Simple GPU based dithering

src/gpu/SkGr.cpp

Index: src/gpu/SkGr.cpp
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index fb2ebdd53fcbbf70c1ed93568f14ed3526b7bce8..c566d6ffaf7b48fd6d48eadda680d2e0c4050f3f 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -13,6 +13,7 @@
 #include "SkPixelRef.h"
 #include "GrResourceCache.h"
 #include "GrGpu.h"
+#include "effects/GrDitherEffect.h"
 #include "GrDrawTargetCaps.h"
 
 #ifndef SK_IGNORE_ETC1_SUPPORT
@@ -465,6 +466,28 @@ void SkPaint2GrPaintNoShader(GrContext* context, const SkPaint& skPaint, bool ju
             }
         }
     }
+

[robertphillips, 2014/06/11 17:20:24]

Wrap this in SK_IGNORE_GPU_DITHER and add it to
gyp/skia_for_chromium_defines.gypi? This will allow us to land this and roll it
into Chromium without having to worry about layout test regressions. Once it is
rolled we can rebaseline the Blink layout tests and remove the flag.

[krajcevski, 2014/06/11 22:33:18]

Done.

+    // If the dither flag is set, then we need to see if the underlying context
+    // supports it. If not, then install a dither effect.
+    if (skPaint.isDither()) {
+        // What are we rendering into?
+        const GrRenderTarget *target = context->getRenderTarget();
+        if (NULL == target) {
+            target = context->getGpu()->getDrawState().getRenderTarget();
+        }
+        SkASSERT(NULL != target);
+
+        if (target->config() == kRGBA_8888_GrPixelConfig ||
+            target->config() == kBGRA_8888_GrPixelConfig) {
+            // The dither flag is set and the target is likely
+            // not going to be dithered by the GPU.
+            SkAutoTUnref<GrEffectRef> effect(GrDitherEffect::Create());
+            if (NULL != effect.get()) {
+                grPaint->addColorEffect(effect);
+                grPaint->setDither(false);
+            }
+        }
+    }
 }
 
 /**