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RGBA4444: harmonize lossless/lossy alpha values

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lossy was rounding with a bias toward opaque:
[232+, 8] -> [15, 1]
now both paths use the range:
[240+, 16] -> [15, 1]

Change-Id: I3da2063b4959b9e9f45bae09e640acc1f43470c5
This commit is contained in:
James Zern 2012-08-11 20:14:33 -07:00 committed by Gerrit Code Review
parent 681cb30ad2
commit fe1958f17d
2 changed files with 2 additions and 7 deletions
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4
src/dec/io.c
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@ -261,7 +261,7 @@ static int EmitAlphaRGBA4444(const VP8Io* const io, WebPDecParams* const p) {
for (j = 0; j < num_rows; ++j) {
for (i = 0; i < mb_w; ++i) {
// Fill in the alpha value (converted to 4 bits).
const uint32_t alpha_value = VP8Clip4Bits(alpha[i]);
const uint32_t alpha_value = alpha[i] >> 4;
alpha_dst[2 * i] = (alpha_dst[2 * i] & 0xf0) | alpha_value;
alpha_mask &= alpha_value;
}
@ -453,7 +453,7 @@ static int ExportAlphaRGBA4444(WebPDecParams* const p, int y_pos) {
WebPRescalerExportRow(&p->scaler_a);
for (i = 0; i < width; ++i) {
// Fill in the alpha value (converted to 4 bits).
const uint32_t alpha_value = VP8Clip4Bits(p->scaler_a.dst[i]);
const uint32_t alpha_value = p->scaler_a.dst[i] >> 4;
alpha_dst[2 * i] = (alpha_dst[2 * i] & 0xf0) | alpha_value;
alpha_mask &= alpha_value;
}

5
src/dsp/yuv.h
View File

@ -90,11 +90,6 @@ static WEBP_INLINE void VP8YuvToRgba(uint8_t y, uint8_t u, uint8_t v,
rgba[3] = 0xff;
}
static WEBP_INLINE uint32_t VP8Clip4Bits(uint8_t c) {
const uint32_t v = (c + 8) >> 4;
return (v > 15) ? 15 : v;
}
// Must be called before everything, to initialize the tables.
void VP8YUVInit(void);