressor

    libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8,

    1/2, 5/8, 3/4, 7/8, 9/8, 5/4, 11/8, 3/2, 13/8, 7/4, 15/8, and 2/1 (only 1/4

    and 1/2 are SIMD-accelerated.)

 -- libjpeg: arithmetic coding

 -- libjpeg: In-memory source and destination managers

    See notes below.

 -- cjpeg: Separate quality settings for luminance and chrominance

    Note that the libpjeg v7+ API was extended to accommodate this feature only

    for convenience purposes.  It has always been possible to implement this

    feature with libjpeg v6b (see rdswitch.c for an example.)

 -- cjpeg: 32-bit BMP support

 -- cjpeg: -rgb option

 -- jpegtran: lossless cropping

 -- jpegtran: -perfect option

 -- jpegtran: forcing width/height when performing lossless crop

 -- rdjpgcom: -raw option

 -- rdjpgcom: locale awareness

 Not supported:

 NOTE:  As of this writing, extensive research has been conducted into the

 usefulness of DCT scaling as a means of data reduction and SmartScale as a

 means of quality improvement.  The reader is invited to peruse the research at

 http://www.libjpeg-turbo.org/About/SmartScale and draw his/her own conclusions,

 but it is the general belief of our project that these features have not

 demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo.

 -- libjpeg: DCT scaling in compressor

    cinfo.scale_num and cinfo.scale_denom are silently ignored.

    There is no technical reason why DCT scaling could not be supported when

    emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see

    below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and

    8/9 would be available, which is of limited usefulness.

 -- libjpeg: SmartScale

    cinfo.block_size is silently ignored.

    SmartScale is an extension to the JPEG format that allows for DCT block

    sizes other than 8x8.  Providing support for this new format would be

    feasible (particularly without full acceleration.)  However, until/unless

    the format becomes either an official industry standard or, at minimum, an

    accepted solution in the community, we are hesitant to implement it, as

    there is no sense of whether or how it might change in the future.  It is

    our belief that SmartScale has not demonstrated sufficient usefulness as a

    lossless format nor as a means of quality enhancement, and thus, our primary

    interest in providing this feature would be as a means of supporting

    additional DCT scaling factors.

 -- libjpeg: Fancy downsampling in compressor

    cinfo.do_fancy_downsampling is silently ignored.

    This requires the DCT scaling feature, which is not supported.

 -- jpegtran: Scaling

    This requires both the DCT scaling and SmartScale features, which are not

    supported.

 -- Lossless RGB JPEG files

    This requires the SmartScale feature, which is not supported.

 What About libjpeg v9?

 ----------------------

 libjpeg v9 introduced yet another field to the JPEG compression structure

 (color_transform), thus making the ABI backward incompatible with that of

 libjpeg v8.  This new field was introduced solely for the purpose of supporting

 lossless SmartScale encoding.  Further, there was actually no reason to extend

 the API in this manner, as the color transform could have just as easily been

 activated by way of a new JPEG colorspace constant, thus preserving backward

 ABI compatibility.

 Our research (see link above) has shown that lossless SmartScale does not

 generally accomplish anything that can't already be accomplished better with

 existing, standard lossless formats.  Thus, at this time, it is our belief that

 there is not sufficient technical justification for software to upgrade from

 libjpeg v8 to libjpeg v9, and therefore, not sufficient technical justification

 for us to emulate the libjpeg v9 ABI.

 =====================================

 In-Memory Source/Destination Managers

 =====================================

 By default, libjpeg-turbo 1.3 and later includes the jpeg_mem_src() and

 jpeg_mem_dest() functions, even when not emulating the libjpeg v8 API/ABI.

 Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8

 API/ABI emulation in order to use the in-memory source/destination managers,

 but several projects requested that those functions be included when emulating

 the libjpeg v6b API/ABI as well.  This allows the use of those functions by

 programs that need them without breaking ABI compatibility for programs that

 don't, and it allows those functions to be provided in the "official"

 libjpeg-turbo binaries.

 Those who are concerned about maintaining strict conformance with the libjpeg

 v6b or v7 API can pass an argument of --without-mem-srcdst to configure or

 an argument of -DWITH_MEM_SRCDST=0 to CMake prior to building libjpeg-turbo.

 This will restore the pre-1.3 behavior, in which jpeg_mem_src() and

 jpeg_mem_dest() are only included when emulating the libjpeg v8 API/ABI.

 On Un*x systems, including the in-memory source/destination managers changes

 the dynamic library version from 62.0.0 to 62.1.0 if using libjpeg v6b API/ABI

 emulation and from 7.0.0 to 7.1.0 if using libjpeg v7 API/ABI emulation.

 Note that, on most Un*x systems, the dynamic linker will not look for a

 function in a library until that function is actually used.  Thus, if a program

 is built against libjpeg-turbo 1.3+ and uses jpeg_mem_src() or jpeg_mem_dest(),

 that program will not fail if run against an older version of libjpeg-turbo or

 against libjpeg v7- until the program actually tries to call jpeg_mem_src() or

 jpeg_mem_dest().  Such is not the case on Windows.  If a program is built

 against the libjpeg-turbo 1.3+ DLL and uses jpeg_mem_src() or jpeg_mem_dest(),

 then it must use the libjpeg-turbo 1.3+ DLL at run time.

 Both cjpeg and djpeg have been extended to allow testing the in-memory

 source/destination manager functions.  See their respective man pages for more

 details.

 *******************************************************************************

 **     Mathematical Compatibility

 *******************************************************************************

 For the most part, libjpeg-turbo should produce identical output to libjpeg

 v6b.  The one exception to this is when using the floating point DCT/IDCT, in

 which case the outputs of libjpeg v6b and libjpeg-turbo are not guaranteed to

 be identical (the accuracy of the floating point DCT/IDCT is constant when

 using libjpeg-turbo's SIMD extensions, but otherwise, it can depend heavily on

 the compiler and compiler settings.)

 While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood, it is

 still using the same algorithms as libjpeg v6b, so there are several specific

 cases in which libjpeg-turbo cannot be expected to produce the same output as

 libjpeg v8:

 -- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8

    implements those scaling algorithms a bit differently than libjpeg v6b does,

    and libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.

 -- When using chrominance subsampling, because libjpeg v8 implements this

    with its DCT/IDCT scaling algorithms rather than with a separate

    downsampling/upsampling algorithm.

 -- When using the floating point IDCT, for the reasons stated above and also

    because the floating point IDCT algorithm was modified in libjpeg v8a to

    improve accuracy.

 -- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or

    "non-smooth") chrominance upsampling, because libjpeg v8 does not support

    merged upsampling with scaling factors > 1.

 *******************************************************************************

 **     Performance Pitfalls

 *******************************************************************************

 ===============

 Restart Markers

 ===============

 The optimized Huffman decoder in libjpeg-turbo does not handle restart markers

 in a way that makes the rest of the libjpeg infrastructure happy, so it is

 necessary to use the slow Huffman decoder when decompressing a JPEG image that

 has restart markers.  This can cause the decompression performance to drop by

 as much as 20%, but the performance will still be much greater than that of

 libjpeg.  Many consumer packages, such as PhotoShop, use restart markers when

 generating JPEG images, so images generated by those programs will experience

 this issue.

 ===============================================

 Fast Integer Forward DCT at High Quality Levels

 ===============================================

 The algorithm used by the SIMD-accelerated quantization function cannot produce

 correct results whenever the fast integer forward DCT is used along with a JPEG

 quality of 98-100.  Thus, libjpeg-turbo must use the non-SIMD quantization

 function in those cases.  This causes performance to drop by as much as 40%.

 It is therefore strongly advised that you use the slow integer forward DCT

 whenever encoding images with a JPEG quality of 98 or higher.