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update the spec for the lossy-alpha compression methods.

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No further experiments are to be expected, so this is quite the
final format so far, pending supplemental feedbacks.

Change-Id: I2a3de025c90b7bb5fdd8792b2b2ccdc2e3753f56
This commit is contained in:
Pascal Massimino 2012-05-24 07:20:11 -07:00
parent e75dc80516
commit 42d61b6def
1 changed files with 20 additions and 0 deletions
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doc/webp-container-spec.txt
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@ -492,6 +492,26 @@ The alpha channel data is losslessly stored as raw data (when
compression method is '0') or compressed using the lossless format compression method is '0') or compressed using the lossless format
(when the compression method is '1'). (when the compression method is '1').
* raw data: consists of a byte sequence of length width * height,
containing all the 8bit transparency values in scan order.
* lossless format compression: the byte sequence is a compressed
image-stream (as described in the specification for lossless format)
of implicit dimension width x height. Note that this image-stream byte
sequence does NOT contain any headers describing the image dimension.
**Rationale**: the dimension is already known from other sources,
so storing it again would be redundant and error-prone.
Once the image-stream is decoded into ARGB color values, following
the process described in the lossless format specification, the
transparency information must be extracted from the *green* channel
of the ARGB quadruplet.
**Rationale**: the green channel is allowed extra transformation
steps in the specification -unlike the other channels- that can
improve compression.
#### Color profile #### Color profile
0 1 2 3 0 1 2 3