Add support for reading the dst pixel value in an effect. Use in a new effect for the kDarken xfer …

src/gpu/gl/GrGLProgramDesc.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrGLProgramDesc.h"
 #include "GrBackendEffectFactory.h"
 #include "GrDrawEffect.h"
 #include "GrEffect.h"
+#include "GrGLShaderBuilder.h"
 #include "GrGpuGL.h"
 
 void GrGLProgramDesc::Build(const GrDrawState& drawState,
                             bool isPoints,
                             GrDrawState::BlendOptFlags blendOpts,
                             GrBlendCoeff srcCoeff,
                             GrBlendCoeff dstCoeff,
                             const GrGpuGL* gpu,
+                            const GrDeviceCoordTexture* dstCopy,
                             GrGLProgramDesc* desc) {
 
     // This should already have been caught
     GrAssert(!(GrDrawState::kSkipDraw_BlendOptFlag & blendOpts));
 
     bool skipCoverage = SkToBool(blendOpts & GrDrawState::kEmitTransBlack_BlendOptFlag);
 
     bool skipColor = SkToBool(blendOpts & (GrDrawState::kEmitTransBlack_BlendOptFlag |
                                            GrDrawState::kEmitCoverage_BlendOptFlag));
 
@@ skipping 43 matching lines @@
     if (skipCoverage) {
         desc->fCoverageInput = kTransBlack_ColorInput;
     } else if (covIsSolidWhite) {
         desc->fCoverageInput = kSolidWhite_ColorInput;
     } else if (GR_GL_NO_CONSTANT_ATTRIBUTES && !requiresAttributeCoverage) {
         desc->fCoverageInput = kUniform_ColorInput;
     } else {
         desc->fCoverageInput = kAttribute_ColorInput;
     }
 
+    bool readsDst = false;
     int lastEnabledStage = -1;
 
     for (int s = 0; s < GrDrawState::kNumStages; ++s) {
 
         bool skip = s < drawState.getFirstCoverageStage() ? skipColor : skipCoverage;
         if (!skip && drawState.isStageEnabled(s)) {
             lastEnabledStage = s;
             const GrEffectRef& effect = *drawState.getStage(s).getEffect();
             const GrBackendEffectFactory& factory = effect->getFactory();
             bool explicitLocalCoords = (drawState.getAttribBindings() &
                                         GrDrawState::kLocalCoords_AttribBindingsBit);
             GrDrawEffect drawEffect(drawState.getStage(s), explicitLocalCoords);
             desc->fEffectKeys[s] = factory.glEffectKey(drawEffect, gpu->glCaps());
+            if (effect->willReadDst()) {
+                readsDst = true;
+            }
         } else {
             desc->fEffectKeys[s] = 0;
         }
     }
 
+    if (readsDst) {
+        GrAssert(NULL != dstCopy);
+        desc->fDstRead = GrGLShaderBuilder::KeyForDstRead(dstCopy->texture(), gpu->glCaps());
+        GrAssert(0 != desc->fDstRead);
+    } else {
+        desc->fDstRead = 0;
+    }
+
     desc->fDualSrcOutput = kNone_DualSrcOutput;
 
     // Currently the experimental GS will only work with triangle prims (and it doesn't do anything
     // other than pass through values from the VS to the FS anyway).
 #if GR_GL_EXPERIMENTAL_GS
 #if 0
     desc->fExperimentalGS = gpu->caps().geometryShaderSupport();
 #else
     desc->fExperimentalGS = false;
 #endif
@@ skipping 77 matching lines @@
     if (requiresAttributeCoverage) {
         GrAssert(desc->fCoverageAttributeIndex < GrDrawState::kVertexAttribCnt);
         GrAssert(GrGLAttribTypeToLayout(vertexAttribs[desc->fCoverageAttributeIndex].fType).fCount == 4);
     }
     if (desc->fAttribBindings & GrDrawState::kLocalCoords_AttribBindingsBit) {
         GrAssert(desc->fLocalCoordsAttributeIndex < GrDrawState::kVertexAttribCnt);
         GrAssert(GrGLAttribTypeToLayout(vertexAttribs[desc->fLocalCoordsAttributeIndex].fType).fCount == 2);
     }
 #endif
 }