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DRC101f09a2010-02-12 22:52:37 +00001*******************************************************************************
2** Background
3*******************************************************************************
4
DRC0f7ff712013-01-23 01:32:25 +00005libjpeg-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.
DRC101f09a2010-02-12 22:52:37 +000012
DRC0f7ff712013-01-23 01:32:25 +000013libjpeg-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.
DRC101f09a2010-02-12 22:52:37 +000018
DRC0f7ff712013-01-23 01:32:25 +000019libjpeg-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.
DRC101f09a2010-02-12 22:52:37 +000025
26
27*******************************************************************************
DRCce1546e2010-02-13 23:06:03 +000028** License
29*******************************************************************************
30
DRC11a122b2012-02-07 00:14:53 +000031Most of libjpeg-turbo inherits the non-restrictive, BSD-style license used by
DRC5039d732013-01-21 23:42:12 +000032libjpeg (see README.) The TurboJPEG wrapper (both C and Java versions) and
DRCb5624ee2011-05-24 14:12:07 +000033associated test programs bear a similar license, which is reproduced below:
DRC1e6b5b42010-03-20 20:00:51 +000034
DRCb5624ee2011-05-24 14:12:07 +000035Redistribution and use in source and binary forms, with or without
36modification, are permitted provided that the following conditions are met:
DRC65e0cd32011-04-26 23:44:37 +000037
DRCb5624ee2011-05-24 14:12:07 +000038- 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.
DRC7c1df0a2011-02-18 02:45:24 +000058
59
60*******************************************************************************
DRC68fef832010-02-16 05:29:10 +000061** Using libjpeg-turbo
DRC101f09a2010-02-12 22:52:37 +000062*******************************************************************************
63
DRC80803ae2011-12-15 13:12:59 +000064libjpeg-turbo includes two APIs that can be used to compress and decompress
DRC9c4590e2011-07-26 09:22:16 +000065JPEG images:
66
DRC11a122b2012-02-07 00:14:53 +000067 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.
DRC9c4590e2011-07-26 09:22:16 +000073
DRC11a122b2012-02-07 00:14:53 +000074 libjpeg API: This is the de facto industry-standard API for compressing and
DRC2c62da32012-01-17 22:55:03 +000075 decompressing JPEG images. It is more difficult to use than the TurboJPEG
DRC5039d732013-01-21 23:42:12 +000076 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.
DRC9c4590e2011-07-26 09:22:16 +000083
DRCd5e964c2013-01-10 11:47:39 +000084======================
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.
DRC9c4590e2011-07-26 09:22:16 +000093
DRC68fef832010-02-16 05:29:10 +000094=============================
95Replacing libjpeg at Run Time
96=============================
DRC101f09a2010-02-12 22:52:37 +000097
DRCd5e964c2013-01-10 11:47:39 +000098Un*x
99----
100
101If a Un*x application is dynamically linked with libjpeg, then you can replace
DRCab4db652011-02-18 04:55:08 +0000102libjpeg with libjpeg-turbo at run time by manipulating LD_LIBRARY_PATH.
DRC68fef832010-02-16 05:29:10 +0000103For instance:
DRC101f09a2010-02-12 22:52:37 +0000104
DRC68fef832010-02-16 05:29:10 +0000105 [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
DRC101f09a2010-02-12 22:52:37 +0000110
DRC68fef832010-02-16 05:29:10 +0000111 [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
DRC101f09a2010-02-12 22:52:37 +0000117
DRCd5e964c2013-01-10 11:47:39 +0000118({lib} = lib32 or lib64, depending on whether you wish to use the 32-bit or the
11964-bit version of libjpeg-turbo.)
DRC101f09a2010-02-12 22:52:37 +0000120
DRCd5e964c2013-01-10 11:47:39 +0000121System administrators can also replace the libjpeg symlinks in /usr/lib* with
DRC11a122b2012-02-07 00:14:53 +0000122links 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.
DRC101f09a2010-02-12 22:52:37 +0000125
DRCd5e964c2013-01-10 11:47:39 +0000126Windows
127-------
DRC7e0b4992010-02-25 05:52:44 +0000128
DRCd5e964c2013-01-10 11:47:39 +0000129If 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
DRCdad4d2e2013-01-11 11:07:37 +0000134libjpeg-turbo binary packages only provide jpeg62.dll. If the application uses
DRCd5e964c2013-01-10 11:47:39 +0000135jpeg7.dll or jpeg8.dll instead, then it will be necessary to build
DRCdad4d2e2013-01-11 11:07:37 +0000136libjpeg-turbo from source (see "libjpeg v7 and v8 API/ABI Emulation" below.)
DRCd5e964c2013-01-10 11:47:39 +0000137
DRCdad4d2e2013-01-11 11:07:37 +0000138The following information is specific to the official libjpeg-turbo binary
139packages for Visual C++:
DRCd5e964c2013-01-10 11:47:39 +0000140
141-- jpeg62.dll requires the Visual C++ 2008 C run-time DLL (msvcr90.dll).
DRC5559c902010-10-18 02:21:10 +0000142msvcr90.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.
DRC0248dd92010-02-25 06:21:12 +0000145
DRCd5e964c2013-01-10 11:47:39 +0000146-- 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
DRCab4db652011-02-18 04:55:08 +0000150jpeg_stdio_src().
DRC101f09a2010-02-12 22:52:37 +0000151
DRCd5e964c2013-01-10 11:47:39 +0000152Mac
153---
154
DRC5559c902010-10-18 02:21:10 +0000155Mac applications typically embed their own copies of the libjpeg dylib inside
DRC68fef832010-02-16 05:29:10 +0000156the (hidden) application bundle, so it is not possible to globally replace
DRCd5e964c2013-01-10 11:47:39 +0000157libjpeg on OS X systems. Replacing the application's version of the libjpeg
158dylib would generally involve copying libjpeg.*.dylib from libjpeg-turbo into
DRC5559c902010-10-18 02:21:10 +0000159the appropriate place in the application bundle and using install_name_tool to
DRCd5e964c2013-01-10 11:47:39 +0000160repoint 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.
DRC101f09a2010-02-12 22:52:37 +0000163
DRC68fef832010-02-16 05:29:10 +0000164========================================
165Using libjpeg-turbo in Your Own Programs
166========================================
DRC101f09a2010-02-12 22:52:37 +0000167
DRC68fef832010-02-16 05:29:10 +0000168For 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
DRCdad4d2e2013-01-11 11:07:37 +0000170libjpeg-turbo. On Un*x systems and Cygwin, you can build against libjpeg-turbo
171instead of libjpeg by setting
DRC101f09a2010-02-12 22:52:37 +0000172
DRC68fef832010-02-16 05:29:10 +0000173 CPATH=/opt/libjpeg-turbo/include
174 and
175 LIBRARY_PATH=/opt/libjpeg-turbo/{lib}
DRC101f09a2010-02-12 22:52:37 +0000176
DRCff95aa62010-06-05 00:22:32 +0000177({lib} = lib32 or lib64, depending on whether you are building a 32-bit or a
17864-bit application.)
DRC101f09a2010-02-12 22:52:37 +0000179
DRC0a1f68e2010-02-24 07:24:26 +0000180If using MinGW, then set
181
DRC3dc1bc22010-05-10 22:18:10 +0000182 CPATH=/c/libjpeg-turbo-gcc[64]/include
DRC0a1f68e2010-02-24 07:24:26 +0000183 and
DRC3dc1bc22010-05-10 22:18:10 +0000184 LIBRARY_PATH=/c/libjpeg-turbo-gcc[64]/lib
DRC0a1f68e2010-02-24 07:24:26 +0000185
186Building against libjpeg-turbo is useful, for instance, if you want to build an
187application that leverages the libjpeg-turbo colorspace extensions (see below.)
DRCdad4d2e2013-01-11 11:07:37 +0000188On 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.
DRC0a1f68e2010-02-24 07:24:26 +0000194
DRCdad4d2e2013-01-11 11:07:37 +0000195To force a Un*x or MinGW application to link against the static version of
196libjpeg-turbo, you can use the following linker options:
DRC101f09a2010-02-12 22:52:37 +0000197
DRC68fef832010-02-16 05:29:10 +0000198 -Wl,-Bstatic -ljpeg -Wl,-Bdynamic
DRC101f09a2010-02-12 22:52:37 +0000199
DRCff95aa62010-06-05 00:22:32 +0000200On OS X, simply add /opt/libjpeg-turbo/lib/libjpeg.a to the linker command
DRCd5e964c2013-01-10 11:47:39 +0000201line.
DRC101f09a2010-02-12 22:52:37 +0000202
DRC68fef832010-02-16 05:29:10 +0000203To build Visual C++ applications using libjpeg-turbo, add
DRCab4db652011-02-18 04:55:08 +0000204c:\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
DRC5559c902010-10-18 02:21:10 +0000206variable, 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.)
DRC101f09a2010-02-12 22:52:37 +0000208
DRC68fef832010-02-16 05:29:10 +0000209=====================
210Colorspace Extensions
211=====================
DRC101f09a2010-02-12 22:52:37 +0000212
DRC80803ae2011-12-15 13:12:59 +0000213libjpeg-turbo includes extensions that allow JPEG images to be compressed
214directly from (and decompressed directly to) buffers that use BGR, BGRX,
DRC67ce3b22011-12-19 02:21:03 +0000215RGBX, XBGR, and XRGB pixel ordering. This is implemented with ten new
DRC68fef832010-02-16 05:29:10 +0000216colorspace constants:
DRC101f09a2010-02-12 22:52:37 +0000217
DRC68fef832010-02-16 05:29:10 +0000218 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 */
DRC67ce3b22011-12-19 02:21:03 +0000224 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 */
DRC101f09a2010-02-12 22:52:37 +0000228
DRC68fef832010-02-16 05:29:10 +0000229Setting 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
DRCb76c8402011-12-19 15:01:55 +0000232the pixel when compressing from/decompressing to an RGB buffer.
DRC101f09a2010-02-12 22:52:37 +0000233
DRC68fef832010-02-16 05:29:10 +0000234Your application can check for the existence of these extensions at compile
235time with:
DRC101f09a2010-02-12 22:52:37 +0000236
DRC68fef832010-02-16 05:29:10 +0000237 #ifdef JCS_EXTENSIONS
DRC101f09a2010-02-12 22:52:37 +0000238
DRCd5e964c2013-01-10 11:47:39 +0000239At 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.
DRC77e39642010-10-12 03:02:31 +0000243
DRC67ce3b22011-12-19 02:21:03 +0000244When 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
DRCb76c8402011-12-19 15:01:55 +0000247byte to be used as an alpha channel, then it should specify JCS_EXT_RGBA,
DRC67ce3b22011-12-19 02:21:03 +0000248JCS_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
DRCb76c8402011-12-19 15:01:55 +0000256jcstest.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
DRCd5e964c2013-01-10 11:47:39 +0000259===================================
260libjpeg v7 and v8 API/ABI Emulation
261===================================
DRC77e39642010-10-12 03:02:31 +0000262
DRCbdbcd142012-02-03 08:55:36 +0000263With libjpeg v7 and v8, new features were added that necessitated extending the
264compression and decompression structures. Unfortunately, due to the exposed
DRC58156992013-01-13 11:05:25 +0000265nature 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
DRCeff4f952013-01-18 06:02:10 +0000270the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs
DRC58156992013-01-13 11:05:25 +0000271in libjpeg-turbo. It should be noted, however, that this feature was added
DRCd5e964c2013-01-10 11:47:39 +0000272primarily 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.)
DRC77e39642010-10-12 03:02:31 +0000277
DRC5559c902010-10-18 02:21:10 +0000278By 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
DRCeff4f952013-01-18 06:02:10 +0000280of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that programs
DRC80803ae2011-12-15 13:12:59 +0000281that are built against libjpeg v7 or v8 can be run with libjpeg-turbo. The
DRC5559c902010-10-18 02:21:10 +0000282following section describes which libjpeg v7+ features are supported and which
283aren't.
DRC77e39642010-10-12 03:02:31 +0000284
DRCd5e964c2013-01-10 11:47:39 +0000285Support for libjpeg v7 and v8 Features:
286---------------------------------------
DRC77e39642010-10-12 03:02:31 +0000287
288Fully supported:
289
DRCbdbcd142012-02-03 08:55:36 +0000290-- 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
DRCac514382013-01-01 11:39:04 +0000295-- libjpeg: arithmetic coding
296
DRCab706232013-01-18 23:42:31 +0000297-- libjpeg: In-memory source and destination managers
298 See notes below.
299
DRCac514382013-01-01 11:39:04 +0000300-- 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
DRC77e39642010-10-12 03:02:31 +0000305-- cjpeg: 32-bit BMP support
306
DRCac514382013-01-01 11:39:04 +0000307-- cjpeg: -rgb option
308
DRC77e39642010-10-12 03:02:31 +0000309-- jpegtran: lossless cropping
310
311-- jpegtran: -perfect option
312
DRCac514382013-01-01 11:39:04 +0000313-- jpegtran: forcing width/height when performing lossless crop
314
DRC77e39642010-10-12 03:02:31 +0000315-- rdjpgcom: -raw option
316
317-- rdjpgcom: locale awareness
318
319
DRC77e39642010-10-12 03:02:31 +0000320Not supported:
321
DRCb87136c2013-01-18 06:12:51 +0000322NOTE: 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
DRC77e39642010-10-12 03:02:31 +0000329-- libjpeg: DCT scaling in compressor
330 cinfo.scale_num and cinfo.scale_denom are silently ignored.
DRCb87136c2013-01-18 06:12:51 +0000331 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.
DRCbdbcd142012-02-03 08:55:36 +0000335
336-- libjpeg: SmartScale
337 cinfo.block_size is silently ignored.
338 SmartScale is an extension to the JPEG format that allows for DCT block
DRCb87136c2013-01-18 06:12:51 +0000339 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.
DRC77e39642010-10-12 03:02:31 +0000348
DRC77e39642010-10-12 03:02:31 +0000349-- libjpeg: Fancy downsampling in compressor
350 cinfo.do_fancy_downsampling is silently ignored.
DRCbdbcd142012-02-03 08:55:36 +0000351 This requires the DCT scaling feature, which is not supported.
DRC77e39642010-10-12 03:02:31 +0000352
DRC77e39642010-10-12 03:02:31 +0000353-- jpegtran: Scaling
DRCbdbcd142012-02-03 08:55:36 +0000354 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.
DRCab4db652011-02-18 04:55:08 +0000359
DRCb87136c2013-01-18 06:12:51 +0000360What 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
DRCab706232013-01-18 23:42:31 +0000378=====================================
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
DRCdc4645d2013-01-19 00:13:57 +0000387the 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.
DRCab706232013-01-18 23:42:31 +0000391
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
DRCdc4645d2013-01-19 00:13:57 +0000402Note 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.
DRCab706232013-01-18 23:42:31 +0000410
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
DRCab4db652011-02-18 04:55:08 +0000415
416*******************************************************************************
DRCd5e964c2013-01-10 11:47:39 +0000417** 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
DRC8940e6c2014-05-11 09:46:28 +0000422which case the outputs of libjpeg v6b and libjpeg-turbo can differ for the
423following reasons:
424
425-- The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so
426 slightly more accurate than the implementation in libjpeg v6b, but not by
427 any amount perceptible to human vision (generally in the range of 0.01 to
428 0.08 dB gain in PNSR.)
429-- When not using the SIMD extensions, then the accuracy of the floating point
430 DCT/IDCT can depend on the compiler and compiler settings.
431
DRCd5e964c2013-01-10 11:47:39 +0000432
433While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood, it is
434still using the same algorithms as libjpeg v6b, so there are several specific
435cases in which libjpeg-turbo cannot be expected to produce the same output as
436libjpeg v8:
437
438-- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8
DRC8940e6c2014-05-11 09:46:28 +0000439 implements those scaling algorithms differently than libjpeg v6b does, and
440 libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.
DRCd5e964c2013-01-10 11:47:39 +0000441
442-- When using chrominance subsampling, because libjpeg v8 implements this
443 with its DCT/IDCT scaling algorithms rather than with a separate
DRC8940e6c2014-05-11 09:46:28 +0000444 downsampling/upsampling algorithm. In our testing, the subsampled/upsampled
445 output of libjpeg v8 is less accurate than that of libjpeg v6b for this
446 reason.
DRCd5e964c2013-01-10 11:47:39 +0000447
448-- When using the floating point IDCT, for the reasons stated above and also
449 because the floating point IDCT algorithm was modified in libjpeg v8a to
450 improve accuracy.
451
452-- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or
453 "non-smooth") chrominance upsampling, because libjpeg v8 does not support
454 merged upsampling with scaling factors > 1.
455
456
457*******************************************************************************
458** Performance Pitfalls
DRCab4db652011-02-18 04:55:08 +0000459*******************************************************************************
460
461===============
462Restart Markers
463===============
464
465The optimized Huffman decoder in libjpeg-turbo does not handle restart markers
DRC11a122b2012-02-07 00:14:53 +0000466in a way that makes the rest of the libjpeg infrastructure happy, so it is
467necessary to use the slow Huffman decoder when decompressing a JPEG image that
468has restart markers. This can cause the decompression performance to drop by
469as much as 20%, but the performance will still be much greater than that of
470libjpeg. Many consumer packages, such as PhotoShop, use restart markers when
471generating JPEG images, so images generated by those programs will experience
472this issue.
DRCab4db652011-02-18 04:55:08 +0000473
474===============================================
475Fast Integer Forward DCT at High Quality Levels
476===============================================
477
478The algorithm used by the SIMD-accelerated quantization function cannot produce
479correct results whenever the fast integer forward DCT is used along with a JPEG
480quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization
481function in those cases. This causes performance to drop by as much as 40%.
482It is therefore strongly advised that you use the slow integer forward DCT
483whenever encoding images with a JPEG quality of 98 or higher.