4 libjpeg-turbo is a JPEG image codec that uses SIMD instructions to accelerate
5 baseline JPEG compression and decompression on x86, x86-64, Arm, PowerPC, and
6 MIPS systems, as well as progressive JPEG compression on x86, x86-64, and Arm
7 systems. On such systems, libjpeg-turbo is generally 2-6x as fast as libjpeg,
8 all else being equal. On other types of systems, libjpeg-turbo can still
9 outperform libjpeg by a significant amount, by virtue of its highly-optimized
10 Huffman coding routines. In many cases, the performance of libjpeg-turbo
11 rivals that of proprietary high-speed JPEG codecs.
13 libjpeg-turbo implements both the traditional libjpeg API as well as the less
14 powerful but more straightforward TurboJPEG API. libjpeg-turbo also features
15 colorspace extensions that allow it to compress from/decompress to 32-bit and
16 big-endian pixel buffers (RGBX, XBGR, etc.), as well as a full-featured Java
19 libjpeg-turbo was originally based on libjpeg/SIMD, an MMX-accelerated
20 derivative of libjpeg v6b developed by Miyasaka Masaru. The TigerVNC and
21 VirtualGL projects made numerous enhancements to the codec in 2009, and in
22 early 2010, libjpeg-turbo spun off into an independent project, with the goal
23 of making high-speed JPEG compression/decompression technology available to a
24 broader range of users and developers.
30 libjpeg-turbo is covered by three compatible BSD-style open source licenses.
31 Refer to [LICENSE.md](LICENSE.md) for a roll-up of license terms.
34 Building libjpeg-turbo
35 ======================
37 Refer to [BUILDING.md](BUILDING.md) for complete instructions.
43 libjpeg-turbo includes two APIs that can be used to compress and decompress
46 - **TurboJPEG API**<br>
47 This API provides an easy-to-use interface for compressing and decompressing
48 JPEG images in memory. It also provides some functionality that would not be
49 straightforward to achieve using the underlying libjpeg API, such as
50 generating planar YUV images and performing multiple simultaneous lossless
51 transforms on an image. The Java interface for libjpeg-turbo is written on
52 top of the TurboJPEG API. The TurboJPEG API is recommended for first-time
53 users of libjpeg-turbo. Refer to [tjexample.c](tjexample.c) and
54 [TJExample.java](java/TJExample.java) for examples of its usage and to
55 <http://libjpeg-turbo.org/Documentation/Documentation> for API documentation.
58 This is the de facto industry-standard API for compressing and decompressing
59 JPEG images. It is more difficult to use than the TurboJPEG API but also
60 more powerful. The libjpeg API implementation in libjpeg-turbo is both
61 API/ABI-compatible and mathematically compatible with libjpeg v6b. It can
62 also optionally be configured to be API/ABI-compatible with libjpeg v7 and v8
63 (see below.) Refer to [cjpeg.c](cjpeg.c) and [djpeg.c](djpeg.c) for examples
64 of its usage and to [libjpeg.txt](libjpeg.txt) for API documentation.
66 There is no significant performance advantage to either API when both are used
67 to perform similar operations.
72 libjpeg-turbo includes extensions that allow JPEG images to be compressed
73 directly from (and decompressed directly to) buffers that use BGR, BGRX,
74 RGBX, XBGR, and XRGB pixel ordering. This is implemented with ten new
77 JCS_EXT_RGB /* red/green/blue */
78 JCS_EXT_RGBX /* red/green/blue/x */
79 JCS_EXT_BGR /* blue/green/red */
80 JCS_EXT_BGRX /* blue/green/red/x */
81 JCS_EXT_XBGR /* x/blue/green/red */
82 JCS_EXT_XRGB /* x/red/green/blue */
83 JCS_EXT_RGBA /* red/green/blue/alpha */
84 JCS_EXT_BGRA /* blue/green/red/alpha */
85 JCS_EXT_ABGR /* alpha/blue/green/red */
86 JCS_EXT_ARGB /* alpha/red/green/blue */
88 Setting `cinfo.in_color_space` (compression) or `cinfo.out_color_space`
89 (decompression) to one of these values will cause libjpeg-turbo to read the
90 red, green, and blue values from (or write them to) the appropriate position in
91 the pixel when compressing from/decompressing to an RGB buffer.
93 Your application can check for the existence of these extensions at compile
98 At run time, attempting to use these extensions with a libjpeg implementation
99 that does not support them will result in a "Bogus input colorspace" error.
100 Applications can trap this error in order to test whether run-time support is
101 available for the colorspace extensions.
103 When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the
104 X byte is undefined, and in order to ensure the best performance, libjpeg-turbo
105 can set that byte to whatever value it wishes. If an application expects the X
106 byte to be used as an alpha channel, then it should specify `JCS_EXT_RGBA`,
107 `JCS_EXT_BGRA`, `JCS_EXT_ABGR`, or `JCS_EXT_ARGB`. When these colorspace
108 constants are used, the X byte is guaranteed to be 0xFF, which is interpreted
111 Your application can check for the existence of the alpha channel colorspace
112 extensions at compile time with:
114 #ifdef JCS_ALPHA_EXTENSIONS
116 [jcstest.c](jcstest.c), located in the libjpeg-turbo source tree, demonstrates
117 how to check for the existence of the colorspace extensions at compile time and
120 libjpeg v7 and v8 API/ABI Emulation
121 -----------------------------------
123 With libjpeg v7 and v8, new features were added that necessitated extending the
124 compression and decompression structures. Unfortunately, due to the exposed
125 nature of those structures, extending them also necessitated breaking backward
126 ABI compatibility with previous libjpeg releases. Thus, programs that were
127 built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is
128 based on the libjpeg v6b code base. Although libjpeg v7 and v8 are not
129 as widely used as v6b, enough programs (including a few Linux distros) made
130 the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs
131 in libjpeg-turbo. It should be noted, however, that this feature was added
132 primarily so that applications that had already been compiled to use libjpeg
133 v7+ could take advantage of accelerated baseline JPEG encoding/decoding
134 without recompiling. libjpeg-turbo does not claim to support all of the
135 libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all
138 By passing an argument of `-DWITH_JPEG7=1` or `-DWITH_JPEG8=1` to `cmake`, you
139 can build a version of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so
140 that programs that are built against libjpeg v7 or v8 can be run with
141 libjpeg-turbo. The following section describes which libjpeg v7+ features are
142 supported and which aren't.
144 ### Support for libjpeg v7 and v8 Features
148 - **libjpeg API: IDCT scaling extensions in decompressor**<br>
149 libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8,
150 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
151 and 1/2 are SIMD-accelerated.)
153 - **libjpeg API: Arithmetic coding**
155 - **libjpeg API: In-memory source and destination managers**<br>
158 - **cjpeg: Separate quality settings for luminance and chrominance**<br>
159 Note that the libpjeg v7+ API was extended to accommodate this feature only
160 for convenience purposes. It has always been possible to implement this
161 feature with libjpeg v6b (see rdswitch.c for an example.)
163 - **cjpeg: 32-bit BMP support**
165 - **cjpeg: `-rgb` option**
167 - **jpegtran: Lossless cropping**
169 - **jpegtran: `-perfect` option**
171 - **jpegtran: Forcing width/height when performing lossless crop**
173 - **rdjpgcom: `-raw` option**
175 - **rdjpgcom: Locale awareness**
180 NOTE: As of this writing, extensive research has been conducted into the
181 usefulness of DCT scaling as a means of data reduction and SmartScale as a
182 means of quality improvement. Readers are invited to peruse the research at
183 <http://www.libjpeg-turbo.org/About/SmartScale> and draw their own conclusions,
184 but it is the general belief of our project that these features have not
185 demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo.
187 - **libjpeg API: DCT scaling in compressor**<br>
188 `cinfo.scale_num` and `cinfo.scale_denom` are silently ignored.
189 There is no technical reason why DCT scaling could not be supported when
190 emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see
191 below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and
192 8/9 would be available, which is of limited usefulness.
194 - **libjpeg API: SmartScale**<br>
195 `cinfo.block_size` is silently ignored.
196 SmartScale is an extension to the JPEG format that allows for DCT block
197 sizes other than 8x8. Providing support for this new format would be
198 feasible (particularly without full acceleration.) However, until/unless
199 the format becomes either an official industry standard or, at minimum, an
200 accepted solution in the community, we are hesitant to implement it, as
201 there is no sense of whether or how it might change in the future. It is
202 our belief that SmartScale has not demonstrated sufficient usefulness as a
203 lossless format nor as a means of quality enhancement, and thus our primary
204 interest in providing this feature would be as a means of supporting
205 additional DCT scaling factors.
207 - **libjpeg API: Fancy downsampling in compressor**<br>
208 `cinfo.do_fancy_downsampling` is silently ignored.
209 This requires the DCT scaling feature, which is not supported.
211 - **jpegtran: Scaling**<br>
212 This requires both the DCT scaling and SmartScale features, which are not
215 - **Lossless RGB JPEG files**<br>
216 This requires the SmartScale feature, which is not supported.
218 ### What About libjpeg v9?
220 libjpeg v9 introduced yet another field to the JPEG compression structure
221 (`color_transform`), thus making the ABI backward incompatible with that of
222 libjpeg v8. This new field was introduced solely for the purpose of supporting
223 lossless SmartScale encoding. Furthermore, there was actually no reason to
224 extend the API in this manner, as the color transform could have just as easily
225 been activated by way of a new JPEG colorspace constant, thus preserving
226 backward ABI compatibility.
228 Our research (see link above) has shown that lossless SmartScale does not
229 generally accomplish anything that can't already be accomplished better with
230 existing, standard lossless formats. Therefore, at this time it is our belief
231 that there is not sufficient technical justification for software projects to
232 upgrade from libjpeg v8 to libjpeg v9, and thus there is not sufficient
233 technical justification for us to emulate the libjpeg v9 ABI.
235 In-Memory Source/Destination Managers
236 -------------------------------------
238 By default, libjpeg-turbo 1.3 and later includes the `jpeg_mem_src()` and
239 `jpeg_mem_dest()` functions, even when not emulating the libjpeg v8 API/ABI.
240 Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8
241 API/ABI emulation in order to use the in-memory source/destination managers,
242 but several projects requested that those functions be included when emulating
243 the libjpeg v6b API/ABI as well. This allows the use of those functions by
244 programs that need them, without breaking ABI compatibility for programs that
245 don't, and it allows those functions to be provided in the "official"
246 libjpeg-turbo binaries.
248 Those who are concerned about maintaining strict conformance with the libjpeg
249 v6b or v7 API can pass an argument of `-DWITH_MEM_SRCDST=0` to `cmake` prior to
250 building libjpeg-turbo. This will restore the pre-1.3 behavior, in which
251 `jpeg_mem_src()` and `jpeg_mem_dest()` are only included when emulating the
254 On Un*x systems, including the in-memory source/destination managers changes
255 the dynamic library version from 62.2.0 to 62.3.0 if using libjpeg v6b API/ABI
256 emulation and from 7.2.0 to 7.3.0 if using libjpeg v7 API/ABI emulation.
258 Note that, on most Un*x systems, the dynamic linker will not look for a
259 function in a library until that function is actually used. Thus, if a program
260 is built against libjpeg-turbo 1.3+ and uses `jpeg_mem_src()` or
261 `jpeg_mem_dest()`, that program will not fail if run against an older version
262 of libjpeg-turbo or against libjpeg v7- until the program actually tries to
263 call `jpeg_mem_src()` or `jpeg_mem_dest()`. Such is not the case on Windows.
264 If a program is built against the libjpeg-turbo 1.3+ DLL and uses
265 `jpeg_mem_src()` or `jpeg_mem_dest()`, then it must use the libjpeg-turbo 1.3+
268 Both cjpeg and djpeg have been extended to allow testing the in-memory
269 source/destination manager functions. See their respective man pages for more
273 Mathematical Compatibility
274 ==========================
276 For the most part, libjpeg-turbo should produce identical output to libjpeg
277 v6b. The one exception to this is when using the floating point DCT/IDCT, in
278 which case the outputs of libjpeg v6b and libjpeg-turbo can differ for the
281 - The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so
282 slightly more accurate than the implementation in libjpeg v6b, but not by
283 any amount perceptible to human vision (generally in the range of 0.01 to
284 0.08 dB gain in PNSR.)
286 - When not using the SIMD extensions, libjpeg-turbo uses the more accurate
287 (and slightly faster) floating point IDCT algorithm introduced in libjpeg
288 v8a as opposed to the algorithm used in libjpeg v6b. It should be noted,
289 however, that this algorithm basically brings the accuracy of the floating
290 point IDCT in line with the accuracy of the accurate integer IDCT. The
291 floating point DCT/IDCT algorithms are mainly a legacy feature, and they do
292 not produce significantly more accuracy than the accurate integer algorithms
293 (to put numbers on this, the typical difference in PNSR between the two
294 algorithms is less than 0.10 dB, whereas changing the quality level by 1 in
295 the upper range of the quality scale is typically more like a 1.0 dB
298 - If the floating point algorithms in libjpeg-turbo are not implemented using
299 SIMD instructions on a particular platform, then the accuracy of the
300 floating point DCT/IDCT can depend on the compiler settings.
302 While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood it is
303 still using the same algorithms as libjpeg v6b, so there are several specific
304 cases in which libjpeg-turbo cannot be expected to produce the same output as
307 - When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8
308 implements those scaling algorithms differently than libjpeg v6b does, and
309 libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.
311 - When using chrominance subsampling, because libjpeg v8 implements this
312 with its DCT/IDCT scaling algorithms rather than with a separate
313 downsampling/upsampling algorithm. In our testing, the subsampled/upsampled
314 output of libjpeg v8 is less accurate than that of libjpeg v6b for this
317 - When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or
318 "non-smooth") chrominance upsampling, because libjpeg v8 does not support
319 merged upsampling with scaling factors > 1.
328 The optimized Huffman decoder in libjpeg-turbo does not handle restart markers
329 in a way that makes the rest of the libjpeg infrastructure happy, so it is
330 necessary to use the slow Huffman decoder when decompressing a JPEG image that
331 has restart markers. This can cause the decompression performance to drop by
332 as much as 20%, but the performance will still be much greater than that of
333 libjpeg. Many consumer packages, such as Photoshop, use restart markers when
334 generating JPEG images, so images generated by those programs will experience
337 Fast Integer Forward DCT at High Quality Levels
338 -----------------------------------------------
340 The algorithm used by the SIMD-accelerated quantization function cannot produce
341 correct results whenever the fast integer forward DCT is used along with a JPEG
342 quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization
343 function in those cases. This causes performance to drop by as much as 40%.
344 It is therefore strongly advised that you use the accurate integer forward DCT
345 whenever encoding images with a JPEG quality of 98 or higher.
348 Memory Debugger Pitfalls
349 ========================
351 Valgrind and Memory Sanitizer (MSan) can generate false positives
352 (specifically, incorrect reports of uninitialized memory accesses) when used
353 with libjpeg-turbo's SIMD extensions. It is generally recommended that the
354 SIMD extensions be disabled, either by passing an argument of `-DWITH_SIMD=0`
355 to `cmake` when configuring the build or by setting the environment variable
356 `JSIMD_FORCENONE` to `1` at run time, when testing libjpeg-turbo with Valgrind,
357 MSan, or other memory debuggers.