diff options
Diffstat (limited to 'media/libjpeg/README.md')
| -rw-r--r--[-rwxr-xr-x] | media/libjpeg/README.md | 100 |
1 files changed, 58 insertions, 42 deletions
diff --git a/media/libjpeg/README.md b/media/libjpeg/README.md index ca8866e06c..01e391ea7c 100755..100644 --- a/media/libjpeg/README.md +++ b/media/libjpeg/README.md @@ -1,13 +1,14 @@ Background ========== -libjpeg-turbo is a JPEG image codec that uses SIMD instructions (MMX, SSE2, -NEON, AltiVec) to accelerate baseline JPEG compression and decompression on -x86, x86-64, ARM, and PowerPC systems. On such systems, libjpeg-turbo is -generally 2-6x as fast as libjpeg, all else being equal. On other types of -systems, libjpeg-turbo can still outperform libjpeg by a significant amount, by -virtue of its highly-optimized Huffman coding routines. In many cases, the -performance of libjpeg-turbo rivals that of proprietary high-speed JPEG codecs. +libjpeg-turbo is a JPEG image codec that uses SIMD instructions to accelerate +baseline JPEG compression and decompression on x86, x86-64, Arm, PowerPC, and +MIPS systems, as well as progressive JPEG compression on x86, x86-64, and Arm +systems. On such systems, libjpeg-turbo is generally 2-6x as fast as libjpeg, +all else being equal. On other types of systems, libjpeg-turbo can still +outperform libjpeg by a significant amount, by virtue of its highly-optimized +Huffman coding routines. In many cases, the performance of libjpeg-turbo +rivals that of proprietary high-speed JPEG codecs. libjpeg-turbo implements both the traditional libjpeg API as well as the less powerful but more straightforward TurboJPEG API. libjpeg-turbo also features @@ -42,21 +43,25 @@ Using libjpeg-turbo libjpeg-turbo includes two APIs that can be used to compress and decompress JPEG images: -- **TurboJPEG API** +- **TurboJPEG API**<br> This API provides an easy-to-use interface for compressing and decompressing JPEG images in memory. It also provides some functionality that would not be straightforward to achieve using the underlying libjpeg API, such as generating planar YUV images and performing multiple simultaneous lossless transforms on an image. The Java interface for libjpeg-turbo is written on - top of the TurboJPEG API. + top of the TurboJPEG API. The TurboJPEG API is recommended for first-time + users of libjpeg-turbo. Refer to [tjexample.c](tjexample.c) and + [TJExample.java](java/TJExample.java) for examples of its usage and to + <http://libjpeg-turbo.org/Documentation/Documentation> for API documentation. -- **libjpeg API** +- **libjpeg API**<br> This is the de facto industry-standard API for compressing and decompressing JPEG images. It is more difficult to use than the TurboJPEG API but also more powerful. The libjpeg API implementation in libjpeg-turbo is both API/ABI-compatible and mathematically compatible with libjpeg v6b. It can also optionally be configured to be API/ABI-compatible with libjpeg v7 and v8 - (see below.) + (see below.) Refer to [cjpeg.c](cjpeg.c) and [djpeg.c](djpeg.c) for examples + of its usage and to [libjpeg.txt](libjpeg.txt) for API documentation. There is no significant performance advantage to either API when both are used to perform similar operations. @@ -130,28 +135,27 @@ without recompiling. libjpeg-turbo does not claim to support all of the libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all cases (see below.) -By passing an argument of `--with-jpeg7` or `--with-jpeg8` to `configure`, or -an argument of `-DWITH_JPEG7=1` or `-DWITH_JPEG8=1` to `cmake`, you can build a -version of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that -programs that are built against libjpeg v7 or v8 can be run with libjpeg-turbo. -The following section describes which libjpeg v7+ features are supported and -which aren't. +By passing an argument of `-DWITH_JPEG7=1` or `-DWITH_JPEG8=1` to `cmake`, you +can build a version of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so +that programs that are built against libjpeg v7 or v8 can be run with +libjpeg-turbo. The following section describes which libjpeg v7+ features are +supported and which aren't. ### Support for libjpeg v7 and v8 Features #### Fully supported -- **libjpeg: IDCT scaling extensions in decompressor** +- **libjpeg API: IDCT scaling extensions in decompressor**<br> libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 7/8, 9/8, 5/4, 11/8, 3/2, 13/8, 7/4, 15/8, and 2/1 (only 1/4 and 1/2 are SIMD-accelerated.) -- **libjpeg: Arithmetic coding** +- **libjpeg API: Arithmetic coding** -- **libjpeg: In-memory source and destination managers** +- **libjpeg API: In-memory source and destination managers**<br> See notes below. -- **cjpeg: Separate quality settings for luminance and chrominance** +- **cjpeg: Separate quality settings for luminance and chrominance**<br> Note that the libpjeg v7+ API was extended to accommodate this feature only for convenience purposes. It has always been possible to implement this feature with libjpeg v6b (see rdswitch.c for an example.) @@ -175,19 +179,19 @@ which aren't. NOTE: As of this writing, extensive research has been conducted into the usefulness of DCT scaling as a means of data reduction and SmartScale as a -means of quality improvement. The reader is invited to peruse the research at -<http://www.libjpeg-turbo.org/About/SmartScale> and draw his/her own conclusions, +means of quality improvement. Readers are invited to peruse the research at +<http://www.libjpeg-turbo.org/About/SmartScale> and draw their own conclusions, but it is the general belief of our project that these features have not demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo. -- **libjpeg: DCT scaling in compressor** +- **libjpeg API: DCT scaling in compressor**<br> `cinfo.scale_num` and `cinfo.scale_denom` are silently ignored. There is no technical reason why DCT scaling could not be supported when emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and 8/9 would be available, which is of limited usefulness. -- **libjpeg: SmartScale** +- **libjpeg API: SmartScale**<br> `cinfo.block_size` is silently ignored. SmartScale is an extension to the JPEG format that allows for DCT block sizes other than 8x8. Providing support for this new format would be @@ -200,15 +204,15 @@ demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo. interest in providing this feature would be as a means of supporting additional DCT scaling factors. -- **libjpeg: Fancy downsampling in compressor** +- **libjpeg API: Fancy downsampling in compressor**<br> `cinfo.do_fancy_downsampling` is silently ignored. This requires the DCT scaling feature, which is not supported. -- **jpegtran: Scaling** +- **jpegtran: Scaling**<br> This requires both the DCT scaling and SmartScale features, which are not supported. -- **Lossless RGB JPEG files** +- **Lossless RGB JPEG files**<br> This requires the SmartScale feature, which is not supported. ### What About libjpeg v9? @@ -226,7 +230,7 @@ generally accomplish anything that can't already be accomplished better with existing, standard lossless formats. Therefore, at this time it is our belief that there is not sufficient technical justification for software projects to upgrade from libjpeg v8 to libjpeg v9, and thus there is not sufficient -echnical justification for us to emulate the libjpeg v9 ABI. +technical justification for us to emulate the libjpeg v9 ABI. In-Memory Source/Destination Managers ------------------------------------- @@ -242,15 +246,14 @@ don't, and it allows those functions to be provided in the "official" libjpeg-turbo binaries. Those who are concerned about maintaining strict conformance with the libjpeg -v6b or v7 API can pass an argument of `--without-mem-srcdst` to `configure` or -an argument of `-DWITH_MEM_SRCDST=0` to `cmake` prior to building -libjpeg-turbo. This will restore the pre-1.3 behavior, in which +v6b or v7 API can pass an argument of `-DWITH_MEM_SRCDST=0` to `cmake` prior to +building libjpeg-turbo. This will restore the pre-1.3 behavior, in which `jpeg_mem_src()` and `jpeg_mem_dest()` are only included when emulating the libjpeg v8 API/ABI. On Un*x systems, including the in-memory source/destination managers changes -the dynamic library version from 62.0.0 to 62.1.0 if using libjpeg v6b API/ABI -emulation and from 7.0.0 to 7.1.0 if using libjpeg v7 API/ABI emulation. +the dynamic library version from 62.2.0 to 62.3.0 if using libjpeg v6b API/ABI +emulation and from 7.2.0 to 7.3.0 if using libjpeg v7 API/ABI emulation. Note that, on most Un*x systems, the dynamic linker will not look for a function in a library until that function is actually used. Thus, if a program @@ -284,12 +287,13 @@ following reasons: (and slightly faster) floating point IDCT algorithm introduced in libjpeg v8a as opposed to the algorithm used in libjpeg v6b. It should be noted, however, that this algorithm basically brings the accuracy of the floating - point IDCT in line with the accuracy of the slow integer IDCT. The floating - point DCT/IDCT algorithms are mainly a legacy feature, and they do not - produce significantly more accuracy than the slow integer algorithms (to put - numbers on this, the typical difference in PNSR between the two algorithms - is less than 0.10 dB, whereas changing the quality level by 1 in the upper - range of the quality scale is typically more like a 1.0 dB difference.) + point IDCT in line with the accuracy of the accurate integer IDCT. The + floating point DCT/IDCT algorithms are mainly a legacy feature, and they do + not produce significantly more accuracy than the accurate integer algorithms + (to put numbers on this, the typical difference in PNSR between the two + algorithms is less than 0.10 dB, whereas changing the quality level by 1 in + the upper range of the quality scale is typically more like a 1.0 dB + difference.) - If the floating point algorithms in libjpeg-turbo are not implemented using SIMD instructions on a particular platform, then the accuracy of the @@ -326,7 +330,7 @@ in a way that makes the rest of the libjpeg infrastructure happy, so it is necessary to use the slow Huffman decoder when decompressing a JPEG image that has restart markers. This can cause the decompression performance to drop by as much as 20%, but the performance will still be much greater than that of -libjpeg. Many consumer packages, such as PhotoShop, use restart markers when +libjpeg. Many consumer packages, such as Photoshop, use restart markers when generating JPEG images, so images generated by those programs will experience this issue. @@ -337,5 +341,17 @@ The algorithm used by the SIMD-accelerated quantization function cannot produce correct results whenever the fast integer forward DCT is used along with a JPEG quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization function in those cases. This causes performance to drop by as much as 40%. -It is therefore strongly advised that you use the slow integer forward DCT +It is therefore strongly advised that you use the accurate integer forward DCT whenever encoding images with a JPEG quality of 98 or higher. + + +Memory Debugger Pitfalls +======================== + +Valgrind and Memory Sanitizer (MSan) can generate false positives +(specifically, incorrect reports of uninitialized memory accesses) when used +with libjpeg-turbo's SIMD extensions. It is generally recommended that the +SIMD extensions be disabled, either by passing an argument of `-DWITH_SIMD=0` +to `cmake` when configuring the build or by setting the environment variable +`JSIMD_FORCENONE` to `1` at run time, when testing libjpeg-turbo with Valgrind, +MSan, or other memory debuggers. |