263 lines
9.4 KiB
Text
263 lines
9.4 KiB
Text
BPG Image library and utilities
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-------------------------------
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1) Quick introduction
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---------------------
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- Edit the Makefile to change the compile options (the default compile
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options should be OK for Linux). Type 'make' to compile and 'make
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install' to install the compiled binaries.
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- bpgview: in order to compile it you need to install the SDL and
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SDL_image libraries.
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- Emscripten usage: in order to generate the Javascript decoder, you
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must install Emscripten and enable its use in the Makefile.
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- An HTML demonstration (with a precompiled Javascript decoder) is
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available in html/index.html (if you use Chrome and want to use
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file:// to access it, launch Chrome with the option
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--allow-file-access-from-files).
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- The BPG file format is specified in doc/bpg_spec.txt.
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2) Compilation and Installation Notes
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-------------------------------------
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2.1) Linux
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----------
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- Edit the Makefile to change the compile options (the default
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compile options should be OK). Type 'make' to compile and 'make
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install' to install the compiled binaries.
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- Use 'make -j N' where N is the number of CPU cores to compile faster.
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- The following packages must be installed: SDL-devel
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SDL_image-devel yasm. It is recommended to use yasm version >= 1.3.0
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to have a faster compilation.
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- Only a 64 bit target is supported because x265 needs it for bit
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depths > 8.
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2.2) Windows
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------------
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- Only cross-compilation from Linux is supported.
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- The following packages need to be installed: mingw64-gcc
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mingw64-libpng mingw64-libjpeg-turbo mingw64-SDL mingw64-SDL_image
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yasm. It is recommended to use yasm version >= 1.3.0 to have a
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faster compilation.
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- Only a 64 bit target is supported because x265 needs it for bit
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depths > 8.
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3) BPG encoder
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--------------
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The BPG command line encoder is 'bpgenc'. It takes JPEG or PNG images
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as input.
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- Speed: by default bpgenc uses the x265. You can compile the much
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slower but more efficient JCTVC encoder and select it with the '-e
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jctvc' option. With x265 you can select the encoding speed with the
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'-m' option (1 = fast, but larger image, 9 = slower but smaller
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image).
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- Bit depth: the default bit depth is 8. You can increase it to 10
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('-b 10' option) to slightly increase the compression ratio. For web
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publishing it is generally not a good idea because the Javascript
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decoder uses more memory. The compiled x265 encoder supports the bit
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depth of 8, 10 and 12. The slower JCTVC encoder can be compiled to
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support higher bit depths (up to 14) by enabling the Makefile
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define: USE_JCTVC_HIGH_BIT_DEPTH.
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- Lossless compression is supported as a bonus thru the HEVC lossless
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capabilities. Use a PNG input in this case unless you know what you
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do ! In case of a JPEG input, the compression is lossless related to
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the JPEG YCbCr data, not the RGB data. In any case, the bit depth
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should match the one of your picture otherwise the file size
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increases a lot. By default the lossless mode sets the bit depth to
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8 bits. The prefered color space is set to "rgb". Notes:
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- lossless mode is less tested that the lossy mode but it usually
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gives better results that PNG on photographic images.
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- the JCTVC encoder gives smaller images than the x265 encoder
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with lossless compression.
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- There is a small difference of interpretation of the quantizer
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parameter (-q option) between the x265 and JCTVC encoder.
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- Color space and chroma format:
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* For JPEG input, the color space of the input image is not
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modified (it is YCbCr, RGB, YCbCrK or CMYK). The chroma is
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subsampled according to the preferred chroma format ('-f'
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option).
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* For PNG input, the input image is converted to the preferred
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color space ('-c' option). Its chroma is then subsampled
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according to the preferred chroma format.
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* grayscale images are kept unmodified.
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- Premultiplied alpha: by default bpgenc uses non-premultiplied alpha
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to preserve the color components. However, premultiplied alpha
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('-premul' option) usually gives a better compression at the expense
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of a loss in the color components. This loss is not an issue if the
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image is not edited.
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- Animations: with the '-a' option, animations can be encoded from a
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sequence of PNG or JPEG images, indexed from 1 or 0. For example:
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./bpgenc -a anim%2d.png -fps 25 -loop 0 -o anim.bpg
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generates an animation from anim01.png, anim02.png, etc... The frame
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rate is specified with '-fps' and the number of loops with '-loop'
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(0 = infinite). If a different delay per image is needed as in some
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animated GIFs, a text file can be specified with the '-delayfile'
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option. It contains one number per image giving its duration in
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centiseconds. All durations are rounded to a multiple of '1/fps', so
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it is important to set a consistent frame rate.
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The necessary frames and delay file can be generated from animated
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GIFs with the ImageMagick tools:
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convert -coalesce anim.gif anim%d.png
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identify -format "%T\n" anim.gif > anim.txt
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In order to reduce the file size, the frame rate can be choosen so
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that most frames have a frame period of 1 (hence if anim.txt
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contains only frame durations of 5 centiseconds, then choose a frame
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rate of 20 frames/s).
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As GIFs use paletted colors and 1 bit transparency, it is always
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better to start from the source material (e.g. PNG files) to have
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the best quality.
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A BPG decoder not supporting animations only displays the first
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frame.
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- By default, bpgenc does not copy the metadata. You can copy them
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with the '-keepmetadata' option. For JPEG input, EXIF, ICCP and XMP
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are copied. For PNG input, ICCP is copied.
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- Objective comparisons: x265 is tuned by default for SSIM. the JCTVC
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encoder is tuned for PSNR only, not for SSIM, so you should use PSNR
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when making objective comparison with other formats.
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4) BPG decoder
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--------------
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The BPG command line decoder is bpgdec. It outputs a PNG or PPM
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image. Use a PPM output to get the fastest speed.
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- With the '-i' option, you have information about the BPG image (and
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no decoded image is output).
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- The '-b' option selects the bit depth (8 or 16) of the PNG
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output. It is independent of the internal BPG bit depth.
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5) BPG viewer
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-------------
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The BPG image viewer uses the SDL library to display BPG images and
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other image formats supported by the SDL_image library. The available
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keys are displayed by launching bpgview without parameters. bpgview
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supports BPG animations.
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6) BPG decoding library
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-----------------------
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BPG images can be decoded in any program with the libbpg
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library.
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The API is not considered stable yet so that the library is only
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provided as a static one.
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Currently there is no similar library for encoding so you should
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invoke the bpgenc utility.
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7) Javascript decoder
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---------------------
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The following Javascript decoders are available, sorted by increasing size:
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> 8 bits animations
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bpgdec8.js no no
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bpgdec.js yes no
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bpgdec8a.js no yes
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The 8 bit only decoders are a little faster and consumes less memory
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(16 MB instead of 32 MB by default, you can change the memory
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configuration in the Makefile if you want to handle larger images).
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The Javascript decoder substitutes all the <img> tags with a source
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having a .bpg extension with a <canvas> tag and decodes the BPG image
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into it. Stylesheets are supported (the 'id' and 'class' attributes
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are preserved). The 'width' and 'height' attributes are supported only
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with pixel units.
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The image data is downloaded with the XMLHttpRequest object. So the
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BPG images and the BPG Javascript decoder must be in the same domain
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unless Cross-Origin Resource Sharing is used.
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When animations are displayed, all the frames are stored in memory, so
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animations with a large number of frames and large resolutions should
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be avoided, as with animated GIFs.
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asm.js gives an interesting speed boost, so we hope that more browsers
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will support this Javascript subset.
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8) FFmpeg modifications
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-----------------------
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- Completed support of chroma_format_idc = 0 (monochrome mode).
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- Fixed RDPCM support (intra predictions).
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- Reduced memory usage for the SAO loop filter.
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- Generated the IDCT coefficients dynamically to reduce the code size.
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- Added a 'dynamic bit depth' mode where all the bit depths from 8 to
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14 are supported without code duplication but slower decoding.
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- Added a modified SPS header to reduce the size of the BPG decoder
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(an alternate solution is to generate standard VPS and SPS headers
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from the BPG header).
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- Added defines to keep only the HEVC intra code and suppress the
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parsing of all the irrelevant NAL units.
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- Stripped FFmpeg from all codecs except HEVC and the necessary
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support code.
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9) x265 modifications
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---------------------
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- Support of monochrome format (some parts not used by BPG may be
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missing).
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- Support of static build.
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10) Licensing
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-------------
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- libbpg and bpgdec are released under the LGPL license (the FFmpeg
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part is under the LGPL, the BPG specific part is released under the
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BSD license).
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- bpgenc is released under the GPL version 2 license. The BPG specific
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code is released under the BSD license. The JCTVC code is released
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under the BSD license. The x265 code is released under the GPL
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version 2 license.
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- BPG relies on the HEVC compression technology which may be protected
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by patents in some countries. Most devices already include or will
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include hardware HEVC support, so we suggest to use it if patents
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are an issue.
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