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bitstream update for lossy alpha compression

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now, we only use 2 bits for the filtering method, and 2 bits
for the compression method.
There's two additional bits which are INFORMATIVE, to specify
whether the source has been pre-processed (level reduction)
during compression. This can be used at decompression time
for some post-processing (see DequantizeLevels()).

New relevant spec excerpt:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      ChunkHeader('ALPH')                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Rsv| P | F | C |     Alpha Bitstream...                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Compression method (C): 2 bits

: The compression method used:

  * `0`: No compression.
  * `1`: Backward reference counts encoded with arithmetic encoder.

Filtering method (F): 2 bits

: The filtering method used:

  * `0`: None.
  * `1`: Horizontal filter.
  * `2`: Vertical filter.
  * `3`: Gradient filter.

Pre-processing (P): 2 bits

: These INFORMATIVE bits are used to signal the pre-processing that has
been performed during compression. The decoder can use this information to
e.g. dither the values or smooth the gradients prior to display.

  * `0`: no pre-processing
  * `1`: level reduction

Decoders are not required to use this information in any specified way.

Reserved (Rsv): 2 bits

: SHOULD be `0`.

Alpha bitstream: _Chunk Size_ - `1` bytes

: Encoded alpha bitstream.

This optional chunk contains encoded alpha data for a single tile.
Either **ALL or NONE** of the tiles must contain this chunk.

The alpha channel data is losslessly stored as raw data (when
compression method is '0') or compressed using the lossless format
(when the compression method is '1').

Change-Id: Ied8f5fb922707a953e6a2b601c69c73e552dda6b
This commit is contained in:
Pascal Massimino
2012-05-22 02:36:22 -07:00
parent d73e63a726
commit 638528cd1e
3 changed files with 69 additions and 33 deletions
Download Patch File Download Diff File Expand all files Collapse all files

34
doc/webp-container-spec.txt
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@ -443,10 +443,17 @@ Notes for tiles containing VP8 data:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ChunkHeader('ALPH') |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | C | Reserved | Alpha Bitstream |
|Rsv| P | F | C | Alpha Bitstream... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Filtering method (F): 4 bits
Compression method (C): 2 bits
: The compression method used:
* `0`: No compression.
* `1`: Backward reference counts encoded with arithmetic encoder.
Filtering method (F): 2 bits
: The filtering method used:
@ -455,28 +462,31 @@ Filtering method (F): 4 bits
* `2`: Vertical filter.
* `3`: Gradient filter.
Compression method (C): 4 bits
Pre-processing (P): 2 bits
: The compression method used:
: These INFORMATIVE bits are used to signal the pre-processing that has
been performed during compression. The decoder can use this information to
e.g. dither the values or smooth the gradients prior to display.
* `0`: No compression.
* `1`: Backward reference counts encoded with arithmetic encoder.
* `0`: no pre-processing
* `1`: level reduction
Reserved: 8 bits
Decoders are not required to use this information in any specified way.
Reserved (Rsv): 2 bits
: SHOULD be `0`.
Alpha bitstream: _Chunk Size_ - `2` bytes
Alpha bitstream: _Chunk Size_ - `1` bytes
: Encoded alpha bitstream.
This optional chunk contains encoded alpha data for a single tile.
Either **ALL or NONE** of the tiles must contain this chunk.
The alpha channel can be encoded either losslessly or with lossy
preprocessing (quantization). After the optional preprocessing, the
alpha values are encoded with a lossless compression method like
zlib.
The alpha channel data is losslessly stored as raw data (when
compression method is '0') or compressed using the lossless format
(when the compression method is '1').
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