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Merge pull request #159 from homm/master

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Precision of alpha composite. Again.
This commit is contained in:
Alex Clark ☺ 2013-03-24 15:31:23 -07:00
commit 887f709b6e
1 changed files with 15 additions and 13 deletions
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28
libImaging/AlphaComposite.c
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@ -69,21 +69,23 @@ ImagingAlphaComposite(Imaging imDst, Imaging imSrc)
// almost equivalent to:
// tmp = a + (2 << (n-1)), ((tmp >> n) + tmp) >> n
// 0xff * 0xff = 16 meaningful bits.
UINT16 blend = dst->a * (255 - src->a);
// Shift 4 bits up, to don't loose blend precision
// on very transparent pixels.
UINT16 outa = (src->a << 4) + (((blend << 4) + (blend >> 4) + 0x80) >> 8);
UINT16 coef1 = (((src->a << 8) - src->a) << 8) / outa; // 12
UINT16 coef2 = (blend << 8) / outa; // 12
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;
UINT32 tmpr = src->r * coef1 + dst->r * coef2 + 0x800;
out->r = ((tmpr >> 8) + tmpr) >> 12;
UINT32 tmpg = src->g * coef1 + dst->g * coef2 + 0x800;
out->g = ((tmpg >> 8) + tmpg) >> 12;
UINT32 tmpb = src->b * coef1 + dst->b * coef2 + 0x800;
out->b = ((tmpb >> 8) + tmpb) >> 12;
out->a = (outa + 0x7) >> 4;
#define SHIFTFORDIV255(a)\
((a >> 8) + a >> 8)
UINT32 tmpr = src->r * coef1 + dst->r * coef2 + (0x80 << 7);
out->r = SHIFTFORDIV255(tmpr) >> 7;
UINT32 tmpg = src->g * coef1 + dst->g * coef2 + (0x80 << 7);
out->g = SHIFTFORDIV255(tmpg) >> 7;
UINT32 tmpb = src->b * coef1 + dst->b * coef2 + (0x80 << 7);
out->b = SHIFTFORDIV255(tmpb) >> 7;
out->a = SHIFTFORDIV255(outa255 + 0x80);
}
dst++; src++; out++;