externals/libjpeg-turbo/README.md

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.