Simple GPU based dithering

src/gpu/effects/GrDitherEffect.cpp

Index: src/gpu/effects/GrDitherEffect.cpp
diff --git a/src/gpu/effects/GrDitherEffect.cpp b/src/gpu/effects/GrDitherEffect.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..be226570fab1b9542b5d8fe9390aaad26e0e6d6a
--- /dev/null
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -0,0 +1,115 @@
+/*
+ * 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 "GrDitherEffect.h"
+
+#include "gl/GrGLEffect.h"
+#include "gl/GrGLSL.h"
+#include "GrTBackendEffectFactory.h"
+
+#include "SkRect.h"
+
+//////////////////////////////////////////////////////////////////////////////
+
+class GLDitherEffect;
+
+class DitherEffect : public GrEffect {
+public:
+    static GrEffectRef* Create() {
+        return CreateEffectRef(AutoEffectUnref(SkNEW(DitherEffect)));
+    }
+
+    virtual ~DitherEffect() {};
+    static const char* Name() { return "Dither"; }
+
+    typedef GLDitherEffect GLEffect;
+
+    virtual void getConstantColorComponents(GrColor* color, uint32_t* validFlags) const SK_OVERRIDE;
+
+    virtual const GrBackendEffectFactory& getFactory() const SK_OVERRIDE {
+        return GrTBackendEffectFactory<DitherEffect>::getInstance();
+    }
+
+private:
+    DitherEffect() {
+        this->setWillReadFragmentPosition();
+    }
+
+    // All dither effects are equal
+    virtual bool onIsEqual(const GrEffect&) const SK_OVERRIDE { return true; }
+
+    GR_DECLARE_EFFECT_TEST;
+
+    typedef GrEffect INHERITED;
+};
+
+void DitherEffect::getConstantColorComponents(GrColor* color, uint32_t* validFlags) const {
+    *validFlags = 0;
+}
+
+//////////////////////////////////////////////////////////////////////////////
+
+GR_DEFINE_EFFECT_TEST(DitherEffect);
+
+GrEffectRef* DitherEffect::TestCreate(SkRandom*,
+                                      GrContext*,
+                                      const GrDrawTargetCaps&,
+                                      GrTexture*[]) {
+    return DitherEffect::Create();
+}
+
+//////////////////////////////////////////////////////////////////////////////
+
+class GLDitherEffect : public GrGLEffect {
+public:
+    GLDitherEffect(const GrBackendEffectFactory&, const GrDrawEffect&);
+
+    virtual void emitCode(GrGLShaderBuilder* builder,
+                          const GrDrawEffect& drawEffect,
+                          EffectKey key,
+                          const char* outputColor,
+                          const char* inputColor,
+                          const TransformedCoordsArray&,
+                          const TextureSamplerArray&) SK_OVERRIDE;
+
+private:
+    typedef GrGLEffect INHERITED;
+};
+
+GLDitherEffect::GLDitherEffect(const GrBackendEffectFactory& factory,
+                               const GrDrawEffect& drawEffect)
+    : INHERITED (factory) {
+}
+
+void GLDitherEffect::emitCode(GrGLShaderBuilder* builder,
+                              const GrDrawEffect& drawEffect,
+                              EffectKey key,
+                              const char* outputColor,
+                              const char* inputColor,
+                              const TransformedCoordsArray&,
+                              const TextureSamplerArray& samplers) {
+    // Generate a random number based on the fragment position. For this
+    // random number generator, we use the "GLSL rand" function
+    // that seems to be floating around on the internet. It works under
+    // the assumption that sin(<big number>) oscillates with high frequency
+    // and sampling it will generate "randomness". Since we're using this
+    // for rendering and not cryptography it should be OK.
+
+    // For each channel c, add the random offset to the pixel to either bump
+    // it up or let it remain constant during quantization.
+    builder->fsCodeAppendf("\t\tfloat r = "
+                           "fract(sin(dot(%s.xy ,vec2(12.9898,78.233))) * 43758.5453);\n",
+                           builder->fragmentPosition());
+    builder->fsCodeAppendf("\t\t%s = (1.0f/255.0f) * vec4(r, r, r, r) + %s;\n",
+                           outputColor, GrGLSLExpr4(inputColor).c_str());
+}
+
+//////////////////////////////////////////////////////////////////////////////
+
+GrEffectRef* GrDitherEffect::Create() {
+    return DitherEffect::Create();
+}