BPG Image Encoder and Decoder
-----------------------------
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 supports by default only 8 bits per component
and does not support monochrome encoding yet (hence no alpha nor
grayscale images can be encoded with it).
- 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 (if you use Chrome and want to use
file:// to access it, launch Chrome with the option
--allow-file-access-from-files).
- The BPG file format is specified in doc/bpg_spec.txt.
2) BPG encoder
--------------
The BPG command line encoder is 'bpgenc'. It takes JPEG or PNG images
as input.
- Speed: by default bpgenc uses the JCTVC encoder which has a high
quality but is slow. If you compiled with x265, you can have a much
faster encoding with the '-e x265' option. With x265 you can also
select the encoding speed with the '-m' option (1 = fast, but larger
image, 9 = slower but smaller image). Warning: x265 does not support
monochrome (and alpha) yet, so you must use the JCTVC encoder for
these cases.
- Bit depth: the default bit depth is 8. You can increase it to 10
('-b 10' option) to slightly increase the compression ratio. For web
publishing it is generally not a good idea because the Javascript
decoder uses more memory.
- 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". Notes:
- lossless mode is less tested that the lossy mode but it usually
gives better results that PNG on photographic images.
- the JCTVC encoder gives smaller images than the x265 encoder
with lossless compression.
- 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, 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.
- Premultiplied alpha: by default bpgenc uses non-premultiplied alpha
to preserve the color components. However, premultiplied alpha
('-premul' option) usually gives a better compression at the expense
of a loss in the color components. This loss is not an issue if the
image is not edited.
- 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) BPG decoder
--------------
The BPG command line decoder is bpgdec. It outputs a PNG or PPM
image. Use a PPM output to get the fastest speed.
- With the '-i' option, you have information about the BPG image (and
no decoded image is output).
- The '-b' option selects the bit depth (8 or 16) of the PNG
output. It is independent of the internal BPG bit depth.
4) BPG decoding library
-----------------------
BPG images can be decoded in any program with the libbpg
library.
The API is not considered stable yet so that the library is only
provided as a static one.
Currently there is no similar library for encoding so you should
invoke the bpgenc utility.
5) Javascript decoder
---------------------
bpgdec.js is a Javascript decoder supporting the BPG file
format. bpgdec8b.js is a specialized version limited to BPG images
using 8 bits per component. It is a little faster and consumes less
memory (16 MB instead of 32 MB by default, you can change the memory
configuration in the Makefile if you want to handle larger images).
The Javascript decoder substitutes all the <img> tags with a source
having a .bpg extension with a <canvas> tag and decodes the BPG image
into it. Stylesheets are supported (the 'id' and 'class' attributes
are preserved). The 'width' and 'height' attributes are supported only
with pixel units.
asm.js gives an interesting speed boost, so we hope that more browser
will support this Javascript subset.
6) FFmpeg modifications
-----------------------
- Completed support of chroma_format_idc = 0 (monochrome mode).
- Fixed RDPCM support (intra predictions).
- Reduced memory usage for the SAO loop filter.
- Generated the IDCT coefficients dynamically to reduce the code size.
- 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.
7) 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.