change srcover (and dstover) to use the faster thought slightly less accurate
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
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 80bafce..7c336c2 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -344,21 +344,24 @@
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]