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

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


*******************************************************************************
**     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.

=====================
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 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 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 slow integer IDCT.  The floating
   point DCT/IDCT algorithms are mainly a legacy feature, and they do not
   produce significantly more accuracy than the slow 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 slow integer forward DCT
whenever encoding images with a JPEG quality of 98 or higher.