Implement support for KHR_blend_equation_advanced

src/gpu/effects/GrCustomXfermode.cpp

Index: src/gpu/effects/GrCustomXfermode.cpp
diff --git a/src/gpu/effects/GrCustomXfermode.cpp b/src/gpu/effects/GrCustomXfermode.cpp
index 2f8a970290fc02aecad7c8167ecbffc249f1e7aa..b15e2c2b9a133ad47bae7cd672a65bec36a2fbee 100644
--- a/src/gpu/effects/GrCustomXfermode.cpp
+++ b/src/gpu/effects/GrCustomXfermode.cpp
@@ -17,6 +17,7 @@
 #include "GrTextureAccess.h"
 #include "SkXfermode.h"
 #include "gl/GrGLCaps.h"
+#include "gl/GrGLGpu.h"
 #include "gl/GrGLProcessor.h"
 #include "gl/GrGLProgramDataManager.h"
 #include "gl/builders/GrGLProgramBuilder.h"
@@ -29,6 +30,27 @@ bool GrCustomXfermode::IsSupportedMode(SkXfermode::Mode mode) {
 // Static helpers
 ///////////////////////////////////////////////////////////////////////////////
 
+static GrBlendEquation hw_blend_equation(SkXfermode::Mode mode) {
+    enum { kOffset = kOverlay_GrBlendEquation - SkXfermode::kOverlay_Mode };
+    return static_cast<GrBlendEquation>(mode + kOffset);
+
+    GR_STATIC_ASSERT(kOverlay_GrBlendEquation == SkXfermode::kOverlay_Mode + kOffset);
+    GR_STATIC_ASSERT(kDarken_GrBlendEquation == SkXfermode::kDarken_Mode + kOffset);
+    GR_STATIC_ASSERT(kLighten_GrBlendEquation == SkXfermode::kLighten_Mode + kOffset);
+    GR_STATIC_ASSERT(kColorDodge_GrBlendEquation == SkXfermode::kColorDodge_Mode + kOffset);
+    GR_STATIC_ASSERT(kColorBurn_GrBlendEquation == SkXfermode::kColorBurn_Mode + kOffset);
+    GR_STATIC_ASSERT(kHardLight_GrBlendEquation == SkXfermode::kHardLight_Mode + kOffset);
+    GR_STATIC_ASSERT(kSoftLight_GrBlendEquation == SkXfermode::kSoftLight_Mode + kOffset);
+    GR_STATIC_ASSERT(kDifference_GrBlendEquation == SkXfermode::kDifference_Mode + kOffset);
+    GR_STATIC_ASSERT(kExclusion_GrBlendEquation == SkXfermode::kExclusion_Mode + kOffset);
+    GR_STATIC_ASSERT(kMultiply_GrBlendEquation == SkXfermode::kMultiply_Mode + kOffset);
+    GR_STATIC_ASSERT(kHSLHue_GrBlendEquation == SkXfermode::kHue_Mode + kOffset);
+    GR_STATIC_ASSERT(kHSLSaturation_GrBlendEquation == SkXfermode::kSaturation_Mode + kOffset);
+    GR_STATIC_ASSERT(kHSLColor_GrBlendEquation == SkXfermode::kColor_Mode + kOffset);
+    GR_STATIC_ASSERT(kHSLLuminosity_GrBlendEquation == SkXfermode::kLuminosity_Mode + kOffset);
+    GR_STATIC_ASSERT(kTotalGrBlendEquationCount == SkXfermode::kLastMode + 1 + kOffset);
+}
+
 static void hard_light(GrGLFragmentBuilder* fsBuilder,
                        const char* final,
                        const char* src,
@@ -510,15 +532,30 @@ public:
                                                const GrDrawTargetCaps& caps) override;
 
     SkXfermode::Mode mode() const { return fMode; }
+    bool hasCoverage() const { return fHasCoverage; }
+    bool hasHWBlendEquation() const { return kInvalid_GrBlendEquation != fHWBlendEquation; }
+
+    GrBlendEquation hwBlendEquation() const {
+        SkASSERT(this->hasHWBlendEquation());
+        return fHWBlendEquation;
+    }
 
 private:
     CustomXP(SkXfermode::Mode mode, const GrDeviceCoordTexture* dstCopy, bool willReadDstColor);
 
     void onGetGLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) const override;
 
+    bool onWillNeedXferBarrier(const GrRenderTarget* rt,
+                               const GrDrawTargetCaps& caps,
+                               GrXferBarrierType* outBarrierType) const override;
+
+    void onGetBlendInfo(BlendInfo*) const override;
+
     bool onIsEqual(const GrXferProcessor& xpBase) const override;
 
     SkXfermode::Mode fMode;
+    bool             fHasCoverage;
+    GrBlendEquation  fHWBlendEquation;
 
     typedef GrXferProcessor INHERITED;
 };
@@ -540,24 +577,48 @@ public:
     GLCustomXP(const GrXferProcessor&) {}
     ~GLCustomXP() override {}
 
-    static void GenKey(const GrXferProcessor& proc, const GrGLSLCaps&, GrProcessorKeyBuilder* b) {
-        uint32_t key = proc.numTextures();
+    static void GenKey(const GrXferProcessor& p, const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) {
+        const CustomXP& xp = p.cast<CustomXP>();
+        uint32_t key = xp.numTextures();
         SkASSERT(key <= 1);
-        key |= proc.cast<CustomXP>().mode() << 1;
+        key |= xp.hasCoverage() << 1;
+        if (xp.hasHWBlendEquation()) {
+            SkASSERT(caps.advBlendEqInteraction() > 0);  // 0 will mean !xp.hasHWBlendEquation().
+            key |= caps.advBlendEqInteraction() << 2;
+        }
+        if (!xp.hasHWBlendEquation() || caps.mustEnableSpecificAdvBlendEqs()) {
+            GR_STATIC_ASSERT(GrGLSLCaps::kLast_AdvBlendEqInteraction < 4);
+            key |= xp.mode() << 4;
+        }
         b->add32(key);
     }
 
 private:
     void onEmitCode(const EmitArgs& args) override {
-        SkXfermode::Mode mode = args.fXP.cast<CustomXP>().mode();
+        const CustomXP& xp = args.fXP.cast<CustomXP>();
         GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
-        const char* dstColor = fsBuilder->dstColor();
 
-        emit_custom_xfermode_code(mode, fsBuilder, args.fOutputPrimary, args.fInputColor, dstColor);
-
-        fsBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
-                               args.fOutputPrimary, args.fOutputPrimary, args.fInputCoverage,
-                               args.fInputCoverage, dstColor);
+        if (xp.hasHWBlendEquation()) {
+            // The blend mode will be implemented in hardware; only output the src color.
+            fsBuilder->enableAdvancedBlendEquationIfNeeded(xp.hwBlendEquation());
+            if (xp.hasCoverage()) {
+                // Do coverage modulation by multiplying it into the src color before blending.
+                // (See getOptimizations())
+                fsBuilder->codeAppendf("%s = %s * %s;",
+                                       args.fOutputPrimary, args.fInputCoverage, args.fInputColor);
+            } else {
+                fsBuilder->codeAppendf("%s = %s;", args.fOutputPrimary, args.fInputColor);
+            }
+        } else {
+            const char* dstColor = fsBuilder->dstColor();
+            emit_custom_xfermode_code(xp.mode(), fsBuilder, args.fOutputPrimary, args.fInputColor,
+                                      dstColor);
+            if (xp.hasCoverage()) {
+                fsBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
+                                       args.fOutputPrimary, args.fOutputPrimary,
+                                       args.fInputCoverage, args.fInputCoverage, dstColor);
+            }
+        }
     }
 
     void onSetData(const GrGLProgramDataManager&, const GrXferProcessor&) override {}
@@ -569,7 +630,10 @@ private:
 
 CustomXP::CustomXP(SkXfermode::Mode mode, const GrDeviceCoordTexture* dstCopy,
                    bool willReadDstColor)
-    : INHERITED(dstCopy, willReadDstColor), fMode(mode) {
+    : INHERITED(dstCopy, willReadDstColor),
+      fMode(mode),
+      fHasCoverage(true),
+      fHWBlendEquation(kInvalid_GrBlendEquation) {
     this->initClassID<CustomXP>();
 }
 
@@ -578,12 +642,15 @@ void CustomXP::onGetGLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder
 }
 
 GrGLXferProcessor* CustomXP::createGLInstance() const {
+    SkASSERT(this->willReadDstColor() != this->hasHWBlendEquation());
     return SkNEW_ARGS(GLCustomXP, (*this));
 }
 
 bool CustomXP::onIsEqual(const GrXferProcessor& other) const {
     const CustomXP& s = other.cast<CustomXP>();
-    return fMode == s.fMode;
+    return fMode == s.fMode &&
+           fHasCoverage == s.fHasCoverage &&
+           fHWBlendEquation == s.fHWBlendEquation;
 }
 
 GrXferProcessor::OptFlags CustomXP::getOptimizations(const GrProcOptInfo& colorPOI,
@@ -591,7 +658,131 @@ GrXferProcessor::OptFlags CustomXP::getOptimizations(const GrProcOptInfo& colorP
                                                        bool doesStencilWrite,
                                                        GrColor* overrideColor,
                                                        const GrDrawTargetCaps& caps) {
-   return GrXferProcessor::kNone_Opt;
+  /*
+    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_Opt;
+    if (colorPOI.allStagesMultiplyInput()) {
+        flags = flags | kCanTweakAlphaForCoverage_OptFlag;
+    }
+    if (coveragePOI.isSolidWhite()) {
+        flags = flags | kIgnoreCoverage_OptFlag;
+        fHasCoverage = false;
+    }
+    if (caps.advancedBlendEquationSupport() && !coveragePOI.isFourChannelOutput()) {
+        // This blend mode can be implemented in hardware.
+        fHWBlendEquation = hw_blend_equation(fMode);
+    }
+    return flags;
+}
+
+bool CustomXP::onWillNeedXferBarrier(const GrRenderTarget* rt,
+                                     const GrDrawTargetCaps& caps,
+                                     GrXferBarrierType* outBarrierType) const {
+    if (this->hasHWBlendEquation() && !caps.advancedCoherentBlendEquationSupport()) {
+        *outBarrierType = kBlend_GrXferBarrierType;
+        return true;
+    }
+    return false;
+}
+
+void CustomXP::onGetBlendInfo(BlendInfo* blendInfo) const {
+    if (this->hasHWBlendEquation()) {
+        blendInfo->fEquation = this->hwBlendEquation();
+    }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -609,6 +800,19 @@ GrCustomXPFactory::onCreateXferProcessor(const GrDrawTargetCaps& caps,
     return CustomXP::Create(fMode, dstCopy, this->willReadDstColor(caps, colorPOI, coveragePOI));
 }
 
+bool GrCustomXPFactory::willReadDstColor(const GrDrawTargetCaps& caps,
+                                         const GrProcOptInfo& colorPOI,
+                                         const GrProcOptInfo& coveragePOI) const {
+    if (!caps.advancedBlendEquationSupport()) {
+        // No hardware support for advanced blend equations; we will need to do it in the shader.
+        return true;
+    }
+    if (coveragePOI.isFourChannelOutput()) {
+        // Advanced blend equations can't tweak alpha for RGB coverage.
+        return true;
+    }
+    return false;
+}
 
 void GrCustomXPFactory::getInvariantOutput(const GrProcOptInfo& colorPOI,
                                                const GrProcOptInfo& coveragePOI,