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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
DRC68fef832010-02-16 05:29:10 +000084=====================
85Colorspace Extensions
86=====================
DRC101f09a2010-02-12 22:52:37 +000087
DRC80803ae2011-12-15 13:12:59 +000088libjpeg-turbo includes extensions that allow JPEG images to be compressed
89directly from (and decompressed directly to) buffers that use BGR, BGRX,
DRC67ce3b22011-12-19 02:21:03 +000090RGBX, XBGR, and XRGB pixel ordering. This is implemented with ten new
DRC68fef832010-02-16 05:29:10 +000091colorspace constants:
DRC101f09a2010-02-12 22:52:37 +000092
DRC68fef832010-02-16 05:29:10 +000093 JCS_EXT_RGB /* red/green/blue */
94 JCS_EXT_RGBX /* red/green/blue/x */
95 JCS_EXT_BGR /* blue/green/red */
96 JCS_EXT_BGRX /* blue/green/red/x */
97 JCS_EXT_XBGR /* x/blue/green/red */
98 JCS_EXT_XRGB /* x/red/green/blue */
DRC67ce3b22011-12-19 02:21:03 +000099 JCS_EXT_RGBA /* red/green/blue/alpha */
100 JCS_EXT_BGRA /* blue/green/red/alpha */
101 JCS_EXT_ABGR /* alpha/blue/green/red */
102 JCS_EXT_ARGB /* alpha/red/green/blue */
DRC101f09a2010-02-12 22:52:37 +0000103
DRC68fef832010-02-16 05:29:10 +0000104Setting cinfo.in_color_space (compression) or cinfo.out_color_space
105(decompression) to one of these values will cause libjpeg-turbo to read the
106red, green, and blue values from (or write them to) the appropriate position in
DRCb76c8402011-12-19 15:01:55 +0000107the pixel when compressing from/decompressing to an RGB buffer.
DRC101f09a2010-02-12 22:52:37 +0000108
DRC68fef832010-02-16 05:29:10 +0000109Your application can check for the existence of these extensions at compile
110time with:
DRC101f09a2010-02-12 22:52:37 +0000111
DRC68fef832010-02-16 05:29:10 +0000112 #ifdef JCS_EXTENSIONS
DRC101f09a2010-02-12 22:52:37 +0000113
DRCd5e964c2013-01-10 11:47:39 +0000114At run time, attempting to use these extensions with a libjpeg implementation
115that does not support them will result in a "Bogus input colorspace" error.
116Applications can trap this error in order to test whether run-time support is
117available for the colorspace extensions.
DRC77e39642010-10-12 03:02:31 +0000118
DRC67ce3b22011-12-19 02:21:03 +0000119When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the
120X byte is undefined, and in order to ensure the best performance, libjpeg-turbo
121can set that byte to whatever value it wishes. If an application expects the X
DRCb76c8402011-12-19 15:01:55 +0000122byte to be used as an alpha channel, then it should specify JCS_EXT_RGBA,
DRC67ce3b22011-12-19 02:21:03 +0000123JCS_EXT_BGRA, JCS_EXT_ABGR, or JCS_EXT_ARGB. When these colorspace constants
124are used, the X byte is guaranteed to be 0xFF, which is interpreted as opaque.
125
126Your application can check for the existence of the alpha channel colorspace
127extensions at compile time with:
128
129 #ifdef JCS_ALPHA_EXTENSIONS
130
DRCb76c8402011-12-19 15:01:55 +0000131jcstest.c, located in the libjpeg-turbo source tree, demonstrates how to check
132for the existence of the colorspace extensions at compile time and run time.
133
DRCd5e964c2013-01-10 11:47:39 +0000134===================================
135libjpeg v7 and v8 API/ABI Emulation
136===================================
DRC77e39642010-10-12 03:02:31 +0000137
DRCbdbcd142012-02-03 08:55:36 +0000138With libjpeg v7 and v8, new features were added that necessitated extending the
139compression and decompression structures. Unfortunately, due to the exposed
DRC58156992013-01-13 11:05:25 +0000140nature of those structures, extending them also necessitated breaking backward
141ABI compatibility with previous libjpeg releases. Thus, programs that were
142built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is
DRC832b1fc2014-03-23 15:21:20 +0000143based on the libjpeg v6b code base. Although libjpeg v7 and v8 are not
DRC58156992013-01-13 11:05:25 +0000144as widely used as v6b, enough programs (including a few Linux distros) made
DRCeff4f952013-01-18 06:02:10 +0000145the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs
DRC58156992013-01-13 11:05:25 +0000146in libjpeg-turbo. It should be noted, however, that this feature was added
DRCd5e964c2013-01-10 11:47:39 +0000147primarily so that applications that had already been compiled to use libjpeg
148v7+ could take advantage of accelerated baseline JPEG encoding/decoding
149without recompiling. libjpeg-turbo does not claim to support all of the
150libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all
151cases (see below.)
DRC77e39642010-10-12 03:02:31 +0000152
DRC5559c902010-10-18 02:21:10 +0000153By passing an argument of --with-jpeg7 or --with-jpeg8 to configure, or an
154argument of -DWITH_JPEG7=1 or -DWITH_JPEG8=1 to cmake, you can build a version
DRCeff4f952013-01-18 06:02:10 +0000155of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that programs
DRC80803ae2011-12-15 13:12:59 +0000156that are built against libjpeg v7 or v8 can be run with libjpeg-turbo. The
DRC5559c902010-10-18 02:21:10 +0000157following section describes which libjpeg v7+ features are supported and which
158aren't.
DRC77e39642010-10-12 03:02:31 +0000159
DRCd5e964c2013-01-10 11:47:39 +0000160Support for libjpeg v7 and v8 Features:
161---------------------------------------
DRC77e39642010-10-12 03:02:31 +0000162
163Fully supported:
164
DRCbdbcd142012-02-03 08:55:36 +0000165-- libjpeg: IDCT scaling extensions in decompressor
166 libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8,
167 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
168 and 1/2 are SIMD-accelerated.)
169
DRCac514382013-01-01 11:39:04 +0000170-- libjpeg: arithmetic coding
171
DRCab706232013-01-18 23:42:31 +0000172-- libjpeg: In-memory source and destination managers
173 See notes below.
174
DRCac514382013-01-01 11:39:04 +0000175-- cjpeg: Separate quality settings for luminance and chrominance
176 Note that the libpjeg v7+ API was extended to accommodate this feature only
177 for convenience purposes. It has always been possible to implement this
178 feature with libjpeg v6b (see rdswitch.c for an example.)
179
DRC77e39642010-10-12 03:02:31 +0000180-- cjpeg: 32-bit BMP support
181
DRCac514382013-01-01 11:39:04 +0000182-- cjpeg: -rgb option
183
DRC77e39642010-10-12 03:02:31 +0000184-- jpegtran: lossless cropping
185
186-- jpegtran: -perfect option
187
DRCac514382013-01-01 11:39:04 +0000188-- jpegtran: forcing width/height when performing lossless crop
189
DRC77e39642010-10-12 03:02:31 +0000190-- rdjpgcom: -raw option
191
192-- rdjpgcom: locale awareness
193
194
DRC77e39642010-10-12 03:02:31 +0000195Not supported:
196
DRCb87136c2013-01-18 06:12:51 +0000197NOTE: As of this writing, extensive research has been conducted into the
198usefulness of DCT scaling as a means of data reduction and SmartScale as a
199means of quality improvement. The reader is invited to peruse the research at
200http://www.libjpeg-turbo.org/About/SmartScale and draw his/her own conclusions,
201but it is the general belief of our project that these features have not
202demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo.
203
DRC77e39642010-10-12 03:02:31 +0000204-- libjpeg: DCT scaling in compressor
205 cinfo.scale_num and cinfo.scale_denom are silently ignored.
DRCb87136c2013-01-18 06:12:51 +0000206 There is no technical reason why DCT scaling could not be supported when
207 emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see
208 below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and
209 8/9 would be available, which is of limited usefulness.
DRCbdbcd142012-02-03 08:55:36 +0000210
211-- libjpeg: SmartScale
212 cinfo.block_size is silently ignored.
213 SmartScale is an extension to the JPEG format that allows for DCT block
DRCb87136c2013-01-18 06:12:51 +0000214 sizes other than 8x8. Providing support for this new format would be
215 feasible (particularly without full acceleration.) However, until/unless
216 the format becomes either an official industry standard or, at minimum, an
217 accepted solution in the community, we are hesitant to implement it, as
218 there is no sense of whether or how it might change in the future. It is
219 our belief that SmartScale has not demonstrated sufficient usefulness as a
220 lossless format nor as a means of quality enhancement, and thus, our primary
221 interest in providing this feature would be as a means of supporting
222 additional DCT scaling factors.
DRC77e39642010-10-12 03:02:31 +0000223
DRC77e39642010-10-12 03:02:31 +0000224-- libjpeg: Fancy downsampling in compressor
225 cinfo.do_fancy_downsampling is silently ignored.
DRCbdbcd142012-02-03 08:55:36 +0000226 This requires the DCT scaling feature, which is not supported.
DRC77e39642010-10-12 03:02:31 +0000227
DRC77e39642010-10-12 03:02:31 +0000228-- jpegtran: Scaling
DRCbdbcd142012-02-03 08:55:36 +0000229 This requires both the DCT scaling and SmartScale features, which are not
230 supported.
231
232-- Lossless RGB JPEG files
233 This requires the SmartScale feature, which is not supported.
DRCab4db652011-02-18 04:55:08 +0000234
DRCb87136c2013-01-18 06:12:51 +0000235What About libjpeg v9?
236----------------------
237
238libjpeg v9 introduced yet another field to the JPEG compression structure
239(color_transform), thus making the ABI backward incompatible with that of
240libjpeg v8. This new field was introduced solely for the purpose of supporting
241lossless SmartScale encoding. Further, there was actually no reason to extend
242the API in this manner, as the color transform could have just as easily been
243activated by way of a new JPEG colorspace constant, thus preserving backward
244ABI compatibility.
245
246Our research (see link above) has shown that lossless SmartScale does not
247generally accomplish anything that can't already be accomplished better with
248existing, standard lossless formats. Thus, at this time, it is our belief that
249there is not sufficient technical justification for software to upgrade from
250libjpeg v8 to libjpeg v9, and therefore, not sufficient technical justification
251for us to emulate the libjpeg v9 ABI.
252
DRCab706232013-01-18 23:42:31 +0000253=====================================
254In-Memory Source/Destination Managers
255=====================================
256
257By default, libjpeg-turbo 1.3 and later includes the jpeg_mem_src() and
258jpeg_mem_dest() functions, even when not emulating the libjpeg v8 API/ABI.
259Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8
260API/ABI emulation in order to use the in-memory source/destination managers,
261but several projects requested that those functions be included when emulating
DRCdc4645d2013-01-19 00:13:57 +0000262the libjpeg v6b API/ABI as well. This allows the use of those functions by
263programs that need them without breaking ABI compatibility for programs that
264don't, and it allows those functions to be provided in the "official"
265libjpeg-turbo binaries.
DRCab706232013-01-18 23:42:31 +0000266
267Those who are concerned about maintaining strict conformance with the libjpeg
268v6b or v7 API can pass an argument of --without-mem-srcdst to configure or
269an argument of -DWITH_MEM_SRCDST=0 to CMake prior to building libjpeg-turbo.
270This will restore the pre-1.3 behavior, in which jpeg_mem_src() and
271jpeg_mem_dest() are only included when emulating the libjpeg v8 API/ABI.
272
273On Un*x systems, including the in-memory source/destination managers changes
274the dynamic library version from 62.0.0 to 62.1.0 if using libjpeg v6b API/ABI
275emulation and from 7.0.0 to 7.1.0 if using libjpeg v7 API/ABI emulation.
276
DRCdc4645d2013-01-19 00:13:57 +0000277Note that, on most Un*x systems, the dynamic linker will not look for a
278function in a library until that function is actually used. Thus, if a program
279is built against libjpeg-turbo 1.3+ and uses jpeg_mem_src() or jpeg_mem_dest(),
280that program will not fail if run against an older version of libjpeg-turbo or
281against libjpeg v7- until the program actually tries to call jpeg_mem_src() or
282jpeg_mem_dest(). Such is not the case on Windows. If a program is built
283against the libjpeg-turbo 1.3+ DLL and uses jpeg_mem_src() or jpeg_mem_dest(),
284then it must use the libjpeg-turbo 1.3+ DLL at run time.
DRCab706232013-01-18 23:42:31 +0000285
286Both cjpeg and djpeg have been extended to allow testing the in-memory
287source/destination manager functions. See their respective man pages for more
288details.
289
DRCab4db652011-02-18 04:55:08 +0000290
291*******************************************************************************
DRCd5e964c2013-01-10 11:47:39 +0000292** Mathematical Compatibility
293*******************************************************************************
294
295For the most part, libjpeg-turbo should produce identical output to libjpeg
296v6b. The one exception to this is when using the floating point DCT/IDCT, in
DRC8940e6c2014-05-11 09:46:28 +0000297which case the outputs of libjpeg v6b and libjpeg-turbo can differ for the
298following reasons:
299
300-- The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so
301 slightly more accurate than the implementation in libjpeg v6b, but not by
302 any amount perceptible to human vision (generally in the range of 0.01 to
303 0.08 dB gain in PNSR.)
DRC715bb412014-05-11 10:09:07 +0000304-- When not using the SIMD extensions, libjpeg-turbo uses the more accurate
305 (and slightly faster) floating point IDCT algorithm introduced in libjpeg
306 v8a as opposed to the algorithm used in libjpeg v6b. It should be noted,
307 however, that this algorithm basically brings the accuracy of the floating
308 point IDCT in line with the accuracy of the slow integer IDCT. The floating
309 point DCT/IDCT algorithms are mainly a legacy feature, and they do not
310 produce significantly more accuracy than the slow integer algorithms (to put
311 numbers on this, the typical difference in PNSR between the two algorithms
312 is less than 0.10 dB, whereas changing the quality level by 1 in the upper
313 range of the quality scale is typically more like a 1.0 dB difference.)
DRC8940e6c2014-05-11 09:46:28 +0000314-- When not using the SIMD extensions, then the accuracy of the floating point
315 DCT/IDCT can depend on the compiler and compiler settings.
316
DRCd5e964c2013-01-10 11:47:39 +0000317While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood, it is
318still using the same algorithms as libjpeg v6b, so there are several specific
319cases in which libjpeg-turbo cannot be expected to produce the same output as
320libjpeg v8:
321
322-- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8
DRC8940e6c2014-05-11 09:46:28 +0000323 implements those scaling algorithms differently than libjpeg v6b does, and
324 libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.
DRCd5e964c2013-01-10 11:47:39 +0000325
326-- When using chrominance subsampling, because libjpeg v8 implements this
327 with its DCT/IDCT scaling algorithms rather than with a separate
DRC8940e6c2014-05-11 09:46:28 +0000328 downsampling/upsampling algorithm. In our testing, the subsampled/upsampled
329 output of libjpeg v8 is less accurate than that of libjpeg v6b for this
330 reason.
DRCd5e964c2013-01-10 11:47:39 +0000331
DRCd5e964c2013-01-10 11:47:39 +0000332-- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or
333 "non-smooth") chrominance upsampling, because libjpeg v8 does not support
334 merged upsampling with scaling factors > 1.
335
336
337*******************************************************************************
338** Performance Pitfalls
DRCab4db652011-02-18 04:55:08 +0000339*******************************************************************************
340
341===============
342Restart Markers
343===============
344
345The optimized Huffman decoder in libjpeg-turbo does not handle restart markers
DRC11a122b2012-02-07 00:14:53 +0000346in a way that makes the rest of the libjpeg infrastructure happy, so it is
347necessary to use the slow Huffman decoder when decompressing a JPEG image that
348has restart markers. This can cause the decompression performance to drop by
349as much as 20%, but the performance will still be much greater than that of
350libjpeg. Many consumer packages, such as PhotoShop, use restart markers when
351generating JPEG images, so images generated by those programs will experience
352this issue.
DRCab4db652011-02-18 04:55:08 +0000353
354===============================================
355Fast Integer Forward DCT at High Quality Levels
356===============================================
357
358The algorithm used by the SIMD-accelerated quantization function cannot produce
359correct results whenever the fast integer forward DCT is used along with a JPEG
360quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization
361function in those cases. This causes performance to drop by as much as 40%.
362It is therefore strongly advised that you use the slow integer forward DCT
363whenever encoding images with a JPEG quality of 98 or higher.