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Fix buggy blend modes.

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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
This commit is contained in:
mtklein 2015-07-14 13:38:28 -07:00 committed by Commit bot
parent 2ecc000536
commit 44cf62c79e
1 changed files with 28 additions and 12 deletions
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40
src/core/Sk4pxXfermode.h
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@ -62,37 +62,53 @@ 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();