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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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21
src/mux/muxinternal.c
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@ -393,12 +393,9 @@ uint8_t* MuxImageEmit(const WebPMuxImage* const wpi, uint8_t* dst) {
//------------------------------------------------------------------------------
// Helper methods for mux.
int MuxHasLosslessImages(const WebPMuxImage* images) {
int MuxHasAlpha(const WebPMuxImage* images) {
while (images != NULL) {
assert(images->img_ != NULL);
if (images->img_->tag_ == kChunks[IDX_VP8L].tag) {
return 1;
}
if (images->has_alpha_) return 1;
images = images->next_;
}
return 0;
@ -512,14 +509,18 @@ WebPMuxError MuxValidate(const WebPMux* const mux) {
if (num_vp8x == 0 && num_images != 1) return WEBP_MUX_INVALID_ARGUMENT;
// ALPHA_FLAG & alpha chunk(s) are consistent.
if (MuxHasLosslessImages(mux->images_)) {
if (MuxHasAlpha(mux->images_)) {
if (num_vp8x > 0) {
// Special case: we have a VP8X chunk as well as some lossless images.
// VP8X chunk is present, so it should contain ALPHA_FLAG.
if (!(flags & ALPHA_FLAG)) return WEBP_MUX_INVALID_ARGUMENT;
}
} else {
err = ValidateChunk(mux, IDX_ALPHA, ALPHA_FLAG, flags, -1, &num_alpha);
} else {
// VP8X chunk is not present, so ALPH chunks should NOT be present either.
err = WebPMuxNumChunks(mux, WEBP_CHUNK_ALPHA, &num_alpha);
if (err != WEBP_MUX_OK) return err;
if (num_alpha > 0) return WEBP_MUX_INVALID_ARGUMENT;
}
} else { // Mux doesn't need alpha. So, ALPHA_FLAG should NOT be present.
if (flags & ALPHA_FLAG) return WEBP_MUX_INVALID_ARGUMENT;
}
// num_fragments & num_images are consistent.