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+*******************************************************************************
+** Background
+*******************************************************************************
+
+libjpeg-turbo is a JPEG image codec that uses SIMD instructions (MMX, SSE2,
+NEON) to accelerate baseline JPEG compression and decompression on x86, x86-64,
+and ARM systems. On such systems, libjpeg-turbo is generally 2-4x as fast as
+libjpeg, all else being equal. On other types of systems, libjpeg-turbo can
+still outperform libjpeg by a significant amount, by virtue of its
+highly-optimized Huffman coding routines. In many cases, the performance of
+libjpeg-turbo rivals that of proprietary high-speed JPEG codecs.
+
+libjpeg-turbo implements both the traditional libjpeg API as well as the less
+powerful but more straightforward TurboJPEG API. libjpeg-turbo also features
+colorspace extensions that allow it to compress from/decompress to 32-bit and
+big-endian pixel buffers (RGBX, XBGR, etc.), as well as a full-featured Java
+interface.
+
+libjpeg-turbo was originally based on libjpeg/SIMD, an MMX-accelerated
+derivative of libjpeg v6b developed by Miyasaka Masaru. The TigerVNC and
+VirtualGL projects made numerous enhancements to the codec in 2009, and in
+early 2010, libjpeg-turbo spun off into an independent project, with the goal
+of making high-speed JPEG compression/decompression technology available to a
+broader range of users and developers.
+
+
+*******************************************************************************
+** License
+*******************************************************************************
+
+Most of libjpeg-turbo inherits the non-restrictive, BSD-style license used by
+libjpeg (see README.) The TurboJPEG wrapper (both C and Java versions) and
+associated test programs bear a similar license, which is reproduced below:
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+- Redistributions of source code must retain the above copyright notice,
+ this list of conditions and the following disclaimer.
+- Redistributions in binary form must reproduce the above copyright notice,
+ this list of conditions and the following disclaimer in the documentation
+ and/or other materials provided with the distribution.
+- Neither the name of the libjpeg-turbo Project nor the names of its
+ contributors may be used to endorse or promote products derived from this
+ software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS",
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGE.
+
+
+*******************************************************************************
+** Using libjpeg-turbo
+*******************************************************************************
+
+libjpeg-turbo includes two APIs that can be used to compress and decompress
+JPEG images:
+
+ TurboJPEG API: This API provides an easy-to-use interface for compressing
+ and decompressing JPEG images in memory. It also provides some functionality
+ that would not be straightforward to achieve using the underlying libjpeg
+ API, such as generating planar YUV images and performing multiple
+ simultaneous lossless transforms on an image. The Java interface for
+ libjpeg-turbo is written on top of the TurboJPEG API.
+
+ libjpeg API: This is the de facto industry-standard API for compressing and
+ decompressing JPEG images. It is more difficult to use than the TurboJPEG
+ API but also more powerful. The libjpeg API implementation in libjpeg-turbo
+ is both API/ABI-compatible and mathematically compatible with libjpeg v6b.
+ It can also optionally be configured to be API/ABI-compatible with libjpeg v7
+ and v8 (see below.)
+
+There is no significant performance advantage to either API when both are used
+to perform similar operations.
+
+======================
+Installation Directory
+======================
+
+This document assumes that libjpeg-turbo will be installed in the default
+directory (/opt/libjpeg-turbo on Un*x and Mac systems and
+c:\libjpeg-turbo[-gcc][64] on Windows systems. If your installation of
+libjpeg-turbo resides in a different directory, then adjust the instructions
+accordingly.
+
+=============================
+Replacing libjpeg at Run Time
+=============================
+
+Un*x
+----
+
+If a Un*x application is dynamically linked with libjpeg, then you can replace
+libjpeg with libjpeg-turbo at run time by manipulating LD_LIBRARY_PATH.
+For instance:
+
+ [Using libjpeg]
+ > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg
+ real 0m0.392s
+ user 0m0.074s
+ sys 0m0.020s
+
+ [Using libjpeg-turbo]
+ > export LD_LIBRARY_PATH=/opt/libjpeg-turbo/{lib}:$LD_LIBRARY_PATH
+ > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg
+ real 0m0.109s
+ user 0m0.029s
+ sys 0m0.010s
+
+({lib} = lib32 or lib64, depending on whether you wish to use the 32-bit or the
+64-bit version of libjpeg-turbo.)
+
+System administrators can also replace the libjpeg symlinks in /usr/lib* with
+links to the libjpeg-turbo dynamic library located in /opt/libjpeg-turbo/{lib}.
+This will effectively accelerate every application that uses the libjpeg
+dynamic library on the system.
+
+Windows
+-------
+
+If a Windows application is dynamically linked with libjpeg, then you can
+replace libjpeg with libjpeg-turbo at run time by backing up the application's
+copy of jpeg62.dll, jpeg7.dll, or jpeg8.dll (assuming the application has its
+own local copy of this library) and copying the corresponding DLL from
+libjpeg-turbo into the application's install directory. The official
+libjpeg-turbo binary packages only provide jpeg62.dll. If the application uses
+jpeg7.dll or jpeg8.dll instead, then it will be necessary to build
+libjpeg-turbo from source (see "libjpeg v7 and v8 API/ABI Emulation" below.)
+
+The following information is specific to the official libjpeg-turbo binary
+packages for Visual C++:
+
+-- jpeg62.dll requires the Visual C++ 2008 C run-time DLL (msvcr90.dll).
+msvcr90.dll ships with more recent versions of Windows, but users of older
+Windows releases can obtain it from the Visual C++ 2008 Redistributable
+Package, which is available as a free download from Microsoft's web site.
+
+-- Features of the libjpeg API that require passing a C run-time structure,
+such as a file handle, from an application to the library will probably not
+work with jpeg62.dll, unless the application is also built to use the Visual
+C++ 2008 C run-time DLL. In particular, this affects jpeg_stdio_dest() and
+jpeg_stdio_src().
+
+Mac
+---
+
+Mac applications typically embed their own copies of the libjpeg dylib inside
+the (hidden) application bundle, so it is not possible to globally replace
+libjpeg on OS X systems. Replacing the application's version of the libjpeg
+dylib would generally involve copying libjpeg.*.dylib from libjpeg-turbo into
+the appropriate place in the application bundle and using install_name_tool to
+repoint the libjpeg-turbo dylib to its new directory. This requires an
+advanced knowledge of OS X and would not survive an upgrade or a re-install of
+the application. Thus, it is not recommended for most users.
+
+========================================
+Using libjpeg-turbo in Your Own Programs
+========================================
+
+For the most part, libjpeg-turbo should work identically to libjpeg, so in
+most cases, an application can be built against libjpeg and then run against
+libjpeg-turbo. On Un*x systems and Cygwin, you can build against libjpeg-turbo
+instead of libjpeg by setting
+
+ CPATH=/opt/libjpeg-turbo/include
+ and
+ LIBRARY_PATH=/opt/libjpeg-turbo/{lib}
+
+({lib} = lib32 or lib64, depending on whether you are building a 32-bit or a
+64-bit application.)
+
+If using MinGW, then set
+
+ CPATH=/c/libjpeg-turbo-gcc[64]/include
+ and
+ LIBRARY_PATH=/c/libjpeg-turbo-gcc[64]/lib
+
+Building against libjpeg-turbo is useful, for instance, if you want to build an
+application that leverages the libjpeg-turbo colorspace extensions (see below.)
+On Un*x systems, you would still need to manipulate LD_LIBRARY_PATH or create
+appropriate symlinks to use libjpeg-turbo at run time. On such systems, you
+can pass -R /opt/libjpeg-turbo/{lib} to the linker to force the use of
+libjpeg-turbo at run time rather than libjpeg (also useful if you want to
+leverage the colorspace extensions), or you can link against the libjpeg-turbo
+static library.
+
+To force a Un*x or MinGW application to link against the static version of
+libjpeg-turbo, you can use the following linker options:
+
+ -Wl,-Bstatic -ljpeg -Wl,-Bdynamic
+
+On OS X, simply add /opt/libjpeg-turbo/lib/libjpeg.a to the linker command
+line.
+
+To build Visual C++ applications using libjpeg-turbo, add
+c:\libjpeg-turbo[64]\include to the system or user INCLUDE environment
+variable and c:\libjpeg-turbo[64]\lib to the system or user LIB environment
+variable, and then link against either jpeg.lib (to use the DLL version of
+libjpeg-turbo) or jpeg-static.lib (to use the static version of libjpeg-turbo.)
+
+=====================
+Colorspace Extensions
+=====================
+
+libjpeg-turbo includes extensions that allow JPEG images to be compressed
+directly from (and decompressed directly to) buffers that use BGR, BGRX,
+RGBX, XBGR, and XRGB pixel ordering. This is implemented with ten new
+colorspace constants:
+
+ JCS_EXT_RGB /* red/green/blue */
+ JCS_EXT_RGBX /* red/green/blue/x */
+ JCS_EXT_BGR /* blue/green/red */
+ JCS_EXT_BGRX /* blue/green/red/x */
+ JCS_EXT_XBGR /* x/blue/green/red */
+ JCS_EXT_XRGB /* x/red/green/blue */
+ JCS_EXT_RGBA /* red/green/blue/alpha */
+ JCS_EXT_BGRA /* blue/green/red/alpha */
+ JCS_EXT_ABGR /* alpha/blue/green/red */
+ JCS_EXT_ARGB /* alpha/red/green/blue */
+
+Setting cinfo.in_color_space (compression) or cinfo.out_color_space
+(decompression) to one of these values will cause libjpeg-turbo to read the
+red, green, and blue values from (or write them to) the appropriate position in
+the pixel when compressing from/decompressing to an RGB buffer.
+
+Your application can check for the existence of these extensions at compile
+time with:
+
+ #ifdef JCS_EXTENSIONS
+
+At run time, attempting to use these extensions with a libjpeg implementation
+that does not support them will result in a "Bogus input colorspace" error.
+Applications can trap this error in order to test whether run-time support is
+available for the colorspace extensions.
+
+When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the
+X byte is undefined, and in order to ensure the best performance, libjpeg-turbo
+can set that byte to whatever value it wishes. If an application expects the X
+byte to be used as an alpha channel, then it should specify JCS_EXT_RGBA,
+JCS_EXT_BGRA, JCS_EXT_ABGR, or JCS_EXT_ARGB. When these colorspace constants
+are used, the X byte is guaranteed to be 0xFF, which is interpreted as opaque.
+
+Your application can check for the existence of the alpha channel colorspace
+extensions at compile time with:
+
+ #ifdef JCS_ALPHA_EXTENSIONS
+
+jcstest.c, located in the libjpeg-turbo source tree, demonstrates how to check
+for the existence of the colorspace extensions at compile time and run time.
+
+===================================
+libjpeg v7 and v8 API/ABI Emulation
+===================================
+
+With libjpeg v7 and v8, new features were added that necessitated extending the
+compression and decompression structures. Unfortunately, due to the exposed
+nature of those structures, extending them also necessitated breaking backward
+ABI compatibility with previous libjpeg releases. Thus, programs that were
+built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is
+based on the libjpeg v6b code base. Although libjpeg v7 and v8 are still not
+as widely used as v6b, enough programs (including a few Linux distros) made
+the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs
+in libjpeg-turbo. It should be noted, however, that this feature was added
+primarily so that applications that had already been compiled to use libjpeg
+v7+ could take advantage of accelerated baseline JPEG encoding/decoding
+without recompiling. libjpeg-turbo does not claim to support all of the
+libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all
+cases (see below.)
+
+By passing an argument of --with-jpeg7 or --with-jpeg8 to configure, or an
+argument of -DWITH_JPEG7=1 or -DWITH_JPEG8=1 to cmake, you can build a version
+of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that programs
+that are built against libjpeg v7 or v8 can be run with libjpeg-turbo. The
+following section describes which libjpeg v7+ features are supported and which
+aren't.
+
+Support for libjpeg v7 and v8 Features:
+---------------------------------------
+
+Fully supported:
+
+-- libjpeg: IDCT scaling extensions in decompressor
+ libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8,
+ 1/2, 5/8, 3/4, 7/8, 9/8, 5/4, 11/8, 3/2, 13/8, 7/4, 15/8, and 2/1 (only 1/4
+ and 1/2 are SIMD-accelerated.)
+
+-- libjpeg: arithmetic coding
+
+-- libjpeg: In-memory source and destination managers
+ See notes below.
+
+-- cjpeg: Separate quality settings for luminance and chrominance
+ Note that the libpjeg v7+ API was extended to accommodate this feature only
+ for convenience purposes. It has always been possible to implement this
+ feature with libjpeg v6b (see rdswitch.c for an example.)
+
+-- cjpeg: 32-bit BMP support
+
+-- cjpeg: -rgb option
+
+-- jpegtran: lossless cropping
+
+-- jpegtran: -perfect option
+
+-- jpegtran: forcing width/height when performing lossless crop
+
+-- rdjpgcom: -raw option
+
+-- rdjpgcom: locale awareness
+
+
+Not supported:
+
+NOTE: As of this writing, extensive research has been conducted into the
+usefulness of DCT scaling as a means of data reduction and SmartScale as a
+means of quality improvement. The reader is invited to peruse the research at
+http://www.libjpeg-turbo.org/About/SmartScale and draw his/her own conclusions,
+but it is the general belief of our project that these features have not
+demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo.
+
+-- libjpeg: DCT scaling in compressor
+ cinfo.scale_num and cinfo.scale_denom are silently ignored.
+ There is no technical reason why DCT scaling could not be supported when
+ emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see
+ below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and
+ 8/9 would be available, which is of limited usefulness.
+
+-- libjpeg: SmartScale
+ cinfo.block_size is silently ignored.
+ SmartScale is an extension to the JPEG format that allows for DCT block
+ sizes other than 8x8. Providing support for this new format would be
+ feasible (particularly without full acceleration.) However, until/unless
+ the format becomes either an official industry standard or, at minimum, an
+ accepted solution in the community, we are hesitant to implement it, as
+ there is no sense of whether or how it might change in the future. It is
+ our belief that SmartScale has not demonstrated sufficient usefulness as a
+ lossless format nor as a means of quality enhancement, and thus, our primary
+ interest in providing this feature would be as a means of supporting
+ additional DCT scaling factors.
+
+-- libjpeg: Fancy downsampling in compressor
+ cinfo.do_fancy_downsampling is silently ignored.
+ This requires the DCT scaling feature, which is not supported.
+
+-- jpegtran: Scaling
+ This requires both the DCT scaling and SmartScale features, which are not
+ supported.
+
+-- Lossless RGB JPEG files
+ This requires the SmartScale feature, which is not supported.
+
+What About libjpeg v9?
+----------------------
+
+libjpeg v9 introduced yet another field to the JPEG compression structure
+(color_transform), thus making the ABI backward incompatible with that of
+libjpeg v8. This new field was introduced solely for the purpose of supporting
+lossless SmartScale encoding. Further, there was actually no reason to extend
+the API in this manner, as the color transform could have just as easily been
+activated by way of a new JPEG colorspace constant, thus preserving backward
+ABI compatibility.
+
+Our research (see link above) has shown that lossless SmartScale does not
+generally accomplish anything that can't already be accomplished better with
+existing, standard lossless formats. Thus, at this time, it is our belief that
+there is not sufficient technical justification for software to upgrade from
+libjpeg v8 to libjpeg v9, and therefore, not sufficient technical justification
+for us to emulate the libjpeg v9 ABI.
+
+=====================================
+In-Memory Source/Destination Managers
+=====================================
+
+By default, libjpeg-turbo 1.3 and later includes the jpeg_mem_src() and
+jpeg_mem_dest() functions, even when not emulating the libjpeg v8 API/ABI.
+Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8
+API/ABI emulation in order to use the in-memory source/destination managers,
+but several projects requested that those functions be included when emulating
+the libjpeg v6b API/ABI as well. This allows the use of those functions by
+programs that need them without breaking ABI compatibility for programs that
+don't, and it allows those functions to be provided in the "official"
+libjpeg-turbo binaries.
+
+Those who are concerned about maintaining strict conformance with the libjpeg
+v6b or v7 API can pass an argument of --without-mem-srcdst to configure or
+an argument of -DWITH_MEM_SRCDST=0 to CMake prior to building libjpeg-turbo.
+This will restore the pre-1.3 behavior, in which jpeg_mem_src() and
+jpeg_mem_dest() are only included when emulating the libjpeg v8 API/ABI.
+
+On Un*x systems, including the in-memory source/destination managers changes
+the dynamic library version from 62.0.0 to 62.1.0 if using libjpeg v6b API/ABI
+emulation and from 7.0.0 to 7.1.0 if using libjpeg v7 API/ABI emulation.
+
+Note that, on most Un*x systems, the dynamic linker will not look for a
+function in a library until that function is actually used. Thus, if a program
+is built against libjpeg-turbo 1.3+ and uses jpeg_mem_src() or jpeg_mem_dest(),
+that program will not fail if run against an older version of libjpeg-turbo or
+against libjpeg v7- until the program actually tries to call jpeg_mem_src() or
+jpeg_mem_dest(). Such is not the case on Windows. If a program is built
+against the libjpeg-turbo 1.3+ DLL and uses jpeg_mem_src() or jpeg_mem_dest(),
+then it must use the libjpeg-turbo 1.3+ DLL at run time.
+
+Both cjpeg and djpeg have been extended to allow testing the in-memory
+source/destination manager functions. See their respective man pages for more
+details.
+
+
+*******************************************************************************
+** Mathematical Compatibility
+*******************************************************************************
+
+For the most part, libjpeg-turbo should produce identical output to libjpeg
+v6b. The one exception to this is when using the floating point DCT/IDCT, in
+which case the outputs of libjpeg v6b and libjpeg-turbo are not guaranteed to
+be identical (the accuracy of the floating point DCT/IDCT is constant when
+using libjpeg-turbo's SIMD extensions, but otherwise, it can depend heavily on
+the compiler and compiler settings.)
+
+While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood, it is
+still using the same algorithms as libjpeg v6b, so there are several specific
+cases in which libjpeg-turbo cannot be expected to produce the same output as
+libjpeg v8:
+
+-- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8
+ implements those scaling algorithms a bit differently than libjpeg v6b does,
+ and libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.
+
+-- When using chrominance subsampling, because libjpeg v8 implements this
+ with its DCT/IDCT scaling algorithms rather than with a separate
+ downsampling/upsampling algorithm.
+
+-- When using the floating point IDCT, for the reasons stated above and also
+ because the floating point IDCT algorithm was modified in libjpeg v8a to
+ improve accuracy.
+
+-- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or
+ "non-smooth") chrominance upsampling, because libjpeg v8 does not support
+ merged upsampling with scaling factors > 1.
+
+
+*******************************************************************************
+** Performance Pitfalls
+*******************************************************************************
+
+===============
+Restart Markers
+===============
+
+The optimized Huffman decoder in libjpeg-turbo does not handle restart markers
+in a way that makes the rest of the libjpeg infrastructure happy, so it is
+necessary to use the slow Huffman decoder when decompressing a JPEG image that
+has restart markers. This can cause the decompression performance to drop by
+as much as 20%, but the performance will still be much greater than that of
+libjpeg. Many consumer packages, such as PhotoShop, use restart markers when
+generating JPEG images, so images generated by those programs will experience
+this issue.
+
+===============================================
+Fast Integer Forward DCT at High Quality Levels
+===============================================
+
+The algorithm used by the SIMD-accelerated quantization function cannot produce
+correct results whenever the fast integer forward DCT is used along with a JPEG
+quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization
+function in those cases. This causes performance to drop by as much as 40%.
+It is therefore strongly advised that you use the slow integer forward DCT
+whenever encoding images with a JPEG quality of 98 or higher.