README-turbo.txt - third_party/libjpeg-turbo - Git at Google

 *******************************************************************************
 **     Background
 *******************************************************************************

 libjpeg-turbo is a derivative of libjpeg that uses SIMD instructions (MMX,
 SSE2, etc.) to accelerate baseline JPEG compression and decompression on x86
 and x86-64 systems.  On such systems, libjpeg-turbo is generally 2-4x as fast
 as the unmodified version of libjpeg, all else being equal.

 libjpeg-turbo was originally based on libjpeg/SIMD by Miyasaka Masaru, but
 the TigerVNC and VirtualGL projects made numerous enhancements to the codec in
 2009, including improved support for Mac OS X, 64-bit support, support for
 32-bit and big-endian pixel formats (RGBX, XBGR, etc.), accelerated Huffman
 encoding/decoding, and various bug fixes.  The goal was to produce a fully
 open-source codec that could replace the partially closed-source TurboJPEG/IPP
 codec used by VirtualGL and TurboVNC.  libjpeg-turbo generally achieves 80-120%
 of the performance of TurboJPEG/IPP.  It is faster in some areas but slower in
 others.

 In early 2010, libjpeg-turbo spun off into its own independent project, with
 the goal of making high-speed JPEG compression/decompression technology
 available to a broader range of users and developers.


 *******************************************************************************
 **     License
 *******************************************************************************

 The TurboJPEG/OSS wrapper, as well as some of the optimizations to the Huffman
 encoder (jchuff.c) and decoder (jdhuff.c), were borrowed from VirtualGL, and
 thus any distribution of libjpeg-turbo that includes those files must, as a
 whole, be subject to the terms of the wxWindows Library Licence, Version 3.1.
 A copy of this license can be found in this directory under LICENSE.txt.  The
 wxWindows Library License is based on the LGPL but includes provisions that
 allow the Library to be statically linked into proprietary libraries and
 applications without requiring the resulting binaries to be distributed under
 the terms of the LGPL.

 The rest of the source code, apart from TurboJPEG/OSS and the Huffman codec
 optimizations, falls under a less restrictive, BSD-style license (see README.)
 You can choose to distribute libjpeg-turbo, as a whole, under this BSD-style
 license by simply removing TurboJPEG/OSS and replacing the optimized jchuff.c
 and jdhuff.c with their unoptimized counterparts from the libjpeg v6b source.


 *******************************************************************************
 **     Using libjpeg-turbo
 *******************************************************************************

 =============================
 Replacing libjpeg at Run Time
 =============================

 If a Unix 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

 NOTE: {lib} can be lib, lib32, lib64, or lib/64, depending on the O/S and
 architecture.

 System administrators can also replace the libjpeg sym links 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.

 The libjpeg-turbo SDK for Visual C++ installs the libjpeg-turbo DLL
 (jpeg62.dll, jpeg7.dll, or jpeg8.dll, depending on whether it was built with
 libjpeg v6b, v7, or v8 emulation) into c:\libjpeg-turbo[64]\bin, and the PATH
 environment variable can be modified such that this directory is searched
 before any others that might contain a libjpeg DLL.  However, if a libjpeg
 DLL exists in an application's install directory, then Windows will load this
 DLL first whenever the application is launched.  Thus, if an application ships
 with jpeg62.dll, jpeg7.dll, or jpeg8.dll, then back up the application's
 version of this DLL and copy c:\libjpeg-turbo[64]\bin\jpeg*.dll into the
 application's install directory to accelerate it.

 The version of the libjpeg-turbo DLL distributed in the libjpeg-turbo SDK for
 Visual C++ 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.

 NOTE:  Features of libjpeg that require passing a C run-time structure, such
 as a file handle, from an application to libjpeg will probably not work with
 the version of the libjpeg-turbo DLL distributed in the libjpeg-turbo SDK for
 Visual C++, 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 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.  If an application uses a shared library version of
 libjpeg, then it may be possible to replace the application's version of it.
 This 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 dylib to the 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.

 =======================
 Replacing TurboJPEG/IPP
 =======================

 libjpeg-turbo is a drop-in replacement for the TurboJPEG/IPP SDK used by
 VirtualGL 2.1.x and TurboVNC 0.6 (and prior.)  libjpeg-turbo contains a wrapper
 library (TurboJPEG/OSS) that emulates the TurboJPEG API using libjpeg-turbo
 instead of the closed-source Intel Performance Primitives.  You can replace the
 TurboJPEG/IPP package on Linux systems with the libjpeg-turbo package in order
 to make existing releases of VirtualGL 2.1.x and TurboVNC 0.x use the new codec
 at run time.  Note that the 64-bit libjpeg-turbo packages contain only 64-bit
 binaries, whereas the TurboJPEG/IPP 64-bit packages contained both 64-bit and
 32-bit binaries.  Thus, to replace a TurboJPEG/IPP 64-bit package, install
 both the 64-bit and 32-bit versions of libjpeg-turbo.

 You can also build the VirtualGL 2.1.x and TurboVNC 0.6 source code with
 the libjpeg-turbo SDK instead of TurboJPEG/IPP.  It should work identically.
 libjpeg-turbo also includes static library versions of TurboJPEG/OSS, which
 are used to build VirtualGL 2.2 and TurboVNC 1.0 and later.

 ========================================
 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 Unix systems (including 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 Linux and Solaris systems, you would still need to manipulate
 LD_LIBRARY_PATH or create appropriate sym links 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 Linux, Solaris, 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 (this also works on Linux and Solaris.)

 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 six 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 */

 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 version of libjpeg
 that doesn't support them will result in a "Bogus input colorspace" error.

 =================================
 libjpeg v7 and v8 API/ABI support
 =================================

 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 are
 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) have
 made the switch that it was desirable to provide support for the libjpeg v7/v8
 API/ABI in libjpeg-turbo.  Although libjpeg-turbo can now be configured as a
 drop-in replacement for libjpeg v7 or v8, it should be noted that not all of
 the features in libjpeg v7 and v8 are supported (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 API/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.

 libjpeg v7 and v8 Features:
 ---------------------------

 Fully supported:

 -- 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

 -- jpegtran: lossless cropping

 -- jpegtran: -perfect option

 -- rdjpgcom: -raw option

 -- rdjpgcom: locale awareness


 Fully supported when using libjpeg v7/v8 emulation:

 -- libjpeg: In-memory source and destination managers


 Not supported:

 -- libjpeg: DCT scaling in compressor
    cinfo.scale_num and cinfo.scale_denom are silently ignored.
    There is no technical reason why DCT scaling cannot be supported, but
    without the SmartScale extension (see below), it would only be able to
    down-scale using ratios of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and 8/9,
    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.  It would be difficult to support this feature while
    retaining backward compatibility with libjpeg v6b.

 -- libjpeg: IDCT scaling extensions in decompressor
    libjpeg-turbo still supports IDCT scaling with scaling factors of 1/2, 1/4,
    and 1/8 (same as libjpeg v6b.)

 -- 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.


 *******************************************************************************
 **     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.
	*******************************************************************************
	** Background
	*******************************************************************************

	libjpeg-turbo is a derivative of libjpeg that uses SIMD instructions (MMX,
	SSE2, etc.) to accelerate baseline JPEG compression and decompression on x86
	and x86-64 systems. On such systems, libjpeg-turbo is generally 2-4x as fast
	as the unmodified version of libjpeg, all else being equal.

	libjpeg-turbo was originally based on libjpeg/SIMD by Miyasaka Masaru, but
	the TigerVNC and VirtualGL projects made numerous enhancements to the codec in
	2009, including improved support for Mac OS X, 64-bit support, support for
	32-bit and big-endian pixel formats (RGBX, XBGR, etc.), accelerated Huffman
	encoding/decoding, and various bug fixes. The goal was to produce a fully
	open-source codec that could replace the partially closed-source TurboJPEG/IPP
	codec used by VirtualGL and TurboVNC. libjpeg-turbo generally achieves 80-120%
	of the performance of TurboJPEG/IPP. It is faster in some areas but slower in
	others.

	In early 2010, libjpeg-turbo spun off into its own independent project, with
	the goal of making high-speed JPEG compression/decompression technology
	available to a broader range of users and developers.


	*******************************************************************************
	** License
	*******************************************************************************

	The TurboJPEG/OSS wrapper, as well as some of the optimizations to the Huffman
	encoder (jchuff.c) and decoder (jdhuff.c), were borrowed from VirtualGL, and
	thus any distribution of libjpeg-turbo that includes those files must, as a
	whole, be subject to the terms of the wxWindows Library Licence, Version 3.1.
	A copy of this license can be found in this directory under LICENSE.txt. The
	wxWindows Library License is based on the LGPL but includes provisions that
	allow the Library to be statically linked into proprietary libraries and
	applications without requiring the resulting binaries to be distributed under
	the terms of the LGPL.

	The rest of the source code, apart from TurboJPEG/OSS and the Huffman codec
	optimizations, falls under a less restrictive, BSD-style license (see README.)
	You can choose to distribute libjpeg-turbo, as a whole, under this BSD-style
	license by simply removing TurboJPEG/OSS and replacing the optimized jchuff.c
	and jdhuff.c with their unoptimized counterparts from the libjpeg v6b source.


	*******************************************************************************
	** Using libjpeg-turbo
	*******************************************************************************

	=============================
	Replacing libjpeg at Run Time
	=============================

	If a Unix 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

	NOTE: {lib} can be lib, lib32, lib64, or lib/64, depending on the O/S and
	architecture.

	System administrators can also replace the libjpeg sym links 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.

	The libjpeg-turbo SDK for Visual C++ installs the libjpeg-turbo DLL
	(jpeg62.dll, jpeg7.dll, or jpeg8.dll, depending on whether it was built with
	libjpeg v6b, v7, or v8 emulation) into c:\libjpeg-turbo[64]\bin, and the PATH
	environment variable can be modified such that this directory is searched
	before any others that might contain a libjpeg DLL. However, if a libjpeg
	DLL exists in an application's install directory, then Windows will load this
	DLL first whenever the application is launched. Thus, if an application ships
	with jpeg62.dll, jpeg7.dll, or jpeg8.dll, then back up the application's
	version of this DLL and copy c:\libjpeg-turbo[64]\bin\jpeg*.dll into the
	application's install directory to accelerate it.

	The version of the libjpeg-turbo DLL distributed in the libjpeg-turbo SDK for
	Visual C++ 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.

	NOTE: Features of libjpeg that require passing a C run-time structure, such
	as a file handle, from an application to libjpeg will probably not work with
	the version of the libjpeg-turbo DLL distributed in the libjpeg-turbo SDK for
	Visual C++, 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 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. If an application uses a shared library version of
	libjpeg, then it may be possible to replace the application's version of it.
	This 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 dylib to the 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.

	=======================
	Replacing TurboJPEG/IPP
	=======================

	libjpeg-turbo is a drop-in replacement for the TurboJPEG/IPP SDK used by
	VirtualGL 2.1.x and TurboVNC 0.6 (and prior.) libjpeg-turbo contains a wrapper
	library (TurboJPEG/OSS) that emulates the TurboJPEG API using libjpeg-turbo
	instead of the closed-source Intel Performance Primitives. You can replace the
	TurboJPEG/IPP package on Linux systems with the libjpeg-turbo package in order
	to make existing releases of VirtualGL 2.1.x and TurboVNC 0.x use the new codec
	at run time. Note that the 64-bit libjpeg-turbo packages contain only 64-bit
	binaries, whereas the TurboJPEG/IPP 64-bit packages contained both 64-bit and
	32-bit binaries. Thus, to replace a TurboJPEG/IPP 64-bit package, install
	both the 64-bit and 32-bit versions of libjpeg-turbo.

	You can also build the VirtualGL 2.1.x and TurboVNC 0.6 source code with
	the libjpeg-turbo SDK instead of TurboJPEG/IPP. It should work identically.
	libjpeg-turbo also includes static library versions of TurboJPEG/OSS, which
	are used to build VirtualGL 2.2 and TurboVNC 1.0 and later.

	========================================
	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 Unix systems (including 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 Linux and Solaris systems, you would still need to manipulate
	LD_LIBRARY_PATH or create appropriate sym links 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 Linux, Solaris, 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 (this also works on Linux and Solaris.)

	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 six 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 */

	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 version of libjpeg
	that doesn't support them will result in a "Bogus input colorspace" error.

	=================================
	libjpeg v7 and v8 API/ABI support
	=================================

	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 are
	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) have
	made the switch that it was desirable to provide support for the libjpeg v7/v8
	API/ABI in libjpeg-turbo. Although libjpeg-turbo can now be configured as a
	drop-in replacement for libjpeg v7 or v8, it should be noted that not all of
	the features in libjpeg v7 and v8 are supported (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 API/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.

	libjpeg v7 and v8 Features:
	---------------------------

	Fully supported:

	-- 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

	-- jpegtran: lossless cropping

	-- jpegtran: -perfect option

	-- rdjpgcom: -raw option

	-- rdjpgcom: locale awareness


	Fully supported when using libjpeg v7/v8 emulation:

	-- libjpeg: In-memory source and destination managers


	Not supported:

	-- libjpeg: DCT scaling in compressor
	cinfo.scale_num and cinfo.scale_denom are silently ignored.
	There is no technical reason why DCT scaling cannot be supported, but
	without the SmartScale extension (see below), it would only be able to
	down-scale using ratios of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and 8/9,
	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. It would be difficult to support this feature while
	retaining backward compatibility with libjpeg v6b.

	-- libjpeg: IDCT scaling extensions in decompressor
	libjpeg-turbo still supports IDCT scaling with scaling factors of 1/2, 1/4,
	and 1/8 (same as libjpeg v6b.)

	-- 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.


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