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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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6
examples/cwebp.c
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@ -497,8 +497,10 @@ static int WriteWebPWithMetadata(FILE* const out,
webp_size -= kVP8XChunkSize;
} else {
const int is_lossless = !memcmp(webp, "VP8L", kTagSize);
// The alpha flag is forced with lossless images.
if (is_lossless) flags |= kAlphaFlag;
if (is_lossless) {
// Presence of alpha is stored in the 29th bit of VP8L data.
if (webp[kChunkHeaderSize + 3] & (1 << 5)) flags |= kAlphaFlag;
}
ok = ok && (fwrite(kVP8XHeader, kChunkHeaderSize, 1, out) == 1);
ok = ok && WriteLE32(out, flags);
ok = ok && WriteLE24(out, picture->width - 1);