Apply coverage in XP base class when using dst reads

src/gpu/gl/GrGLXferProcessor.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gl/GrGLXferProcessor.h"
 
 #include "GrXferProcessor.h"
 #include "gl/builders/GrGLFragmentShaderBuilder.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 
 void GrGLXferProcessor::emitCode(const EmitArgs& args) {
+    if (!args.fXP.willReadDstColor()) {
+        this->emitOutputsForBlendState(args);
+        return;
+    }
+
+    GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
+    const char* dstColor = fsBuilder->dstColor();
+
     if (args.fXP.getDstTexture()) {
         bool topDown = kTopLeft_GrSurfaceOrigin == args.fXP.getDstTexture()->origin();
 
-        GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
-
         if (args.fXP.readsCoverage()) {
             // We don't think any shaders actually output negative coverage, but just as a safety
             // check for floating point precision errors we compare with <= here
             fsBuilder->codeAppendf("if (all(lessThanEqual(%s, vec4(0)))) {"
                                    "    discard;"
                                    "}", args.fInputCoverage);
         }
 
-        const char* dstColor = fsBuilder->dstColor();
-
         const char* dstTopLeftName;
         const char* dstCoordScaleName;
 
         fDstTopLeftUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
                                               kVec2f_GrSLType,
                                               kDefault_GrSLPrecision,
                                               "DstTextureUpperLeft",
                                               &dstTopLeftName);
         fDstScaleUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
                                             kVec2f_GrSLType,
                                             kDefault_GrSLPrecision,
                                             "DstTextureCoordScale",
                                             &dstCoordScaleName);
         const char* fragPos = fsBuilder->fragmentPosition();
 
         fsBuilder->codeAppend("// Read color from copy of the destination.\n");
         fsBuilder->codeAppendf("vec2 _dstTexCoord = (%s.xy - %s) * %s;",
                                fragPos, dstTopLeftName, dstCoordScaleName);
 
         if (!topDown) {
             fsBuilder->codeAppend("_dstTexCoord.y = 1.0 - _dstTexCoord.y;");
         }
 
         fsBuilder->codeAppendf("vec4 %s = ", dstColor);
         fsBuilder->appendTextureLookup(args.fSamplers[0], "_dstTexCoord", kVec2f_GrSLType);
         fsBuilder->codeAppend(";");
     }
 
-    this->onEmitCode(args);
+    this->emitBlendCodeForDstRead(args.fPB, args.fInputColor, dstColor, args.fOutputPrimary,
+                                  args.fXP);
+
+    // Apply coverage.
+    if (args.fXP.readsCoverage()) {
+        fsBuilder->codeAppendf("%s = %s * %s + (vec4(1.0) - %s) * %s;",
+                               args.fOutputPrimary, args.fInputCoverage,
+                               args.fOutputPrimary, args.fInputCoverage, dstColor);
+    }
 }
 
 void GrGLXferProcessor::setData(const GrGLProgramDataManager& pdm, const GrXferProcessor& xp) {
     if (xp.getDstTexture()) {
         if (fDstTopLeftUni.isValid()) {
             pdm.set2f(fDstTopLeftUni, static_cast<GrGLfloat>(xp.dstTextureOffset().fX),
                       static_cast<GrGLfloat>(xp.dstTextureOffset().fY));
             pdm.set2f(fDstScaleUni, 1.f / xp.getDstTexture()->width(),
                       1.f / xp.getDstTexture()->height());
         } else {
             SkASSERT(!fDstScaleUni.isValid());
         }
     } else {
         SkASSERT(!fDstTopLeftUni.isValid());
         SkASSERT(!fDstScaleUni.isValid());
     }
     this->onSetData(pdm, xp);
 }