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authormtklein <mtklein@chromium.org>2015-07-14 13:38:28 -0700
committerSteve Kondik <steve@cyngn.com>2016-06-27 17:35:22 -0700
commit6c050ffafcd95b17288464d4ccb42e1690e49efd (patch)
treec9d3e29e93ea0ee02b94582f96f497bf4918caaa
parentaf10be129ecd076eb9fbf48cbbabf5c189f660e1 (diff)
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Fix buggy blend modes.
The problem turns out to be not over- or underflow like I thought, but that I used different math for x*y/255 for alphas and colors, sometimes resulting in colors where alpha was one less than the maximum color component, which is not a valid SkPMColor. To be safe, I've switched over all four of these similar complex modes to use exact math everywhere. They now match the byte-by-byte code in SkXfermode.cpp exactly. This will slow down Darken and Lighten by about 2x. I plan to follow up with a CL to see if I can eek out some speed there, and another CL to add asserts that Sk4px code always writes valid SkPMColors. BUG=skia:4052 Review URL: https://codereview.chromium.org/1241683003
-rw-r--r--src/core/Sk4pxXfermode.h40
1 files changed, 28 insertions, 12 deletions
diff --git a/src/core/Sk4pxXfermode.h b/src/core/Sk4pxXfermode.h
index 2b724ca820..61a9f99473 100644
--- a/src/core/Sk4pxXfermode.h
+++ b/src/core/Sk4pxXfermode.h
@@ -60,37 +60,53 @@ XFERMODE(Exclusion) {
return (s - p) + (d - p.zeroAlphas());
}
-XFERMODE(HardLight) {
- auto alphas = SrcOver::Xfer(s,d);
+// We take care to use exact math for these next few modes where alphas
+// and colors are calculated using significantly different math. We need
+// to preserve premul invariants, and exact math makes this easier.
+//
+// TODO: Some of these implementations might be able to be sped up a bit
+// while maintaining exact math, but let's follow up with that.
+XFERMODE(HardLight) {
auto sa = s.alphas(),
da = d.alphas();
+ auto srcover = s + (d * sa.inv()).div255();
+
auto isLite = ((sa-s) < s).widenLoHi();
- auto dark = s*d << 1,
- lite = sa*da - ((da-d)*(sa-s) << 1),
+ auto lite = sa*da - ((da-d)*(sa-s) << 1),
+ dark = s*d << 1,
both = s*da.inv() + d*sa.inv();
+ auto alphas = srcover;
auto colors = (both + isLite.thenElse(lite, dark)).div255();
return alphas.zeroColors() + colors.zeroAlphas();
}
XFERMODE(Overlay) { return HardLight::Xfer(d,s); }
XFERMODE(Darken) {
- auto sda = s.approxMulDiv255(d.alphas()),
- dsa = d.approxMulDiv255(s.alphas());
- auto srcover = s + (d - dsa),
- dstover = d + (s - sda);
+ auto sa = s.alphas(),
+ da = d.alphas();
+
+ auto sda = (s*da).div255(),
+ dsa = (d*sa).div255();
+
+ auto srcover = s + (d * sa.inv()).div255(),
+ dstover = d + (s * da.inv()).div255();
auto alphas = srcover,
colors = (sda < dsa).thenElse(srcover, dstover);
return alphas.zeroColors() + colors.zeroAlphas();
}
XFERMODE(Lighten) {
- auto sda = s.approxMulDiv255(d.alphas()),
- dsa = d.approxMulDiv255(s.alphas());
- auto srcover = s + (d - dsa),
- dstover = d + (s - sda);
+ auto sa = s.alphas(),
+ da = d.alphas();
+
+ auto sda = (s*da).div255(),
+ dsa = (d*sa).div255();
+
+ auto srcover = s + (d * sa.inv()).div255(),
+ dstover = d + (s * da.inv()).div255();
auto alphas = srcover,
colors = (dsa < sda).thenElse(srcover, dstover);
return alphas.zeroColors() + colors.zeroAlphas();