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Merge pull request #172 from homm/alpha-blending-enhancement

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Fix compilation on windows and one more thing
This commit is contained in:
Alex Clark ☺ 2013-03-29 05:17:48 -07:00
commit 9aff1c354b
1 changed files with 6 additions and 5 deletions
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11
libImaging/AlphaComposite.c
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@ -69,21 +69,22 @@ ImagingAlphaComposite(Imaging imDst, Imaging imSrc)
// almost equivalent to:
// tmp = a + (2 << (n-1)), ((tmp >> n) + tmp) >> n
UINT32 tmpr, tmpg, tmpb;
UINT16 blend = dst->a * (255 - src->a);
UINT16 outa255 = src->a * 255 + blend;
// There we use 7 bits for precision.
// We could use more, but we go beyond 32 bits.
UINT16 coef1 = src->a * 255 * 255 * 128 / outa255;
UINT16 coef2 = blend * 255 * 128 / outa255;
UINT16 coef2 = 255 * 128 - coef1;
#define SHIFTFORDIV255(a)\
((a >> 8) + a >> 8)
((((a) >> 8) + a) >> 8)
UINT32 tmpr = src->r * coef1 + dst->r * coef2 + (0x80 << 7);
tmpr = src->r * coef1 + dst->r * coef2 + (0x80 << 7);
out->r = SHIFTFORDIV255(tmpr) >> 7;
UINT32 tmpg = src->g * coef1 + dst->g * coef2 + (0x80 << 7);
tmpg = src->g * coef1 + dst->g * coef2 + (0x80 << 7);
out->g = SHIFTFORDIV255(tmpg) >> 7;
UINT32 tmpb = src->b * coef1 + dst->b * coef2 + (0x80 << 7);
tmpb = src->b * coef1 + dst->b * coef2 + (0x80 << 7);
out->b = SHIFTFORDIV255(tmpb) >> 7;
out->a = SHIFTFORDIV255(outa255 + 0x80);
}