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341 lines
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341 lines
16 KiB
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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NEON, AltiVec) to accelerate baseline JPEG compression and decompression on |
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x86, x86-64, ARM, and PowerPC 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. |
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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.) |
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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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