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change srcover (and dstover) to use the faster thought slightly less accurate

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computation of the scale factor (+1 instead of SkAlpha255To256). This has the
upside of guaranteeing that if the dst is opaque (alpha==255), it will always
stay opaque.



git-svn-id: http://skia.googlecode.com/svn/trunk@104 2bbb7eff-a529-9590-31e7-b0007b416f81
This commit is contained in:
reed@android.com 2009-03-03 20:31:12 +00:00
parent d66efc273e
commit 1116fb25e5
1 changed files with 13 additions and 10 deletions
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23
src/core/SkXfermode.cpp
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@ -344,21 +344,24 @@ static SkPMColor dst_modeproc(SkPMColor src, SkPMColor dst) {
return dst;
}
// kSrcOver_Mode, //!< [Sa + (1 - Sa)*Da, Sc + (1 - Sa)*Dc]
// kSrcOver_Mode, //!< [Sa + Da - Sa*Da, Sc + (1 - Sa)*Dc]
static SkPMColor srcover_modeproc(SkPMColor src, SkPMColor dst) {
#if 0
// this is the old, more-correct way, but it doesn't guarantee that dst==255
// will always stay opaque
return src + SkAlphaMulQ(dst, SkAlpha255To256(255 - SkGetPackedA32(src)));
#else
// this is slightly faster, but more importantly guarantees that dst==255
// will always stay opaque
return src + SkAlphaMulQ(dst, 256 - SkGetPackedA32(src));
#endif
}
// kDstOver_Mode, //!< [Sa + (1 - Sa)*Da, Dc + (1 - Da)*Sc]
// kDstOver_Mode, //!< [Sa + Da - Sa*Da, Dc + (1 - Da)*Sc]
static SkPMColor dstover_modeproc(SkPMColor src, SkPMColor dst) {
unsigned sa = SkGetPackedA32(src);
unsigned da = SkGetPackedA32(dst);
unsigned ida = 255 - da;
return SkPackARGB32(sa + da - SkAlphaMulAlpha(sa, da),
SkGetPackedR32(dst) + SkAlphaMulAlpha(ida, SkGetPackedR32(src)),
SkGetPackedG32(dst) + SkAlphaMulAlpha(ida, SkGetPackedG32(src)),
SkGetPackedB32(dst) + SkAlphaMulAlpha(ida, SkGetPackedB32(src)));
// this is the reverse of srcover, just flipping src and dst
// see srcover's comment about the 256 for opaqueness guarantees
return dst + SkAlphaMulQ(src, 256 - SkGetPackedA32(dst));
}
// kSrcIn_Mode, //!< [Sa * Da, Sc * Da]