Edit File: README-turbo.txt
******************************************************************************* ** 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.