1 files changed, 85 insertions, 0 deletions
diff --git a/src/gpu/effects/GrDitherEffect.fp b/src/gpu/effects/GrDitherEffect.fp
new file mode 100644
index 0000000000..ed6c0e6b70
--- /dev/null
+++ b/src/gpu/effects/GrDitherEffect.fp
@@ -0,0 +1,85 @@
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+// This controls the range of values added to color channels
+layout(key) in int rangeType;
+
+@make {
+    static std::unique_ptr<GrFragmentProcessor> Make(GrPixelConfig dstConfig) {
+        int rangeType;
+        switch (dstConfig) {
+            case kGray_8_GrPixelConfig:
+            case kGray_8_as_Lum_GrPixelConfig:
+            case kGray_8_as_Red_GrPixelConfig:
+            case kRGBA_8888_GrPixelConfig:
+            case kRGB_888_GrPixelConfig:
+            case kBGRA_8888_GrPixelConfig:
+                rangeType = 0;
+                break;
+            case kRGB_565_GrPixelConfig:
+                rangeType = 1;
+                break;
+            case kRGBA_4444_GrPixelConfig:
+                rangeType = 2;
+                break;
+            case kUnknown_GrPixelConfig:
+            case kSRGBA_8888_GrPixelConfig:
+            case kSBGRA_8888_GrPixelConfig:
+            case kRGBA_1010102_GrPixelConfig:
+            case kAlpha_half_GrPixelConfig:
+            case kAlpha_half_as_Red_GrPixelConfig:
+            case kRGBA_float_GrPixelConfig:
+            case kRG_float_GrPixelConfig:
+            case kRGBA_half_GrPixelConfig:
+            case kAlpha_8_GrPixelConfig:
+            case kAlpha_8_as_Alpha_GrPixelConfig:
+            case kAlpha_8_as_Red_GrPixelConfig:
+                return nullptr;
+        }
+        return std::unique_ptr<GrFragmentProcessor>(new GrDitherEffect(rangeType));
+    }
+}
+
+void main() {
+    half value;
+    half range;
+    @switch (rangeType) {
+        case 0:
+            range = 1.0 / 255.0;
+            break;
+        case 1:
+            range = 1.0 / 63.0;
+            break;
+        default:
+            // Experimentally this looks better than the expected value of 1/15.
+            range = 1.0 / 15.0;
+            break;
+    }
+    @if (sk_Caps.integerSupport) {
+        // This ordered-dither code is lifted from the cpu backend.
+        uint x = uint(sk_FragCoord.x);
+        uint y = uint(sk_FragCoord.y);
+        uint m = (y & 1) << 5 | (x & 1) << 4 |
+                 (y & 2) << 2 | (x & 2) << 1 |
+                 (y & 4) >> 1 | (x & 4) >> 2;
+        value = half(m) * 1.0 / 64.0 - 63.0 / 128.0;
+    } else {
+        // Simulate the integer effect used above using step/mod. For speed, simulates a 4x4
+        // dither pattern rather than an 8x8 one.
+        half4 modValues = mod(sk_FragCoord.xyxy, half4(2.0, 2.0, 4.0, 4.0));
+        half4 stepValues = step(modValues, half4(1.0, 1.0, 2.0, 2.0));
+        value = dot(stepValues, half4(8.0 / 16.0, 4.0 / 16.0, 2.0 / 16.0, 1.0 / 16.0)) - 15.0 / 32.0;
+    }
+    // For each color channel, add the random offset to the channel value and then clamp
+    // between 0 and alpha to keep the color premultiplied.
+    sk_OutColor = half4(clamp(sk_InColor.rgb + value * range, 0, sk_InColor.a), sk_InColor.a);
+}
+
+@test(testData) {
+    float range = testData->fRandom->nextRangeF(0.001f, 0.05f);
+    return std::unique_ptr<GrFragmentProcessor>(new GrDitherEffect(range));
+}