# Android differences from upstream chromium

Android builds with `C_ARITH_CODING_SUPPORTED` and `D_ARITH_CODING_SUPPORTED`,
added in
https://android-review.googlesource.com/c/platform/external/libjpeg-turbo/+/291407/
to support a variant of jpeg files using arithmetic (instead of Huffman)
encoding. This variant isn't often used because of a lack of support in many
viewers (e.g. Chromium), but Android really values backwards compatibility, and
this might break some users. Android probably only needs to keep
`D_ARITH_CODING_SUPPORTED`, but vendor code might also be encoding by setting
jpeg_compress_struct.arith_code to true, so we enable both to ensure full
backwards compatibility since it's not really costing us anything.
We `#define` these in jconfig.h rather than in Android.bp so that they're
correctly exported to any *users* (in particular, jerror.h only conditionally
defines the corresponding error codes if these `#define`s are present).

Name: libjpeg-turbo
URL: https://github.com/libjpeg-turbo/libjpeg-turbo/
Version: 2.1.0
License: Custom license
License File: LICENSE.md
Security Critical: yes
License Android Compatible: yes

Description:
This consists of the components:
* libjpeg-turbo 2.1.0
* This file (README.chromium)
* A build file (BUILD.gn)
* An OWNERS file
* A codereview.settings file
* Patched header files used by Chromium
* Deleted unused directories: cmakescripts, doc, fuzz, java, release,
  sharedlib, simd/loongson, simd/mips, simd/powerpc, and win
* Deleted unused files: appveyor.yml, CMakeLists.txt, doxygen.config,
  doxygen-extra.css, .gitattributes, md5/CMakeLists.txt, md5/md5cmp.c,
  simd/CMakeLists.txt, tjexample.c, tjexampletest.in, tjexampletest.java.in and
  .travis.yml
* Deleted legacy Arm Neon assembly files (supporting old compiler versions that
  do not generate performant code from intrinsics):
  simd/arm/aarch32/jsimd_neon.S, simd/arm/aarch64/jsimd_neon.S.

This libjpeg-turbo can replace our libjpeg-6b without any modifications in the
Chromium code.

Same as our copy of libjpeg-6b, this libjpeg-turbo also added a new file
jpeglibmangler.h and included it from jpeglib.h that changes the names of all
externally visible functions to chromium_* so that we can avoid conflicts that
arise when system libraries attempt to use our libjpeg. Also, we applied the
following changes which are not merged to upstream:

* Configuration files jconfig.h, jconfigint.h and neon-compat.h were generated
  and then altered manually to be compatible on all of Chromium's platforms.
  http://crbug.com/608347
* Fix static const data duplication of jpeg_nbits_table. A unique copy
  was in the jchuff.obj and jcphuff.obj resulting in an added 65k in
  .rdata in chrome.dll and chrome_child.dll.  Declaring extern const
  in the header instead of static const and moving the definition to
  a new .c file fixes this so only one copy is referenced. Also added
  extern wrappers around usage in asm files. The jpeg_nbits_table.inc
  file was also deleted. It was also necessary to give this table hidden
  visibility to avoid invalid relocations (ignored by ld but rejected by
  lld) arising from attempts to reference the table from assembler on
  32-bit x86. This only affects shared libraries, but that's important
  for downstream Android builds.
* Patches to enable running the upstream unit tests through GTest.
  The upstream unit tests are defined here under the section 'TESTS':
  https://github.com/libjpeg-turbo/libjpeg-turbo/blob/master/CMakeLists.txt
  These changes are tracked by Chromium issue: https://crbug.com/993876
  - Refactor tjunittest.c to provide test interface
  - Move tjunittest logs from stdout to stderr
  - Refactor tjbench.c to provide test interface
  - Move tbench logs from stdout to stderr
  - Write tjunittest output files to sdcard on Android
  - Refactor cjpeg.c to provide test interface
  - Refactor jpegtran.c to provide test interface
  - Add input JPEG images for djpeg and jpegtran tests
  - Refactor djpeg.c to provide test interface
  A new gtest directory contains GTest wrappers (and associated utilities) for
  each of tjunittest, tjbench, cjpeg, djpeg and jpegtran.

Refer to working-with-nested-repos [1] for details of how to setup your git
svn client to update the code (for making local changes, cherry picking from
upstream, etc).

[1] https://www.chromium.org/developers/how-tos/get-the-code/working-with-nested-repos

libjpeg-turbo note:  This file has been modified by The libjpeg-turbo Project
to include only information relevant to libjpeg-turbo, to wordsmith certain
sections, and to remove impolitic language that existed in the libjpeg v8
README.  It is included only for reference.  Please see README.md for
information specific to libjpeg-turbo.


The Independent JPEG Group's JPEG software
==========================================

This distribution contains a release of the Independent JPEG Group's free JPEG
software.  You are welcome to redistribute this software and to use it for any
purpose, subject to the conditions under LEGAL ISSUES, below.

This software is the work of Tom Lane, Guido Vollbeding, Philip Gladstone,
Bill Allombert, Jim Boucher, Lee Crocker, Bob Friesenhahn, Ben Jackson,
Julian Minguillon, Luis Ortiz, George Phillips, Davide Rossi, Ge' Weijers,
and other members of the Independent JPEG Group.

IJG is not affiliated with the ISO/IEC JTC1/SC29/WG1 standards committee
(also known as JPEG, together with ITU-T SG16).


DOCUMENTATION ROADMAP
=====================

This file contains the following sections:

OVERVIEW            General description of JPEG and the IJG software.
LEGAL ISSUES        Copyright, lack of warranty, terms of distribution.
REFERENCES          Where to learn more about JPEG.
ARCHIVE LOCATIONS   Where to find newer versions of this software.
FILE FORMAT WARS    Software *not* to get.
TO DO               Plans for future IJG releases.

Other documentation files in the distribution are:

User documentation:
  usage.txt         Usage instructions for cjpeg, djpeg, jpegtran,
                    rdjpgcom, and wrjpgcom.
  *.1               Unix-style man pages for programs (same info as usage.txt).
  wizard.txt        Advanced usage instructions for JPEG wizards only.
  change.log        Version-to-version change highlights.
Programmer and internal documentation:
  libjpeg.txt       How to use the JPEG library in your own programs.
  example.txt       Sample code for calling the JPEG library.
  structure.txt     Overview of the JPEG library's internal structure.
  coderules.txt     Coding style rules --- please read if you contribute code.

Please read at least usage.txt.  Some information can also be found in the JPEG
FAQ (Frequently Asked Questions) article.  See ARCHIVE LOCATIONS below to find
out where to obtain the FAQ article.

If you want to understand how the JPEG code works, we suggest reading one or
more of the REFERENCES, then looking at the documentation files (in roughly
the order listed) before diving into the code.


OVERVIEW
========

This package contains C software to implement JPEG image encoding, decoding,
and transcoding.  JPEG (pronounced "jay-peg") is a standardized compression
method for full-color and grayscale images.  JPEG's strong suit is compressing
photographic images or other types of images that have smooth color and
brightness transitions between neighboring pixels.  Images with sharp lines or
other abrupt features may not compress well with JPEG, and a higher JPEG
quality may have to be used to avoid visible compression artifacts with such
images.

JPEG is lossy, meaning that the output pixels are not necessarily identical to
the input pixels.  However, on photographic content and other "smooth" images,
very good compression ratios can be obtained with no visible compression
artifacts, and extremely high compression ratios are possible if you are
willing to sacrifice image quality (by reducing the "quality" setting in the
compressor.)

This software implements JPEG baseline, extended-sequential, and progressive
compression processes.  Provision is made for supporting all variants of these
processes, although some uncommon parameter settings aren't implemented yet.
We have made no provision for supporting the hierarchical or lossless
processes defined in the standard.

We provide a set of library routines for reading and writing JPEG image files,
plus two sample applications "cjpeg" and "djpeg", which use the library to
perform conversion between JPEG and some other popular image file formats.
The library is intended to be reused in other applications.

In order to support file conversion and viewing software, we have included
considerable functionality beyond the bare JPEG coding/decoding capability;
for example, the color quantization modules are not strictly part of JPEG
decoding, but they are essential for output to colormapped file formats or
colormapped displays.  These extra functions can be compiled out of the
library if not required for a particular application.

We have also included "jpegtran", a utility for lossless transcoding between
different JPEG processes, and "rdjpgcom" and "wrjpgcom", two simple
applications for inserting and extracting textual comments in JFIF files.

The emphasis in designing this software has been on achieving portability and
flexibility, while also making it fast enough to be useful.  In particular,
the software is not intended to be read as a tutorial on JPEG.  (See the
REFERENCES section for introductory material.)  Rather, it is intended to
be reliable, portable, industrial-strength code.  We do not claim to have
achieved that goal in every aspect of the software, but we strive for it.

We welcome the use of this software as a component of commercial products.
No royalty is required, but we do ask for an acknowledgement in product
documentation, as described under LEGAL ISSUES.


LEGAL ISSUES
============

In plain English:

1. We don't promise that this software works.  (But if you find any bugs,
   please let us know!)
2. You can use this software for whatever you want.  You don't have to pay us.
3. You may not pretend that you wrote this software.  If you use it in a
   program, you must acknowledge somewhere in your documentation that
   you've used the IJG code.

In legalese:

The authors make NO WARRANTY or representation, either express or implied,
with respect to this software, its quality, accuracy, merchantability, or
fitness for a particular purpose.  This software is provided "AS IS", and you,
its user, assume the entire risk as to its quality and accuracy.

This software is copyright (C) 1991-2020, Thomas G. Lane, Guido Vollbeding.
All Rights Reserved except as specified below.

Permission is hereby granted to use, copy, modify, and distribute this
software (or portions thereof) for any purpose, without fee, subject to these
conditions:
(1) If any part of the source code for this software is distributed, then this
README file must be included, with this copyright and no-warranty notice
unaltered; and any additions, deletions, or changes to the original files
must be clearly indicated in accompanying documentation.
(2) If only executable code is distributed, then the accompanying
documentation must state that "this software is based in part on the work of
the Independent JPEG Group".
(3) Permission for use of this software is granted only if the user accepts
full responsibility for any undesirable consequences; the authors accept
NO LIABILITY for damages of any kind.

These conditions apply to any software derived from or based on the IJG code,
not just to the unmodified library.  If you use our work, you ought to
acknowledge us.

Permission is NOT granted for the use of any IJG author's name or company name
in advertising or publicity relating to this software or products derived from
it.  This software may be referred to only as "the Independent JPEG Group's
software".

We specifically permit and encourage the use of this software as the basis of
commercial products, provided that all warranty or liability claims are
assumed by the product vendor.


REFERENCES
==========

We recommend reading one or more of these references before trying to
understand the innards of the JPEG software.

The best short technical introduction to the JPEG compression algorithm is
        Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
        Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
(Adjacent articles in that issue discuss MPEG motion picture compression,
applications of JPEG, and related topics.)  If you don't have the CACM issue
handy, a PDF file containing a revised version of Wallace's article is
available at http://www.ijg.org/files/Wallace.JPEG.pdf.  The file (actually
a preprint for an article that appeared in IEEE Trans. Consumer Electronics)
omits the sample images that appeared in CACM, but it includes corrections
and some added material.  Note: the Wallace article is copyright ACM and IEEE,
and it may not be used for commercial purposes.

A somewhat less technical, more leisurely introduction to JPEG can be found in
"The Data Compression Book" by Mark Nelson and Jean-loup Gailly, published by
M&T Books (New York), 2nd ed. 1996, ISBN 1-55851-434-1.  This book provides
good explanations and example C code for a multitude of compression methods
including JPEG.  It is an excellent source if you are comfortable reading C
code but don't know much about data compression in general.  The book's JPEG
sample code is far from industrial-strength, but when you are ready to look
at a full implementation, you've got one here...

The best currently available description of JPEG is the textbook "JPEG Still
Image Data Compression Standard" by William B. Pennebaker and Joan L.
Mitchell, published by Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
Price US$59.95, 638 pp.  The book includes the complete text of the ISO JPEG
standards (DIS 10918-1 and draft DIS 10918-2).

The original JPEG standard is divided into two parts, Part 1 being the actual
specification, while Part 2 covers compliance testing methods.  Part 1 is
titled "Digital Compression and Coding of Continuous-tone Still Images,
Part 1: Requirements and guidelines" and has document numbers ISO/IEC IS
10918-1, ITU-T T.81.  Part 2 is titled "Digital Compression and Coding of
Continuous-tone Still Images, Part 2: Compliance testing" and has document
numbers ISO/IEC IS 10918-2, ITU-T T.83.

The JPEG standard does not specify all details of an interchangeable file
format.  For the omitted details, we follow the "JFIF" conventions, revision
1.02.  JFIF version 1 has been adopted as ISO/IEC 10918-5 (05/2013) and
Recommendation ITU-T T.871 (05/2011): Information technology - Digital
compression and coding of continuous-tone still images: JPEG File Interchange
Format (JFIF).  It is available as a free download in PDF file format from
https://www.iso.org/standard/54989.html and http://www.itu.int/rec/T-REC-T.871.
A PDF file of the older JFIF 1.02 specification is available at
http://www.w3.org/Graphics/JPEG/jfif3.pdf.

The TIFF 6.0 file format specification can be obtained from
http://mirrors.ctan.org/graphics/tiff/TIFF6.ps.gz.  The JPEG incorporation
scheme found in the TIFF 6.0 spec of 3-June-92 has a number of serious
problems.  IJG does not recommend use of the TIFF 6.0 design (TIFF Compression
tag 6).  Instead, we recommend the JPEG design proposed by TIFF Technical Note
#2 (Compression tag 7).  Copies of this Note can be obtained from
http://www.ijg.org/files/.  It is expected that the next revision
of the TIFF spec will replace the 6.0 JPEG design with the Note's design.
Although IJG's own code does not support TIFF/JPEG, the free libtiff library
uses our library to implement TIFF/JPEG per the Note.


ARCHIVE LOCATIONS
=================

The "official" archive site for this software is www.ijg.org.
The most recent released version can always be found there in
directory "files".

The JPEG FAQ (Frequently Asked Questions) article is a source of some
general information about JPEG.  It is available at
http://www.faqs.org/faqs/jpeg-faq.


FILE FORMAT COMPATIBILITY
=========================

This software implements ITU T.81 | ISO/IEC 10918 with some extensions from
ITU T.871 | ISO/IEC 10918-5 (JPEG File Interchange Format-- see REFERENCES).
Informally, the term "JPEG image" or "JPEG file" most often refers to JFIF or
a subset thereof, but there are other formats containing the name "JPEG" that
are incompatible with the DCT-based JPEG standard or with JFIF (for instance,
JPEG 2000 and JPEG XR).  This software therefore does not support these
formats.  Indeed, one of the original reasons for developing this free software
was to help force convergence on a common, interoperable format standard for
JPEG files.

JFIF is a minimal or "low end" representation.  TIFF/JPEG (TIFF revision 6.0 as
modified by TIFF Technical Note #2) can be used for "high end" applications
that need to record a lot of additional data about an image.


TO DO
=====

Please send bug reports, offers of help, etc. to jpeg-info@jpegclub.org.

Background
==========

libjpeg-turbo is a JPEG image codec that uses SIMD instructions to accelerate
baseline JPEG compression and decompression on x86, x86-64, Arm, PowerPC, and
MIPS systems, as well as progressive JPEG compression on x86, x86-64, and Arm
systems.  On such systems, libjpeg-turbo is generally 2-6x 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
=======

libjpeg-turbo is covered by three compatible BSD-style open source licenses.
Refer to [LICENSE.md](LICENSE.md) for a roll-up of license terms.


Building libjpeg-turbo
======================

Refer to [BUILDING.md](BUILDING.md) for complete instructions.


Using libjpeg-turbo
===================

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

- **TurboJPEG API**<br>
  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.  The TurboJPEG API is recommended for first-time
  users of libjpeg-turbo.  Refer to [tjexample.c](tjexample.c) and
  [TJExample.java](java/TJExample.java) for examples of its usage and to
  <http://libjpeg-turbo.org/Documentation/Documentation> for API documentation.

- **libjpeg API**<br>
  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.)  Refer to [cjpeg.c](cjpeg.c) and [djpeg.c](djpeg.c) for examples
  of its usage and to [libjpeg.txt](libjpeg.txt) for API documentation.

There is no significant performance advantage to either API when both are used
to perform similar operations.

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](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 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 `-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 API: IDCT scaling extensions in decompressor**<br>
  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 API: Arithmetic coding**

- **libjpeg API: In-memory source and destination managers**<br>
  See notes below.

- **cjpeg: Separate quality settings for luminance and chrominance**<br>
  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.  Readers are invited to peruse the research at
<http://www.libjpeg-turbo.org/About/SmartScale> and draw their 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 API: DCT scaling in compressor**<br>
  `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 API: SmartScale**<br>
  `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 API: Fancy downsampling in compressor**<br>
  `cinfo.do_fancy_downsampling` is silently ignored.
  This requires the DCT scaling feature, which is not supported.

- **jpegtran: Scaling**<br>
  This requires both the DCT scaling and SmartScale features, which are not
  supported.

- **Lossless RGB JPEG files**<br>
  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.  Furthermore, 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.  Therefore, at this time it is our belief
that there is not sufficient technical justification for software projects to
upgrade from libjpeg v8 to libjpeg v9, and thus there is 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 `-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 62.2.0 to 62.3.0 if using libjpeg v6b API/ABI
emulation and from 7.2.0 to 7.3.0 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 can differ for the
following reasons:

- The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so
  slightly more accurate than the implementation in libjpeg v6b, but not by
  any amount perceptible to human vision (generally in the range of 0.01 to
  0.08 dB gain in PNSR.)

- When not using the SIMD extensions, libjpeg-turbo uses the more accurate
  (and slightly faster) floating point IDCT algorithm introduced in libjpeg
  v8a as opposed to the algorithm used in libjpeg v6b.  It should be noted,
  however, that this algorithm basically brings the accuracy of the floating
  point IDCT in line with the accuracy of the accurate integer IDCT.  The
  floating point DCT/IDCT algorithms are mainly a legacy feature, and they do
  not produce significantly more accuracy than the accurate integer algorithms
  (to put numbers on this, the typical difference in PNSR between the two
  algorithms is less than 0.10 dB, whereas changing the quality level by 1 in
  the upper range of the quality scale is typically more like a 1.0 dB
  difference.)

- If the floating point algorithms in libjpeg-turbo are not implemented using
  SIMD instructions on a particular platform, then the accuracy of the
  floating point DCT/IDCT can depend on the 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 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.  In our testing, the subsampled/upsampled
  output of libjpeg v8 is less accurate than that of libjpeg v6b for this
  reason.

- 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 accurate integer forward DCT
whenever encoding images with a JPEG quality of 98 or higher.


Memory Debugger Pitfalls
========================

Valgrind and Memory Sanitizer (MSan) can generate false positives
(specifically, incorrect reports of uninitialized memory accesses) when used
with libjpeg-turbo's SIMD extensions.  It is generally recommended that the
SIMD extensions be disabled, either by passing an argument of `-DWITH_SIMD=0`
to `cmake` when configuring the build or by setting the environment variable
`JSIMD_FORCENONE` to `1` at run time, when testing libjpeg-turbo with Valgrind,
MSan, or other memory debuggers.