GrXferProcessor::OptFlags
PorterDuffXferProcessor::onGetOptimizations(const GrProcOptInfo& colorPOI,
const GrProcOptInfo& coveragePOI,
bool doesStencilWrite,
GrColor* overrideColor,
const GrCaps& caps) {
GrXferProcessor::OptFlags optFlags = GrXferProcessor::kNone_OptFlags;
if (!fBlendFormula.modifiesDst()) {
if (!doesStencilWrite) {
optFlags |= GrXferProcessor::kSkipDraw_OptFlag;
}
optFlags |= (GrXferProcessor::kIgnoreColor_OptFlag |
GrXferProcessor::kIgnoreCoverage_OptFlag |
GrXferProcessor::kCanTweakAlphaForCoverage_OptFlag);
} else {
if (!fBlendFormula.usesInputColor()) {
optFlags |= GrXferProcessor::kIgnoreColor_OptFlag;
}
if (coveragePOI.isSolidWhite()) {
optFlags |= GrXferProcessor::kIgnoreCoverage_OptFlag;
}
if (colorPOI.allStagesMultiplyInput() && fBlendFormula.canTweakAlphaForCoverage()) {
optFlags |= GrXferProcessor::kCanTweakAlphaForCoverage_OptFlag;
}
}
return optFlags;
}
GrXferProcessor::OptFlags CustomXP::onGetOptimizations(const GrProcOptInfo& colorPOI,
const GrProcOptInfo& coveragePOI,
bool doesStencilWrite,
GrColor* overrideColor,
const GrCaps& caps) {
/*
Most the optimizations we do here are based on tweaking alpha for coverage.
The general SVG blend equation is defined in the spec as follows:
Dca' = B(Sc, Dc) * Sa * Da + Y * Sca * (1-Da) + Z * Dca * (1-Sa)
Da' = X * Sa * Da + Y * Sa * (1-Da) + Z * Da * (1-Sa)
(Note that Sca, Dca indicate RGB vectors that are premultiplied by alpha,
and that B(Sc, Dc) is a mode-specific function that accepts non-multiplied
RGB colors.)
For every blend mode supported by this class, i.e. the "advanced" blend
modes, X=Y=Z=1 and this equation reduces to the PDF blend equation.
It can be shown that when X=Y=Z=1, these equations can modulate alpha for
coverage.
== Color ==
We substitute Y=Z=1 and define a blend() function that calculates Dca' in
terms of premultiplied alpha only:
blend(Sca, Dca, Sa, Da) = {Dca : if Sa == 0,
Sca : if Da == 0,
B(Sca/Sa, Dca/Da) * Sa * Da + Sca * (1-Da) + Dca * (1-Sa) : if Sa,Da != 0}
And for coverage modulation, we use a post blend src-over model:
Dca'' = f * blend(Sca, Dca, Sa, Da) + (1-f) * Dca
(Where f is the fractional coverage.)
Next we show that canTweakAlphaForCoverage() is true by proving the
following relationship:
blend(f*Sca, Dca, f*Sa, Da) == f * blend(Sca, Dca, Sa, Da) + (1-f) * Dca
General case (f,Sa,Da != 0):
f * blend(Sca, Dca, Sa, Da) + (1-f) * Dca
= f * (B(Sca/Sa, Dca/Da) * Sa * Da + Sca * (1-Da) + Dca * (1-Sa)) + (1-f) * Dca [Sa,Da != 0, definition of blend()]
= B(Sca/Sa, Dca/Da) * f*Sa * Da + f*Sca * (1-Da) + f*Dca * (1-Sa) + Dca - f*Dca
= B(Sca/Sa, Dca/Da) * f*Sa * Da + f*Sca - f*Sca * Da + f*Dca - f*Dca * Sa + Dca - f*Dca
= B(Sca/Sa, Dca/Da) * f*Sa * Da + f*Sca - f*Sca * Da - f*Dca * Sa + Dca
= B(Sca/Sa, Dca/Da) * f*Sa * Da + f*Sca * (1-Da) - f*Dca * Sa + Dca
= B(Sca/Sa, Dca/Da) * f*Sa * Da + f*Sca * (1-Da) + Dca * (1 - f*Sa)
= B(f*Sca/f*Sa, Dca/Da) * f*Sa * Da + f*Sca * (1-Da) + Dca * (1 - f*Sa) [f!=0]
= blend(f*Sca, Dca, f*Sa, Da) [definition of blend()]
Corner cases (Sa=0, Da=0, and f=0):
Sa=0: f * blend(Sca, Dca, Sa, Da) + (1-f) * Dca
= f * Dca + (1-f) * Dca [Sa=0, definition of blend()]
= Dca
= blend(0, Dca, 0, Da) [definition of blend()]
= blend(f*Sca, Dca, f*Sa, Da) [Sa=0]
Da=0: f * blend(Sca, Dca, Sa, Da) + (1-f) * Dca
= f * Sca + (1-f) * Dca [Da=0, definition of blend()]
= f * Sca [Da=0]
= blend(f*Sca, 0, f*Sa, 0) [definition of blend()]
= blend(f*Sca, Dca, f*Sa, Da) [Da=0]
f=0: f * blend(Sca, Dca, Sa, Da) + (1-f) * Dca
= Dca [f=0]
= blend(0, Dca, 0, Da) [definition of blend()]
= blend(f*Sca, Dca, f*Sa, Da) [f=0]
== Alpha ==
We substitute X=Y=Z=1 and define a blend() function that calculates Da':
blend(Sa, Da) = Sa * Da + Sa * (1-Da) + Da * (1-Sa)
= Sa * Da + Sa - Sa * Da + Da - Da * Sa
= Sa + Da - Sa * Da
We use the same model for coverage modulation as we did with color:
Da'' = f * blend(Sa, Da) + (1-f) * Da
And show that canTweakAlphaForCoverage() is true by proving the following
relationship:
blend(f*Sa, Da) == f * blend(Sa, Da) + (1-f) * Da
f * blend(Sa, Da) + (1-f) * Da
= f * (Sa + Da - Sa * Da) + (1-f) * Da
= f*Sa + f*Da - f*Sa * Da + Da - f*Da
= f*Sa - f*Sa * Da + Da
= f*Sa + Da - f*Sa * Da
= blend(f*Sa, Da)
*/
OptFlags flags = kNone_OptFlags;
if (colorPOI.allStagesMultiplyInput()) {
flags |= kCanTweakAlphaForCoverage_OptFlag;
}
if (this->hasHWBlendEquation() && coveragePOI.isSolidWhite()) {
flags |= kIgnoreCoverage_OptFlag;
}
return flags;
}
GrXferProcessor::OptFlags
PorterDuffXferProcessor::internalGetOptimizations(const GrProcOptInfo& colorPOI,
const GrProcOptInfo& coveragePOI,
bool doesStencilWrite) {
if (this->willReadDstColor()) {
return GrXferProcessor::kNone_Opt;
}
bool srcAIsOne = colorPOI.isOpaque();
bool hasCoverage = !coveragePOI.isSolidWhite();
bool dstCoeffIsOne = kOne_GrBlendCoeff == fDstBlend ||
(kSA_GrBlendCoeff == fDstBlend && srcAIsOne);
bool dstCoeffIsZero = kZero_GrBlendCoeff == fDstBlend ||
(kISA_GrBlendCoeff == fDstBlend && srcAIsOne);
// When coeffs are (0,1) there is no reason to draw at all, unless
// stenciling is enabled. Having color writes disabled is effectively
// (0,1).
if ((kZero_GrBlendCoeff == fSrcBlend && dstCoeffIsOne)) {
if (doesStencilWrite) {
return GrXferProcessor::kIgnoreColor_OptFlag |
GrXferProcessor::kSetCoverageDrawing_OptFlag;
} else {
fDstBlend = kOne_GrBlendCoeff;
return GrXferProcessor::kSkipDraw_OptFlag;
}
}
// if we don't have coverage we can check whether the dst
// has to read at all. If not, we'll disable blending.
if (!hasCoverage) {
if (dstCoeffIsZero) {
if (kOne_GrBlendCoeff == fSrcBlend) {
// if there is no coverage and coeffs are (1,0) then we
// won't need to read the dst at all, it gets replaced by src
fDstBlend = kZero_GrBlendCoeff;
return GrXferProcessor::kNone_Opt;
} else if (kZero_GrBlendCoeff == fSrcBlend) {
// if the op is "clear" then we don't need to emit a color
// or blend, just write transparent black into the dst.
fSrcBlend = kOne_GrBlendCoeff;
fDstBlend = kZero_GrBlendCoeff;
return GrXferProcessor::kIgnoreColor_OptFlag |
GrXferProcessor::kIgnoreCoverage_OptFlag;
}
}
} else {
// check whether coverage can be safely rolled into alpha
// of if we can skip color computation and just emit coverage
if (can_tweak_alpha_for_coverage(fDstBlend)) {
if (colorPOI.allStagesMultiplyInput()) {
return GrXferProcessor::kSetCoverageDrawing_OptFlag |
GrXferProcessor::kCanTweakAlphaForCoverage_OptFlag;
} else {
return GrXferProcessor::kSetCoverageDrawing_OptFlag;
}
}
if (dstCoeffIsZero) {
if (kZero_GrBlendCoeff == fSrcBlend) {
// the source color is not included in the blend
// the dst coeff is effectively zero so blend works out to:
// (c)(0)D + (1-c)D = (1-c)D.
fDstBlend = kISA_GrBlendCoeff;
return GrXferProcessor::kIgnoreColor_OptFlag |
GrXferProcessor::kSetCoverageDrawing_OptFlag;
} else if (srcAIsOne) {
// the dst coeff is effectively zero so blend works out to:
// cS + (c)(0)D + (1-c)D = cS + (1-c)D.
// If Sa is 1 then we can replace Sa with c
// and set dst coeff to 1-Sa.
fDstBlend = kISA_GrBlendCoeff;
if (colorPOI.allStagesMultiplyInput()) {
return GrXferProcessor::kSetCoverageDrawing_OptFlag |
GrXferProcessor::kCanTweakAlphaForCoverage_OptFlag;
} else {
return GrXferProcessor::kSetCoverageDrawing_OptFlag;
}
}
} else if (dstCoeffIsOne) {
// the dst coeff is effectively one so blend works out to:
// cS + (c)(1)D + (1-c)D = cS + D.
fDstBlend = kOne_GrBlendCoeff;
if (colorPOI.allStagesMultiplyInput()) {
return GrXferProcessor::kSetCoverageDrawing_OptFlag |
GrXferProcessor::kCanTweakAlphaForCoverage_OptFlag;
} else {
return GrXferProcessor::kSetCoverageDrawing_OptFlag;
}
return GrXferProcessor::kSetCoverageDrawing_OptFlag;
}
}
return GrXferProcessor::kNone_Opt;
}