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libbpg-0.9.3
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README
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README
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BPG Image Encoder and Decoder
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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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- x265 usage: for much increased compression speed (but lower
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quality), you can compile and install x265 and then enable its use
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in the Makefile. x265 supports by default only 8 bits per component
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and does not support monochrome encoding yet (hence no alpha nor
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grayscale images can be encoded with it).
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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) 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 JCTVC encoder which has a high
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quality but is slow. If you compiled with x265, you can have a much
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faster encoding with the '-e x265' option. With x265 you can also
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select the encoding speed with the '-m' option (1 = fast, but larger
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image, 9 = slower but smaller image). Warning: x265 does not support
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monochrome (and alpha) yet, so you must use the JCTVC encoder for
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these cases.
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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.
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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 difference of interpretation of the quantizer parameter
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(-q option) between the x265 and JCTVC encoder. The default value is
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optimized for the JCTVC encoder, not for x265. We will try to align
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the x265 value to JCTVC in the future.
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- By default, the JCTVC encoder is limited to a precision of 12
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bits. You can enable high bit depths (up to 14) by enabling the
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Makefile define: USE_JCTVC_HIGH_BIT_DEPTH. The encoder is sligthly
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slower in this case.
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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). Images with vertically subsampled chroma are currently
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not supported.
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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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- 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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3) 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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4) 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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5) Javascript decoder
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---------------------
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bpgdec.js is a Javascript decoder supporting the BPG file
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format. bpgdec8b.js is a specialized version limited to BPG images
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using 8 bits per component. It is a little faster and consumes less
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memory (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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asm.js gives an interesting speed boost, so we hope that more browser
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will support this Javascript subset.
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6) 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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(the solution instead 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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7) Licensing
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------------
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- libbpg and bpgenc 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 BSD license (it includes the JCTVC code
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which is released under the BSD license. The BPG specific part is
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released under the BSD 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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