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Stricter check for presence of alpha when writing lossless images

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Earlier, all lossless images were assumed to contain alpha.
Now, we use the 'alpha_is_used' bit from the VP8L bitstream to determine
the
same.

Detecting an absence of alpha can sometimes lead to much more efficient
rendering, especially for animated images.

Related: refine mux code to read width/height/has_alpha information only
once
per frame/fragment. This avoid frequent calls to VP8(L)GetInfo().

Change-Id: I4e0eef4db7d94425396c7dff6ca5599d5bca8297
This commit is contained in:
Urvang Joshi
2013-07-19 11:55:09 -07:00
parent a03c3516cb
commit 8ba1bf61a0
5 changed files with 88 additions and 98 deletions
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16
src/mux/muxi.h
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@ -48,6 +48,9 @@ struct WebPMuxImage {
WebPChunk* header_; // Corresponds to WEBP_CHUNK_ANMF/WEBP_CHUNK_FRGM.
WebPChunk* alpha_; // Corresponds to WEBP_CHUNK_ALPHA.
WebPChunk* img_; // Corresponds to WEBP_CHUNK_IMAGE.
int width_;
int height_;
int has_alpha_; // Through ALPH chunk or as part of VP8L.
int is_partial_; // True if only some of the chunks are filled.
WebPMuxImage* next_;
};
@ -148,11 +151,6 @@ static WEBP_INLINE size_t ChunkDiskSize(const WebPChunk* chunk) {
// Write out the given list of chunks into 'dst'.
uint8_t* ChunkListEmit(const WebPChunk* chunk_list, uint8_t* dst);
// Get the width, height and has_alpha info of image stored in 'image_chunk'.
WebPMuxError MuxGetImageInfo(const WebPChunk* const image_chunk,
int* const width, int* const height,
int* const has_alpha);
//------------------------------------------------------------------------------
// MuxImage object management.
@ -170,6 +168,10 @@ WebPMuxImage* MuxImageDelete(WebPMuxImage* const wpi);
// If id == WEBP_CHUNK_NIL, all images will be matched.
int MuxImageCount(const WebPMuxImage* wpi_list, WebPChunkId id);
// Update width/height/has_alpha info from chunks within wpi.
// Also remove ALPH chunk if not needed.
int MuxImageFinalize(WebPMuxImage* const wpi);
// Check if given ID corresponds to an image related chunk.
static WEBP_INLINE int IsWPI(WebPChunkId id) {
switch (id) {
@ -200,8 +202,8 @@ uint8_t* MuxImageEmit(const WebPMuxImage* const wpi, uint8_t* dst);
//------------------------------------------------------------------------------
// Helper methods for mux.
// Checks if the given image list contains at least one lossless image.
int MuxHasLosslessImages(const WebPMuxImage* images);
// Checks if the given image list contains at least one image with alpha.
int MuxHasAlpha(const WebPMuxImage* images);
// Write out RIFF header into 'data', given total data size 'size'.
uint8_t* MuxEmitRiffHeader(uint8_t* const data, size_t size);