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

src/gpu/SkGpuDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGpuDevice.h"
 
 #include "effects/GrTextureDomainEffect.h"
@@ skipping 475 matching lines @@
 // color once while converting to GrPaint and then ignored.
 inline bool skPaint2GrPaintNoShader(SkGpuDevice* dev,
                                     const SkPaint& skPaint,
                                     bool justAlpha,
                                     bool constantColor,
                                     GrPaint* grPaint) {
 
     grPaint->setDither(skPaint.isDither());
     grPaint->setAntiAlias(skPaint.isAntiAlias());
 
-    SkXfermode::Coeff sm = SkXfermode::kOne_Coeff;
-    SkXfermode::Coeff dm = SkXfermode::kISA_Coeff;
+    SkXfermode::Coeff sm;
+    SkXfermode::Coeff dm;
 
     SkXfermode* mode = skPaint.getXfermode();
     GrEffectRef* xferEffect = NULL;
-    if (SkXfermode::AsNewEffect(mode, dev->context(), &xferEffect, &sm, &dm)) {
-        SkSafeUnref(grPaint->colorStage(kXfermodeEffectIdx)->setEffect(xferEffect));
+    if (SkXfermode::AsNewEffectOrCoeff(mode, dev->context(), &xferEffect, &sm, &dm)) {
+        if (NULL != xferEffect) {
+            grPaint->colorStage(kXfermodeEffectIdx)->setEffect(xferEffect)->unref();
+            // This may not be the right place to have this logic but we set the GPU blend to
+            // src-over so that fractional coverage will be accounted for correctly.
+            sm = SkXfermode::kOne_Coeff;
+            dm = SkXfermode::kISA_Coeff;
+        }
     } else {
         //SkDEBUGCODE(SkDebugf("Unsupported xfer mode.\n");)
 #if 0
         return false;
+#else
+        // Fall back to src-over
+        sm = SkXfermode::kOne_Coeff;
+        dm = SkXfermode::kISA_Coeff;
 #endif
     }
     grPaint->setBlendFunc(sk_blend_to_grblend(sm), sk_blend_to_grblend(dm));
 
     if (justAlpha) {
         uint8_t alpha = skPaint.getAlpha();
         grPaint->setColor(GrColorPackRGBA(alpha, alpha, alpha, alpha));
         // justAlpha is currently set to true only if there is a texture,
         // so constantColor should not also be true.
         GrAssert(!constantColor);
@@ skipping 1331 matching lines @@
                          GrTexture* texture,
                          bool needClear)
     : SkDevice(make_bitmap(context, texture->asRenderTarget())) {
 
     GrAssert(texture && texture->asRenderTarget());
     // This constructor is called from onCreateCompatibleDevice. It has locked the RT in the texture
     // cache. We pass true for the third argument so that it will get unlocked.
     this->initFromRenderTarget(context, texture->asRenderTarget(), true);
     fNeedClear = needClear;
 }