1******************************************************************************* 2** Background 3******************************************************************************* 4 5libjpeg-turbo is a JPEG image codec that uses SIMD instructions (MMX, SSE2, 6NEON) to accelerate baseline JPEG compression and decompression on x86, x86-64, 7and ARM systems. On such systems, libjpeg-turbo is generally 2-4x as fast as 8libjpeg, all else being equal. On other types of systems, libjpeg-turbo can 9still outperform libjpeg by a significant amount, by virtue of its 10highly-optimized Huffman coding routines. In many cases, the performance of 11libjpeg-turbo rivals that of proprietary high-speed JPEG codecs. 12 13libjpeg-turbo implements both the traditional libjpeg API as well as the less 14powerful but more straightforward TurboJPEG API. libjpeg-turbo also features 15colorspace extensions that allow it to compress from/decompress to 32-bit and 16big-endian pixel buffers (RGBX, XBGR, etc.), as well as a full-featured Java 17interface. 18 19libjpeg-turbo was originally based on libjpeg/SIMD, an MMX-accelerated 20derivative of libjpeg v6b developed by Miyasaka Masaru. The TigerVNC and 21VirtualGL projects made numerous enhancements to the codec in 2009, and in 22early 2010, libjpeg-turbo spun off into an independent project, with the goal 23of making high-speed JPEG compression/decompression technology available to a 24broader range of users and developers. 25 26 27******************************************************************************* 28** License 29******************************************************************************* 30 31Most of libjpeg-turbo inherits the non-restrictive, BSD-style license used by 32libjpeg (see README.) The TurboJPEG wrapper (both C and Java versions) and 33associated test programs bear a similar license, which is reproduced below: 34 35Redistribution and use in source and binary forms, with or without 36modification, are permitted provided that the following conditions are met: 37 38- Redistributions of source code must retain the above copyright notice, 39 this list of conditions and the following disclaimer. 40- Redistributions in binary form must reproduce the above copyright notice, 41 this list of conditions and the following disclaimer in the documentation 42 and/or other materials provided with the distribution. 43- Neither the name of the libjpeg-turbo Project nor the names of its 44 contributors may be used to endorse or promote products derived from this 45 software without specific prior written permission. 46 47THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS", 48AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 49IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 50ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE 51LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 52CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 53SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 54INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 55CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 56ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 57POSSIBILITY OF SUCH DAMAGE. 58 59 60******************************************************************************* 61** Using libjpeg-turbo 62******************************************************************************* 63 64libjpeg-turbo includes two APIs that can be used to compress and decompress 65JPEG images: 66 67 TurboJPEG API: This API provides an easy-to-use interface for compressing 68 and decompressing JPEG images in memory. It also provides some functionality 69 that would not be straightforward to achieve using the underlying libjpeg 70 API, such as generating planar YUV images and performing multiple 71 simultaneous lossless transforms on an image. The Java interface for 72 libjpeg-turbo is written on top of the TurboJPEG API. 73 74 libjpeg API: This is the de facto industry-standard API for compressing and 75 decompressing JPEG images. It is more difficult to use than the TurboJPEG 76 API but also more powerful. The libjpeg API implementation in libjpeg-turbo 77 is both API/ABI-compatible and mathematically compatible with libjpeg v6b. 78 It can also optionally be configured to be API/ABI-compatible with libjpeg v7 79 and v8 (see below.) 80 81There is no significant performance advantage to either API when both are used 82to perform similar operations. 83 84====================== 85Installation Directory 86====================== 87 88This document assumes that libjpeg-turbo will be installed in the default 89directory (/opt/libjpeg-turbo on Un*x and Mac systems and 90c:\libjpeg-turbo[-gcc][64] on Windows systems. If your installation of 91libjpeg-turbo resides in a different directory, then adjust the instructions 92accordingly. 93 94============================= 95Replacing libjpeg at Run Time 96============================= 97 98Un*x 99---- 100 101If a Un*x application is dynamically linked with libjpeg, then you can replace 102libjpeg with libjpeg-turbo at run time by manipulating LD_LIBRARY_PATH. 103For instance: 104 105 [Using libjpeg] 106 > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg 107 real 0m0.392s 108 user 0m0.074s 109 sys 0m0.020s 110 111 [Using libjpeg-turbo] 112 > export LD_LIBRARY_PATH=/opt/libjpeg-turbo/{lib}:$LD_LIBRARY_PATH 113 > time cjpeg <vgl_5674_0098.ppm >vgl_5674_0098.jpg 114 real 0m0.109s 115 user 0m0.029s 116 sys 0m0.010s 117 118({lib} = lib32 or lib64, depending on whether you wish to use the 32-bit or the 11964-bit version of libjpeg-turbo.) 120 121System administrators can also replace the libjpeg symlinks in /usr/lib* with 122links to the libjpeg-turbo dynamic library located in /opt/libjpeg-turbo/{lib}. 123This will effectively accelerate every application that uses the libjpeg 124dynamic library on the system. 125 126Windows 127------- 128 129If a Windows application is dynamically linked with libjpeg, then you can 130replace libjpeg with libjpeg-turbo at run time by backing up the application's 131copy of jpeg62.dll, jpeg7.dll, or jpeg8.dll (assuming the application has its 132own local copy of this library) and copying the corresponding DLL from 133libjpeg-turbo into the application's install directory. The official 134libjpeg-turbo binary packages only provide jpeg62.dll. If the application uses 135jpeg7.dll or jpeg8.dll instead, then it will be necessary to build 136libjpeg-turbo from source (see "libjpeg v7 and v8 API/ABI Emulation" below.) 137 138The following information is specific to the official libjpeg-turbo binary 139packages for Visual C++: 140 141-- jpeg62.dll requires the Visual C++ 2008 C run-time DLL (msvcr90.dll). 142msvcr90.dll ships with more recent versions of Windows, but users of older 143Windows releases can obtain it from the Visual C++ 2008 Redistributable 144Package, which is available as a free download from Microsoft's web site. 145 146-- Features of the libjpeg API that require passing a C run-time structure, 147such as a file handle, from an application to the library will probably not 148work with jpeg62.dll, unless the application is also built to use the Visual 149C++ 2008 C run-time DLL. In particular, this affects jpeg_stdio_dest() and 150jpeg_stdio_src(). 151 152Mac 153--- 154 155Mac applications typically embed their own copies of the libjpeg dylib inside 156the (hidden) application bundle, so it is not possible to globally replace 157libjpeg on OS X systems. Replacing the application's version of the libjpeg 158dylib would generally involve copying libjpeg.*.dylib from libjpeg-turbo into 159the appropriate place in the application bundle and using install_name_tool to 160repoint the libjpeg-turbo dylib to its new directory. This requires an 161advanced knowledge of OS X and would not survive an upgrade or a re-install of 162the application. Thus, it is not recommended for most users. 163 164======================================== 165Using libjpeg-turbo in Your Own Programs 166======================================== 167 168For the most part, libjpeg-turbo should work identically to libjpeg, so in 169most cases, an application can be built against libjpeg and then run against 170libjpeg-turbo. On Un*x systems and Cygwin, you can build against libjpeg-turbo 171instead of libjpeg by setting 172 173 CPATH=/opt/libjpeg-turbo/include 174 and 175 LIBRARY_PATH=/opt/libjpeg-turbo/{lib} 176 177({lib} = lib32 or lib64, depending on whether you are building a 32-bit or a 17864-bit application.) 179 180If using MinGW, then set 181 182 CPATH=/c/libjpeg-turbo-gcc[64]/include 183 and 184 LIBRARY_PATH=/c/libjpeg-turbo-gcc[64]/lib 185 186Building against libjpeg-turbo is useful, for instance, if you want to build an 187application that leverages the libjpeg-turbo colorspace extensions (see below.) 188On Un*x systems, you would still need to manipulate LD_LIBRARY_PATH or create 189appropriate symlinks to use libjpeg-turbo at run time. On such systems, you 190can pass -R /opt/libjpeg-turbo/{lib} to the linker to force the use of 191libjpeg-turbo at run time rather than libjpeg (also useful if you want to 192leverage the colorspace extensions), or you can link against the libjpeg-turbo 193static library. 194 195To force a Un*x or MinGW application to link against the static version of 196libjpeg-turbo, you can use the following linker options: 197 198 -Wl,-Bstatic -ljpeg -Wl,-Bdynamic 199 200On OS X, simply add /opt/libjpeg-turbo/lib/libjpeg.a to the linker command 201line. 202 203To build Visual C++ applications using libjpeg-turbo, add 204c:\libjpeg-turbo[64]\include to the system or user INCLUDE environment 205variable and c:\libjpeg-turbo[64]\lib to the system or user LIB environment 206variable, and then link against either jpeg.lib (to use the DLL version of 207libjpeg-turbo) or jpeg-static.lib (to use the static version of libjpeg-turbo.) 208 209===================== 210Colorspace Extensions 211===================== 212 213libjpeg-turbo includes extensions that allow JPEG images to be compressed 214directly from (and decompressed directly to) buffers that use BGR, BGRX, 215RGBX, XBGR, and XRGB pixel ordering. This is implemented with ten new 216colorspace constants: 217 218 JCS_EXT_RGB /* red/green/blue */ 219 JCS_EXT_RGBX /* red/green/blue/x */ 220 JCS_EXT_BGR /* blue/green/red */ 221 JCS_EXT_BGRX /* blue/green/red/x */ 222 JCS_EXT_XBGR /* x/blue/green/red */ 223 JCS_EXT_XRGB /* x/red/green/blue */ 224 JCS_EXT_RGBA /* red/green/blue/alpha */ 225 JCS_EXT_BGRA /* blue/green/red/alpha */ 226 JCS_EXT_ABGR /* alpha/blue/green/red */ 227 JCS_EXT_ARGB /* alpha/red/green/blue */ 228 229Setting cinfo.in_color_space (compression) or cinfo.out_color_space 230(decompression) to one of these values will cause libjpeg-turbo to read the 231red, green, and blue values from (or write them to) the appropriate position in 232the pixel when compressing from/decompressing to an RGB buffer. 233 234Your application can check for the existence of these extensions at compile 235time with: 236 237 #ifdef JCS_EXTENSIONS 238 239At run time, attempting to use these extensions with a libjpeg implementation 240that does not support them will result in a "Bogus input colorspace" error. 241Applications can trap this error in order to test whether run-time support is 242available for the colorspace extensions. 243 244When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the 245X byte is undefined, and in order to ensure the best performance, libjpeg-turbo 246can set that byte to whatever value it wishes. If an application expects the X 247byte to be used as an alpha channel, then it should specify JCS_EXT_RGBA, 248JCS_EXT_BGRA, JCS_EXT_ABGR, or JCS_EXT_ARGB. When these colorspace constants 249are used, the X byte is guaranteed to be 0xFF, which is interpreted as opaque. 250 251Your application can check for the existence of the alpha channel colorspace 252extensions at compile time with: 253 254 #ifdef JCS_ALPHA_EXTENSIONS 255 256jcstest.c, located in the libjpeg-turbo source tree, demonstrates how to check 257for the existence of the colorspace extensions at compile time and run time. 258 259=================================== 260libjpeg v7 and v8 API/ABI Emulation 261=================================== 262 263With libjpeg v7 and v8, new features were added that necessitated extending the 264compression and decompression structures. Unfortunately, due to the exposed 265nature of those structures, extending them also necessitated breaking backward 266ABI compatibility with previous libjpeg releases. Thus, programs that were 267built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is 268based on the libjpeg v6b code base. Although libjpeg v7 and v8 are still not 269as widely used as v6b, enough programs (including a few Linux distros) made 270the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs 271in libjpeg-turbo. It should be noted, however, that this feature was added 272primarily so that applications that had already been compiled to use libjpeg 273v7+ could take advantage of accelerated baseline JPEG encoding/decoding 274without recompiling. libjpeg-turbo does not claim to support all of the 275libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all 276cases (see below.) 277 278By passing an argument of --with-jpeg7 or --with-jpeg8 to configure, or an 279argument of -DWITH_JPEG7=1 or -DWITH_JPEG8=1 to cmake, you can build a version 280of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that programs 281that are built against libjpeg v7 or v8 can be run with libjpeg-turbo. The 282following section describes which libjpeg v7+ features are supported and which 283aren't. 284 285Support for libjpeg v7 and v8 Features: 286--------------------------------------- 287 288Fully supported: 289 290-- libjpeg: IDCT scaling extensions in decompressor 291 libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8, 292 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 1/4 293 and 1/2 are SIMD-accelerated.) 294 295-- libjpeg: arithmetic coding 296 297-- libjpeg: In-memory source and destination managers 298 See notes below. 299 300-- cjpeg: Separate quality settings for luminance and chrominance 301 Note that the libpjeg v7+ API was extended to accommodate this feature only 302 for convenience purposes. It has always been possible to implement this 303 feature with libjpeg v6b (see rdswitch.c for an example.) 304 305-- cjpeg: 32-bit BMP support 306 307-- cjpeg: -rgb option 308 309-- jpegtran: lossless cropping 310 311-- jpegtran: -perfect option 312 313-- jpegtran: forcing width/height when performing lossless crop 314 315-- rdjpgcom: -raw option 316 317-- rdjpgcom: locale awareness 318 319 320Not supported: 321 322NOTE: As of this writing, extensive research has been conducted into the 323usefulness of DCT scaling as a means of data reduction and SmartScale as a 324means of quality improvement. The reader is invited to peruse the research at 325http://www.libjpeg-turbo.org/About/SmartScale and draw his/her own conclusions, 326but it is the general belief of our project that these features have not 327demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo. 328 329-- libjpeg: DCT scaling in compressor 330 cinfo.scale_num and cinfo.scale_denom are silently ignored. 331 There is no technical reason why DCT scaling could not be supported when 332 emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see 333 below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and 334 8/9 would be available, which is of limited usefulness. 335 336-- libjpeg: SmartScale 337 cinfo.block_size is silently ignored. 338 SmartScale is an extension to the JPEG format that allows for DCT block 339 sizes other than 8x8. Providing support for this new format would be 340 feasible (particularly without full acceleration.) However, until/unless 341 the format becomes either an official industry standard or, at minimum, an 342 accepted solution in the community, we are hesitant to implement it, as 343 there is no sense of whether or how it might change in the future. It is 344 our belief that SmartScale has not demonstrated sufficient usefulness as a 345 lossless format nor as a means of quality enhancement, and thus, our primary 346 interest in providing this feature would be as a means of supporting 347 additional DCT scaling factors. 348 349-- libjpeg: Fancy downsampling in compressor 350 cinfo.do_fancy_downsampling is silently ignored. 351 This requires the DCT scaling feature, which is not supported. 352 353-- jpegtran: Scaling 354 This requires both the DCT scaling and SmartScale features, which are not 355 supported. 356 357-- Lossless RGB JPEG files 358 This requires the SmartScale feature, which is not supported. 359 360What About libjpeg v9? 361---------------------- 362 363libjpeg v9 introduced yet another field to the JPEG compression structure 364(color_transform), thus making the ABI backward incompatible with that of 365libjpeg v8. This new field was introduced solely for the purpose of supporting 366lossless SmartScale encoding. Further, there was actually no reason to extend 367the API in this manner, as the color transform could have just as easily been 368activated by way of a new JPEG colorspace constant, thus preserving backward 369ABI compatibility. 370 371Our research (see link above) has shown that lossless SmartScale does not 372generally accomplish anything that can't already be accomplished better with 373existing, standard lossless formats. Thus, at this time, it is our belief that 374there is not sufficient technical justification for software to upgrade from 375libjpeg v8 to libjpeg v9, and therefore, not sufficient technical justification 376for us to emulate the libjpeg v9 ABI. 377 378===================================== 379In-Memory Source/Destination Managers 380===================================== 381 382By default, libjpeg-turbo 1.3 and later includes the jpeg_mem_src() and 383jpeg_mem_dest() functions, even when not emulating the libjpeg v8 API/ABI. 384Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8 385API/ABI emulation in order to use the in-memory source/destination managers, 386but several projects requested that those functions be included when emulating 387the libjpeg v6b API/ABI as well. This allows the use of those functions by 388programs that need them without breaking ABI compatibility for programs that 389don't, and it allows those functions to be provided in the "official" 390libjpeg-turbo binaries. 391 392Those who are concerned about maintaining strict conformance with the libjpeg 393v6b or v7 API can pass an argument of --without-mem-srcdst to configure or 394an argument of -DWITH_MEM_SRCDST=0 to CMake prior to building libjpeg-turbo. 395This will restore the pre-1.3 behavior, in which jpeg_mem_src() and 396jpeg_mem_dest() are only included when emulating the libjpeg v8 API/ABI. 397 398On Un*x systems, including the in-memory source/destination managers changes 399the dynamic library version from 62.0.0 to 62.1.0 if using libjpeg v6b API/ABI 400emulation and from 7.0.0 to 7.1.0 if using libjpeg v7 API/ABI emulation. 401 402Note that, on most Un*x systems, the dynamic linker will not look for a 403function in a library until that function is actually used. Thus, if a program 404is built against libjpeg-turbo 1.3+ and uses jpeg_mem_src() or jpeg_mem_dest(), 405that program will not fail if run against an older version of libjpeg-turbo or 406against libjpeg v7- until the program actually tries to call jpeg_mem_src() or 407jpeg_mem_dest(). Such is not the case on Windows. If a program is built 408against the libjpeg-turbo 1.3+ DLL and uses jpeg_mem_src() or jpeg_mem_dest(), 409then it must use the libjpeg-turbo 1.3+ DLL at run time. 410 411Both cjpeg and djpeg have been extended to allow testing the in-memory 412source/destination manager functions. See their respective man pages for more 413details. 414 415 416******************************************************************************* 417** Mathematical Compatibility 418******************************************************************************* 419 420For the most part, libjpeg-turbo should produce identical output to libjpeg 421v6b. The one exception to this is when using the floating point DCT/IDCT, in 422which case the outputs of libjpeg v6b and libjpeg-turbo are not guaranteed to 423be identical (the accuracy of the floating point DCT/IDCT is constant when 424using libjpeg-turbo's SIMD extensions, but otherwise, it can depend heavily on 425the compiler and compiler settings.) 426 427While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood, it is 428still using the same algorithms as libjpeg v6b, so there are several specific 429cases in which libjpeg-turbo cannot be expected to produce the same output as 430libjpeg v8: 431 432-- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8 433 implements those scaling algorithms a bit differently than libjpeg v6b does, 434 and libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior. 435 436-- When using chrominance subsampling, because libjpeg v8 implements this 437 with its DCT/IDCT scaling algorithms rather than with a separate 438 downsampling/upsampling algorithm. 439 440-- When using the floating point IDCT, for the reasons stated above and also 441 because the floating point IDCT algorithm was modified in libjpeg v8a to 442 improve accuracy. 443 444-- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or 445 "non-smooth") chrominance upsampling, because libjpeg v8 does not support 446 merged upsampling with scaling factors > 1. 447 448 449******************************************************************************* 450** Performance Pitfalls 451******************************************************************************* 452 453=============== 454Restart Markers 455=============== 456 457The optimized Huffman decoder in libjpeg-turbo does not handle restart markers 458in a way that makes the rest of the libjpeg infrastructure happy, so it is 459necessary to use the slow Huffman decoder when decompressing a JPEG image that 460has restart markers. This can cause the decompression performance to drop by 461as much as 20%, but the performance will still be much greater than that of 462libjpeg. Many consumer packages, such as PhotoShop, use restart markers when 463generating JPEG images, so images generated by those programs will experience 464this issue. 465 466=============================================== 467Fast Integer Forward DCT at High Quality Levels 468=============================================== 469 470The algorithm used by the SIMD-accelerated quantization function cannot produce 471correct results whenever the fast integer forward DCT is used along with a JPEG 472quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization 473function in those cases. This causes performance to drop by as much as 40%. 474It is therefore strongly advised that you use the slow integer forward DCT 475whenever encoding images with a JPEG quality of 98 or higher. 476