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347 строки
16 KiB
Plaintext
Background
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==========
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libjpeg-turbo is a JPEG image codec that uses SIMD instructions (MMX, SSE2,
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AVX2, NEON, AltiVec) to accelerate baseline JPEG compression and decompression
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on x86, x86-64, ARM, and PowerPC systems, as well as progressive JPEG
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compression on x86 and x86-64 systems. On such systems, libjpeg-turbo is
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generally 2-6x as fast as libjpeg, all else being equal. On other types of
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systems, libjpeg-turbo can still outperform libjpeg by a significant amount, by
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virtue of its highly-optimized Huffman coding routines. In many cases, the
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performance of libjpeg-turbo rivals that of proprietary high-speed JPEG codecs.
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libjpeg-turbo implements both the traditional libjpeg API as well as the less
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powerful but more straightforward TurboJPEG API. libjpeg-turbo also features
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colorspace extensions that allow it to compress from/decompress to 32-bit and
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big-endian pixel buffers (RGBX, XBGR, etc.), as well as a full-featured Java
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interface.
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libjpeg-turbo was originally based on libjpeg/SIMD, an MMX-accelerated
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derivative of libjpeg v6b developed by Miyasaka Masaru. The TigerVNC and
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VirtualGL projects made numerous enhancements to the codec in 2009, and in
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early 2010, libjpeg-turbo spun off into an independent project, with the goal
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of making high-speed JPEG compression/decompression technology available to a
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broader range of users and developers.
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License
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=======
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libjpeg-turbo is covered by three compatible BSD-style open source licenses.
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Refer to [LICENSE.md](LICENSE.md) for a roll-up of license terms.
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Building libjpeg-turbo
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======================
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Refer to [BUILDING.md](BUILDING.md) for complete instructions.
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Using libjpeg-turbo
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===================
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libjpeg-turbo includes two APIs that can be used to compress and decompress
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JPEG images:
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- **TurboJPEG API**<br>
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This API provides an easy-to-use interface for compressing and decompressing
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JPEG images in memory. It also provides some functionality that would not be
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straightforward to achieve using the underlying libjpeg API, such as
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generating planar YUV images and performing multiple simultaneous lossless
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transforms on an image. The Java interface for libjpeg-turbo is written on
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top of the TurboJPEG API. The TurboJPEG API is recommended for first-time
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users of libjpeg-turbo. Refer to [tjexample.c](tjexample.c) and
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[TJExample.java](java/TJExample.java) for examples of its usage and to
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<http://libjpeg-turbo.org/Documentation/Documentation> for API documentation.
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- **libjpeg API**<br>
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This is the de facto industry-standard API for compressing and decompressing
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JPEG images. It is more difficult to use than the TurboJPEG API but also
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more powerful. The libjpeg API implementation in libjpeg-turbo is both
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API/ABI-compatible and mathematically compatible with libjpeg v6b. It can
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also optionally be configured to be API/ABI-compatible with libjpeg v7 and v8
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(see below.) Refer to [cjpeg.c](cjpeg.c) and [djpeg.c](djpeg.c) for examples
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of its usage and to [libjpeg.txt](libjpeg.txt) for API documentation.
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There is no significant performance advantage to either API when both are used
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to perform similar operations.
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Colorspace Extensions
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---------------------
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libjpeg-turbo includes extensions that allow JPEG images to be compressed
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directly from (and decompressed directly to) buffers that use BGR, BGRX,
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RGBX, XBGR, and XRGB pixel ordering. This is implemented with ten new
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colorspace constants:
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JCS_EXT_RGB /* red/green/blue */
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JCS_EXT_RGBX /* red/green/blue/x */
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JCS_EXT_BGR /* blue/green/red */
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JCS_EXT_BGRX /* blue/green/red/x */
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JCS_EXT_XBGR /* x/blue/green/red */
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JCS_EXT_XRGB /* x/red/green/blue */
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JCS_EXT_RGBA /* red/green/blue/alpha */
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JCS_EXT_BGRA /* blue/green/red/alpha */
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JCS_EXT_ABGR /* alpha/blue/green/red */
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JCS_EXT_ARGB /* alpha/red/green/blue */
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Setting `cinfo.in_color_space` (compression) or `cinfo.out_color_space`
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(decompression) to one of these values will cause libjpeg-turbo to read the
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red, green, and blue values from (or write them to) the appropriate position in
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the pixel when compressing from/decompressing to an RGB buffer.
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Your application can check for the existence of these extensions at compile
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time with:
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#ifdef JCS_EXTENSIONS
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At run time, attempting to use these extensions with a libjpeg implementation
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that does not support them will result in a "Bogus input colorspace" error.
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Applications can trap this error in order to test whether run-time support is
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available for the colorspace extensions.
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When using the RGBX, BGRX, XBGR, and XRGB colorspaces during decompression, the
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X byte is undefined, and in order to ensure the best performance, libjpeg-turbo
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can set that byte to whatever value it wishes. If an application expects the X
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byte to be used as an alpha channel, then it should specify `JCS_EXT_RGBA`,
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`JCS_EXT_BGRA`, `JCS_EXT_ABGR`, or `JCS_EXT_ARGB`. When these colorspace
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constants are used, the X byte is guaranteed to be 0xFF, which is interpreted
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as opaque.
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Your application can check for the existence of the alpha channel colorspace
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extensions at compile time with:
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#ifdef JCS_ALPHA_EXTENSIONS
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[jcstest.c](jcstest.c), located in the libjpeg-turbo source tree, demonstrates
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how to check for the existence of the colorspace extensions at compile time and
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run time.
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libjpeg v7 and v8 API/ABI Emulation
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-----------------------------------
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With libjpeg v7 and v8, new features were added that necessitated extending the
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compression and decompression structures. Unfortunately, due to the exposed
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nature of those structures, extending them also necessitated breaking backward
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ABI compatibility with previous libjpeg releases. Thus, programs that were
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built to use libjpeg v7 or v8 did not work with libjpeg-turbo, since it is
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based on the libjpeg v6b code base. Although libjpeg v7 and v8 are not
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as widely used as v6b, enough programs (including a few Linux distros) made
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the switch that there was a demand to emulate the libjpeg v7 and v8 ABIs
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in libjpeg-turbo. It should be noted, however, that this feature was added
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primarily so that applications that had already been compiled to use libjpeg
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v7+ could take advantage of accelerated baseline JPEG encoding/decoding
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without recompiling. libjpeg-turbo does not claim to support all of the
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libjpeg v7+ features, nor to produce identical output to libjpeg v7+ in all
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cases (see below.)
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By passing an argument of `--with-jpeg7` or `--with-jpeg8` to `configure`, or
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an argument of `-DWITH_JPEG7=1` or `-DWITH_JPEG8=1` to `cmake`, you can build a
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version of libjpeg-turbo that emulates the libjpeg v7 or v8 ABI, so that
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programs that are built against libjpeg v7 or v8 can be run with libjpeg-turbo.
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The following section describes which libjpeg v7+ features are supported and
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which aren't.
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### Support for libjpeg v7 and v8 Features
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#### Fully supported
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- **libjpeg: IDCT scaling extensions in decompressor**<br>
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libjpeg-turbo supports IDCT scaling with scaling factors of 1/8, 1/4, 3/8,
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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
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and 1/2 are SIMD-accelerated.)
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- **libjpeg: Arithmetic coding**
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- **libjpeg: In-memory source and destination managers**<br>
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See notes below.
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- **cjpeg: Separate quality settings for luminance and chrominance**<br>
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Note that the libpjeg v7+ API was extended to accommodate this feature only
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for convenience purposes. It has always been possible to implement this
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feature with libjpeg v6b (see rdswitch.c for an example.)
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- **cjpeg: 32-bit BMP support**
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- **cjpeg: `-rgb` option**
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- **jpegtran: Lossless cropping**
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- **jpegtran: `-perfect` option**
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- **jpegtran: Forcing width/height when performing lossless crop**
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- **rdjpgcom: `-raw` option**
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- **rdjpgcom: Locale awareness**
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#### Not supported
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NOTE: As of this writing, extensive research has been conducted into the
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usefulness of DCT scaling as a means of data reduction and SmartScale as a
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means of quality improvement. The reader is invited to peruse the research at
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<http://www.libjpeg-turbo.org/About/SmartScale> and draw his/her own conclusions,
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but it is the general belief of our project that these features have not
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demonstrated sufficient usefulness to justify inclusion in libjpeg-turbo.
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- **libjpeg: DCT scaling in compressor**<br>
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`cinfo.scale_num` and `cinfo.scale_denom` are silently ignored.
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There is no technical reason why DCT scaling could not be supported when
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emulating the libjpeg v7+ API/ABI, but without the SmartScale extension (see
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below), only scaling factors of 1/2, 8/15, 4/7, 8/13, 2/3, 8/11, 4/5, and
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8/9 would be available, which is of limited usefulness.
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- **libjpeg: SmartScale**<br>
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`cinfo.block_size` is silently ignored.
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SmartScale is an extension to the JPEG format that allows for DCT block
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sizes other than 8x8. Providing support for this new format would be
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feasible (particularly without full acceleration.) However, until/unless
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the format becomes either an official industry standard or, at minimum, an
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accepted solution in the community, we are hesitant to implement it, as
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there is no sense of whether or how it might change in the future. It is
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our belief that SmartScale has not demonstrated sufficient usefulness as a
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lossless format nor as a means of quality enhancement, and thus our primary
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interest in providing this feature would be as a means of supporting
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additional DCT scaling factors.
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- **libjpeg: Fancy downsampling in compressor**<br>
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`cinfo.do_fancy_downsampling` is silently ignored.
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This requires the DCT scaling feature, which is not supported.
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- **jpegtran: Scaling**<br>
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This requires both the DCT scaling and SmartScale features, which are not
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supported.
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- **Lossless RGB JPEG files**<br>
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This requires the SmartScale feature, which is not supported.
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### What About libjpeg v9?
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libjpeg v9 introduced yet another field to the JPEG compression structure
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(`color_transform`), thus making the ABI backward incompatible with that of
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libjpeg v8. This new field was introduced solely for the purpose of supporting
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lossless SmartScale encoding. Furthermore, there was actually no reason to
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extend the API in this manner, as the color transform could have just as easily
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been activated by way of a new JPEG colorspace constant, thus preserving
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backward ABI compatibility.
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Our research (see link above) has shown that lossless SmartScale does not
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generally accomplish anything that can't already be accomplished better with
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existing, standard lossless formats. Therefore, at this time it is our belief
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that there is not sufficient technical justification for software projects to
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upgrade from libjpeg v8 to libjpeg v9, and thus there is not sufficient
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technical justification for us to emulate the libjpeg v9 ABI.
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In-Memory Source/Destination Managers
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-------------------------------------
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By default, libjpeg-turbo 1.3 and later includes the `jpeg_mem_src()` and
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`jpeg_mem_dest()` functions, even when not emulating the libjpeg v8 API/ABI.
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Previously, it was necessary to build libjpeg-turbo from source with libjpeg v8
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API/ABI emulation in order to use the in-memory source/destination managers,
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but several projects requested that those functions be included when emulating
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the libjpeg v6b API/ABI as well. This allows the use of those functions by
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programs that need them, without breaking ABI compatibility for programs that
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don't, and it allows those functions to be provided in the "official"
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libjpeg-turbo binaries.
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Those who are concerned about maintaining strict conformance with the libjpeg
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v6b or v7 API can pass an argument of `--without-mem-srcdst` to `configure` or
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an argument of `-DWITH_MEM_SRCDST=0` to `cmake` prior to building
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libjpeg-turbo. This will restore the pre-1.3 behavior, in which
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`jpeg_mem_src()` and `jpeg_mem_dest()` are only included when emulating the
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libjpeg v8 API/ABI.
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On Un*x systems, including the in-memory source/destination managers changes
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the dynamic library version from 62.1.0 to 62.2.0 if using libjpeg v6b API/ABI
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emulation and from 7.1.0 to 7.2.0 if using libjpeg v7 API/ABI emulation.
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Note that, on most Un*x systems, the dynamic linker will not look for a
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function in a library until that function is actually used. Thus, if a program
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is built against libjpeg-turbo 1.3+ and uses `jpeg_mem_src()` or
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`jpeg_mem_dest()`, that program will not fail if run against an older version
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of libjpeg-turbo or against libjpeg v7- until the program actually tries to
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call `jpeg_mem_src()` or `jpeg_mem_dest()`. Such is not the case on Windows.
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If a program is built against the libjpeg-turbo 1.3+ DLL and uses
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`jpeg_mem_src()` or `jpeg_mem_dest()`, then it must use the libjpeg-turbo 1.3+
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DLL at run time.
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Both cjpeg and djpeg have been extended to allow testing the in-memory
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source/destination manager functions. See their respective man pages for more
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details.
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Mathematical Compatibility
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==========================
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For the most part, libjpeg-turbo should produce identical output to libjpeg
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v6b. The one exception to this is when using the floating point DCT/IDCT, in
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which case the outputs of libjpeg v6b and libjpeg-turbo can differ for the
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following reasons:
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- The SSE/SSE2 floating point DCT implementation in libjpeg-turbo is ever so
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slightly more accurate than the implementation in libjpeg v6b, but not by
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any amount perceptible to human vision (generally in the range of 0.01 to
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0.08 dB gain in PNSR.)
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- When not using the SIMD extensions, libjpeg-turbo uses the more accurate
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(and slightly faster) floating point IDCT algorithm introduced in libjpeg
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v8a as opposed to the algorithm used in libjpeg v6b. It should be noted,
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however, that this algorithm basically brings the accuracy of the floating
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point IDCT in line with the accuracy of the slow integer IDCT. The floating
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point DCT/IDCT algorithms are mainly a legacy feature, and they do not
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produce significantly more accuracy than the slow integer algorithms (to put
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numbers on this, the typical difference in PNSR between the two algorithms
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is less than 0.10 dB, whereas changing the quality level by 1 in the upper
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range of the quality scale is typically more like a 1.0 dB difference.)
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- If the floating point algorithms in libjpeg-turbo are not implemented using
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SIMD instructions on a particular platform, then the accuracy of the
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floating point DCT/IDCT can depend on the compiler settings.
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While libjpeg-turbo does emulate the libjpeg v8 API/ABI, under the hood it is
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still using the same algorithms as libjpeg v6b, so there are several specific
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cases in which libjpeg-turbo cannot be expected to produce the same output as
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libjpeg v8:
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- When decompressing using scaling factors of 1/2 and 1/4, because libjpeg v8
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implements those scaling algorithms differently than libjpeg v6b does, and
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libjpeg-turbo's SIMD extensions are based on the libjpeg v6b behavior.
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- When using chrominance subsampling, because libjpeg v8 implements this
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with its DCT/IDCT scaling algorithms rather than with a separate
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downsampling/upsampling algorithm. In our testing, the subsampled/upsampled
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output of libjpeg v8 is less accurate than that of libjpeg v6b for this
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reason.
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- When decompressing using a scaling factor > 1 and merged (AKA "non-fancy" or
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"non-smooth") chrominance upsampling, because libjpeg v8 does not support
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merged upsampling with scaling factors > 1.
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Performance Pitfalls
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====================
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Restart Markers
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---------------
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The optimized Huffman decoder in libjpeg-turbo does not handle restart markers
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in a way that makes the rest of the libjpeg infrastructure happy, so it is
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necessary to use the slow Huffman decoder when decompressing a JPEG image that
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has restart markers. This can cause the decompression performance to drop by
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as much as 20%, but the performance will still be much greater than that of
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libjpeg. Many consumer packages, such as PhotoShop, use restart markers when
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generating JPEG images, so images generated by those programs will experience
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this issue.
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Fast Integer Forward DCT at High Quality Levels
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-----------------------------------------------
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The algorithm used by the SIMD-accelerated quantization function cannot produce
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correct results whenever the fast integer forward DCT is used along with a JPEG
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quality of 98-100. Thus, libjpeg-turbo must use the non-SIMD quantization
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function in those cases. This causes performance to drop by as much as 40%.
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It is therefore strongly advised that you use the slow integer forward DCT
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whenever encoding images with a JPEG quality of 98 or higher.
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