Implement support for KHR_blend_equation_advanced

src/gpu/effects/GrAdvancedEquationXPFactory.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "effects/GrAdvancedEquationXPFactory.h"
+
+#include "GrDrawTargetCaps.h"
+#include "GrGpu.h"
+#include "gl/GrGLXferProcessor.h"
+#include "gl/builders/GrGLFragmentShaderBuilder.h"
+#include "gl/builders/GrGLProgramBuilder.h"
+
+bool GrAdvancedEquationXPFactory::IsSupported(const GrContext* context, SkXfermode::Mode mode) {
+    if (!context->getGpu()->caps()->advancedBlendEquationSupport()) {
+        return false;
+    }
+    if (mode > SkXfermode::kLastMode) {
+        return false;
+    }
+    return mode == SkXfermode::kScreen_Mode || mode > SkXfermode::kLastCoeffMode;
+}
+
+template<GrBlendEquation Equation> GrXPFactory* GrAdvancedEquationXPFactory::TakeFactoryRef() {
+    GR_STATIC_ASSERT(GrBlendEquationIsAdvanced(Equation));
+    static GrAdvancedEquationXPFactory* factory;
+    if (!factory) {
+        factory = SkNEW_ARGS(GrAdvancedEquationXPFactory, (Equation));
+    }
+    return SkRef(factory);
+}
+
+GrXPFactory* GrAdvancedEquationXPFactory::Create(const GrContext* context, SkXfermode::Mode mode) {
+    if (!context->getGpu()->caps()->advancedBlendEquationSupport()) {
+        return NULL;
+    }
+
+    switch (mode) {
+        default: break;
+        case SkXfermode::kScreen_Mode: return TakeFactoryRef<kScreen_GrBlendEquation>();
+        case SkXfermode::kOverlay_Mode: return TakeFactoryRef<kOverlay_GrBlendEquation>();
+        case SkXfermode::kDarken_Mode: return TakeFactoryRef<kDarken_GrBlendEquation>();
+        case SkXfermode::kLighten_Mode: return TakeFactoryRef<kLighten_GrBlendEquation>();
+        case SkXfermode::kColorDodge_Mode: return TakeFactoryRef<kColorDodge_GrBlendEquation>();
+        case SkXfermode::kColorBurn_Mode: return TakeFactoryRef<kColorBurn_GrBlendEquation>();
+        case SkXfermode::kHardLight_Mode: return TakeFactoryRef<kHardLight_GrBlendEquation>();
+        case SkXfermode::kSoftLight_Mode: return TakeFactoryRef<kSoftLight_GrBlendEquation>();
+        case SkXfermode::kDifference_Mode: return TakeFactoryRef<kDifference_GrBlendEquation>();
+        case SkXfermode::kExclusion_Mode: return TakeFactoryRef<kExclusion_GrBlendEquation>();
+        case SkXfermode::kMultiply_Mode: return TakeFactoryRef<kMultiply_GrBlendEquation>();
+        case SkXfermode::kHue_Mode: return TakeFactoryRef<kHSLHue_GrBlendEquation>();
+        case SkXfermode::kSaturation_Mode: return TakeFactoryRef<kHSLSaturation_GrBlendEquation>();
+        case SkXfermode::kColor_Mode: return TakeFactoryRef<kHSLColor_GrBlendEquation>();
+        case SkXfermode::kLuminosity_Mode: return TakeFactoryRef<kHSLLuminosity_GrBlendEquation>();
+    }
+
+    return NULL;
+}
+
+bool GrAdvancedEquationXPFactory::canTweakAlphaForCoverage() const {
+   /*
+    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 "advanced" blend mode, X=Y=Z=1 and this equation reduces to the
+    PDF blend equation.
+
+    It can be shown that when X=Y=Z=1, canTweakAlphaForCoverage() is true.

[Mark Kilgard, 2015/04/03 18:30:03]

great to see this analysis

+
+
+    == 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, Skia uses 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
+
+    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]
+
+    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()]

[Mark Kilgard, 2015/04/03 18:30:04]

great analysis

I think you should motivate the "Corner cases" analysis better by putting the
"General case" analysis first.

Then note the "General case" analysis has expressions dividing by Sa, Da, and f.
 This could be a problem if any of Sa, Da, or f were zero.

Since these values may be zero, that motivations the "Corners cases" analysis.

[Chris Dalton, 2015/04/17 08:37:18]

Done.

+
+
+    == 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)
+   */
+
+    return true;
+}
+
+bool GrAdvancedEquationXPFactory::supportsRGBCoverage(GrColor knownColor,
+                                                      uint32_t knownColorFlags) const {
+   /*
+    We do coverage modulation by multiplying it into the src color before
+    blending. The GPU runs the blend equation *after* the fragment shader, so
+    it is not within our control to apply coverage at that point. This model
+    is mathematically correct, but the proof lies in the following property:
+
+      f*Sca / f*Sa == Sc
+
+    (That is, the general case, f,Sa,Da != 0. See canTweakAlphaForCoverage().)
+
+    But when coverage is unique per channel, this property no longer holds:

[Mark Kilgard, 2015/04/03 18:30:03]

"when coverage is unique per channel" ??

huh?  What does that even mean?  Is something odd happening such as "ClearType"
style rendering?

[Chris Dalton, 2015/04/17 08:37:19]

Yeah, "RGB coverage" in the context of GrXferProcessor means "ClearType" style
rendering, where each color channel has a unique coverage.

+
+      fc*Sca / fa*Sa == fc/fa * Sc != Sc  [fc!=fa]
+
+    The only time it is still true is when Sc == 0.

[Chris Dalton, 2015/03/31 10:51:49]

This is most likely wrong. I'll update the comment.

Question: With RGB coverage is what is the alpha channel's coverage? Is it 1? Or
is it something like the pixel-wide coverage?

+
+    (It would also work out if we could know the dst color was trans black.)

[Mark Kilgard, 2015/04/03 18:30:03]

I'm assume by "trans black" you mean transparent black or (0,0,0,0) for [Dca,Da]

FYI, Adobe uses the helpful phrasing "cleared to glass" for this condition.

+   */
+
+    return kRGB_GrColorComponentFlags == (knownColorFlags & kRGB_GrColorComponentFlags) &&
+           0 == (knownColor & ~(0xffu << GrColor_SHIFT_A)); // RGB=0

[Mark Kilgard, 2015/04/03 18:30:03]

Is there not some constant to use instead of 0xffu?

Might be GrColor_ONE or such?

I'm assuming this expression is something about coverage not being unique by
channel but I don't really understand the motivation for such a situation in
Skia.

How often is this expression false?  Rarely I assume?

[Chris Dalton, 2015/04/17 08:37:19]

That code is completely gone now.

+}
+
+bool GrAdvancedEquationXPFactory::willBlendCoherently(const GrDrawTargetCaps& caps) const {
+    return GrDrawTargetCaps::kAdvancedCoherent_BlendEquationSupport == caps.blendEquationSupport();
+}
+
+GrXferProcessor* GrAdvancedEquationXPFactory::onCreateXferProcessor(
+                                                        const GrDrawTargetCaps& caps,
+                                                        const GrProcOptInfo& colorPOI,
+                                                        const GrProcOptInfo& coveragePOI,

[Mark Kilgard, 2015/04/03 18:30:03]

wish I knew what "XP" stands for

can I assume POI is "point of interest"?

[Chris Dalton, 2015/04/17 08:37:19]

XP = Xfer Processor = An object that controls the end of the fragment shader +
GL blend state to do blending.

POI = ProcOptInfo = Invariant data about the color/coverage outputs that can be
used to make optimizations in the shader.

+                                                        const GrDeviceCoordTexture* dstCopy) const {
+    SkASSERT(caps.advancedBlendEquationSupport());
+    SkASSERT(!dstCopy || !dstCopy->texture());
+
+    if (coveragePOI.isFourChannelOutput() && !this->supportsRGBCoverage(colorPOI.color(),
+                                                                        colorPOI.validFlags())) {
+        return NULL;
+    }
+    if (!coveragePOI.isSolidWhite()) {
+        return SkRef(&fXferProcessorWithCoverage);
+    }
+    return SkRef(&fXferProcessor);
+}
+
+void GrAdvancedEquationXPFactory::XferProcessorBase::onGetGLProcessorKey(const GrGLCaps&,
+                                                                     GrProcessorKeyBuilder*) const {
+    // Our XP's don't emit variable fragment code. (HasCoverage is accounted for by the class ID.)
+    return;
+}
+
+template</*HasCoverage = true*/>
+GrGLXferProcessor* GrAdvancedEquationXPFactory::XferProcessor<true>::createGLInstance() const {
+    class GLXferProcessorWithCoverage : public GrGLXferProcessor {
+    private:
+        void onEmitCode(const EmitArgs& args) override {
+            // We do coverage modulation by multiplying it into the src color before blending.
+            // (See canTweakAlphaForCoverage())
+            GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+            fsBuilder->codeAppendf("%s = %s * %s;",
+                                   args.fOutputPrimary, args.fInputColor, args.fInputCoverage);
+        }
+        void onSetData(const GrGLProgramDataManager&, const GrXferProcessor&) override {}
+    };
+    return SkNEW(GLXferProcessorWithCoverage);
+}
+
+template</*HasCoverage = false*/>
+GrGLXferProcessor* GrAdvancedEquationXPFactory::XferProcessor<false>::createGLInstance() const {
+    class GLXferProcessor : public GrGLXferProcessor {
+    private:
+        void onEmitCode(const EmitArgs& args) override {
+            GrGLFPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+            fsBuilder->codeAppendf("%s = %s;", args.fOutputPrimary, args.fInputColor);
+        }
+        void onSetData(const GrGLProgramDataManager&, const GrXferProcessor&) override {}
+    };
+    return SkNEW(GLXferProcessor);
+}
+
+void GrAdvancedEquationXPFactory::XferProcessorBase::onGetBlendInfo(BlendInfo* blendInfo) const {
+    SkASSERT(GrBlendEquationIsAdvanced(fEquation));
+    blendInfo->fEquation = fEquation;
+}
+
+
+GR_DEFINE_XP_FACTORY_TEST(GrAdvancedEquationXPFactory);
+
+GrXPFactory* GrAdvancedEquationXPFactory::TestCreate(SkRandom* random,
+                                                     GrContext* context,
+                                                     const GrDrawTargetCaps& caps,
+                                                     GrTexture*[]) {
+    int mode = random->nextRangeU(SkXfermode::kLastCoeffMode, SkXfermode::kLastMode);
+    if (mode == SkXfermode::kLastCoeffMode) {
+        mode = SkXfermode::kScreen_Mode;
+    }
+    return GrAdvancedEquationXPFactory::Create(context, static_cast<SkXfermode::Mode>(mode));
+}