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 c4720b58c85ade590aa0ad252e8b0f1da999cba2..f2f04017f5e470609952f5d807cb4b40cd13f20c 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
@@ -458,6 +459,30 @@ void SkPaint2GrPaintNoShader(GrContext* context, const SkPaint& skPaint, GrColor
             }
         }
     }
+
+#ifndef SK_IGNORE_GPU_DITHER
+    // 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) {

[bsalomon, 2014/06/12 16:24:11]

I'd like to understand how this happens. Falling back to the draw state's RT
doesn't seem right.

+            target = context->getGpu()->getDrawState().getRenderTarget();
+        }
+
+        if (NULL != target &&
+            (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);
+            }
+        }
+    }
+#endif
 }
 
 /**