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EXPERIMENTAL: add support for alpha channel

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This is a (minor) bitstream change: if the 'color_space' bit is set to '1'
(which is normally an undefined/invalid behaviour), we add extra data at the
end of partition #0 (so-called 'extensions')

Namely, we add the size of the extension data as 3 bytes (little-endian),
followed by a set of bits telling which extensions we're incorporating.
The data then _preceeds_ this trailing tags.

This is all experimental, and you'll need to have
'#define WEBP_EXPERIMENTAL_FEATURES' in webp/types.h to enable this code
(at your own risk! :))

Still, this hack produces almost-valid WebP file for decoders that don't
check this color_space bit. In particular, previous 'dwebp' (and for instance
Chrome) will recognize this files and decode them, but without the alpha
of course. Other decoder will just see random extra stuff at the end of
partition #0.

To experiment with the alpha-channel, you need to compile on Unix platform
and use PNGs for input/output.

If 'alpha.png' is a source with alpha channel, then you can try (on Unix):

  cwebp alpha.png -o alpha.webp
  dwebp alpha.webp -o test.png

cwebp now has a '-noalpha' flag to ignore any alpha information from the
source, if present.

More hacking and experimenting welcome!

Change-Id: I3c7b1fd8411c9e7a9f77690e898479ad85c52f3e
This commit is contained in:
Pascal Massimino
2011-04-25 16:58:04 -07:00
parent cfbf88a6c4
commit 2ab4b72f53
27 changed files with 477 additions and 59 deletions
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9
src/webp/decode.h
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@ -178,11 +178,10 @@ VP8StatusCode WebPIAppend(WebPIDecoder* const idec, const uint8_t* data,
VP8StatusCode WebPIUpdate(WebPIDecoder* const idec, const uint8_t* data,
uint32_t data_size);
// Returns the r/g/b/(a) image decoded so far. Returns NULL if output params
// are not initialized yet. The r/g/b/(a) output type corresponds to the mode
// specified in WebPINew()/WebPINewRGB(). *last_y is the index of last decoded
// row in raster scan order. Some pointers (*last_y, *width etc.) can be NULL if
// corresponding information is not needed.
// Returns the RGB image decoded so far. Returns NULL if output params are not
// initialized yet. *last_y is the index of last decoded row in raster scan
// order. Some pointers (*last_y, *width etc.) can be NULL if corresponding
// information is not needed.
uint8_t* WebPIDecGetRGB(const WebPIDecoder* const idec, int *last_y,
int* width, int* height, int* stride);