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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
(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
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