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..002a60865c2a82eea335eb8e2822ddad3c7d9b42
--- /dev/null
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -0,0 +1,126 @@
+/*
+ * 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) {
+}
+

[robertphillips, 2014/06/11 14:15:50]

// for each channel c:
// 1. Compute quantized colors [low, high] that c is between
// 2. Pick high by flipping a coin weighted by (c - low)

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

Done.

+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.
+    builder->fsCodeAppendf("\t\tfloat r = "

[robertphillips, 2014/06/11 14:14:32]

magic numbers?

[egdaniel, 2014/06/11 14:26:24]

Probably should add a comment to describe it, but this one line rand number
generator is often used in glsl shaders. Basically it creates a zig zag function
then the sin adds some extra noise all the zig zags (if I remember right). But
yeah at least some explanation should be added. 

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

I added a comment for this.

+                           "fract(sin(dot(%s.xy ,vec2(12.9898,78.233))) * 43758.5453);\n",
+                           builder->fragmentPosition());
+
+    // For each channel, weight the neighboring pixel values based on thier
+    // distance from the input channel

[robertphillips, 2014/06/11 14:14:32]

kSuffixes ?

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

Done.

+    static const char *suffixes[4] = { "r", "g", "b", "a" };
+    for (int i = 0; i < 4; ++i) {
+        builder->fsCodeAppendf("\t\tfloat dithered_%s;\n", suffixes[i]);
+        builder->fsCodeAppendf("\t\t{\n");
+        builder->fsCodeAppendf("\t\t\tfloat channel = 255.0f * %s.%s;\n",
+                               inputColor, suffixes[i]);
+        builder->fsCodeAppendf("\t\t\tfloat low = floor(channel);\n");
+        builder->fsCodeAppendf("\t\t\tfloat c = channel-low;\n");
+        builder->fsCodeAppendf("\t\t\tfloat high = low + 1;\n");
+        builder->fsCodeAppendf("\t\t\tfloat t = float(c > r);\n");
+        builder->fsCodeAppendf("\t\t\tdithered_%s = (low*(1-t) + high*t)/255.0f;\n", suffixes[i]);
+        builder->fsCodeAppendf("\t\t}\n");
+    }
+
+    builder->fsCodeAppendf("\t\t%s = vec4(", outputColor);
+    for (int i = 0; i < 4; ++i) {
+        builder->fsCodeAppendf("dithered_%s%s", suffixes[i], (i < 3)? ", " : ");\n");
+    }
+}
+
+//////////////////////////////////////////////////////////////////////////////
+
+GrEffectRef* GrDitherEffect::Create() {
+    return DitherEffect::Create();
+}