bgp-image-format

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bgp-image-format

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BPG (Better Portable Graphics) is a new image format based on HEVC. Developed by Fabrice Bellard. http://bellard.org/bpg/

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BPG Image Encoder and Decoder
-----------------------------

BPG is a library developed by [Fabrice Bellard](http://bellard.org).
The official website of the project is http://bellard.org/bpg

1) Quick introduction
---------------------

- Edit the Makefile to change the compile options (the default compile
  options should be OK for Linux). Type 'make' to compile and 'make
  install' to install the compiled binaries.

- x265 usage: for much increased compression speed (but lower
  quality), you can compile and install x265 and then enable its use
  in the Makefile. x265 is used when the compression level ('-m'
  option of bpgenc) is < 9. Unfortunately, x265 does not support yet
  the monochrome format or the lossless mode, so the JCTVC code is
  still required in these cases. With a default x265 compilation, only
  8 bit depth is supported (hence you must force a bit depth of 8 with
  '-b 8').

- Emscripten usage: in order to generate the Javascript decoder, you
  must install Emscripten and enable its use in the Makefile.

- An HTML demonstration (with a precompiled Javascript decoder) is
  available in html/index.html. The Javascript decoder has a hardcoded
  maximum memory usage of 32MB, which is enough for 1920x1080
  pictures. You can modify it in the Makefile to optimize it for your
  needs.

- The BPG file format is specified in doc/bpg_spec.txt.

2) BPG encoding tips
--------------------

- bpgenc is slow because it uses by default the reference HEVC
  encoder. It can also use x265 but currently x265 does not support
  all the features (in particular the monochrome mode) and gives a
  lower quality. It is expected that x265 (or another HEVC encoder)
  will become quickly better.

- A bit depth of 10 is used by default in order to slightly increase
  the compression ratio (rounding errors are reduced). You can still
  force the wanted bit depth with the '-b' option.

- Lossless compression is supported as a bonus thru the HEVC lossless
  capabilities. Use a PNG input in this case unless you know what you
  do ! In case of a JPEG input, the compression is lossless related to
  the JPEG YCbCr data, not the RGB data. In any case, the bit depth
  should match the one of your picture otherwise the file size
  increases a lot. By default the lossless mode sets the bit depth to
  8 bits. The prefered color space is set to "rgb". Note: lossless
  mode is less tested that the lossy mode but it usually gives better
  results that PNG on photographic images.

- There is a difference of interpretation of the quantizer parameter
  (-q option) between the x265 and JCTVC encoder. The default value is
  optimized for the JCTVC encoder, not for x265. We will try to align
  the x265 value to JCTVC in the future.

- By default, the JCTVC encoder is limited to a precision of 12
  bits. You can enable high bit depths (up to 14) by enabling the
  Makefile define: USE_JCTVC_HIGH_BIT_DEPTH. The encoder is sligthly
  slower in this case.

- Color space and chroma format:

* For JPEG input, the color space of the input image is not
      modified (it is YCbCr, RGB, YCbCrK or CMYK). The chroma is
      subsampled according to the preferred chroma format ('-f'
      option). Images with vertically subsampled chroma are currently
      not supported.

* For PNG input, grayscale images are not modified. For RGB
      images, the input image is converted to the preferred color
      space ('-c' option). Its chroma is then subsampled according to
      the preferred chroma format.

* grayscale images are kept unmodified.

- By default, bpgenc does not copy the metadata. You can copy them
  with the '-keepmetadata' option. For JPEG input, EXIF, ICCP and XMP
  are copied. For PNG input, ICCP is copied.

3) FFmpeg modifications
-----------------------

- Completed support of chroma_format_idc = 0 (monochrome mode).

- Fixed RDPCM support (intra predictions).

- Added a 'dynamic bit depth' mode where all the bit depths from 8 to
  14 are supported without code duplication but slower decoding.

- Added a modified SPS header to reduce the size of the BPG decoder
  (the solution instead is to generate standard VPS and SPS headers
  from the BPG header).

- Added defines to keep only the HEVC intra code and suppress the
  parsing of all the irrelevant NAL units.

- Stripped FFmpeg from all codecs except HEVC and the necessary
  support code.

- Generated the IDCT coefficients dynamically to reduce the code size.

4) Licensing
------------

- libbpg and bpgenc are released under the LGPL license (the FFmpeg
    part is under the LGPL, the BPG specific part is released under
    the BSD license).

- bpgenc is released under the BSD license (it includes the JCTVC
    code which is released under the BSD license. The BPG specific
    part is released under the BSD license).

- BPG relies on the HEVC compression technology which may be
    protected by patents in some countries. Most devices already
    include or will include hardware HEVC support, so we suggest to
    use it if patents are an issue.

bgp-image-format

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