Revert "Improvements to GPU dither."

This reverts commit 1806e33e6a09b1361de4925d32389a01e0e2c7e7.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Improvements to GPU dither.
> 
> 1) Makes the range of the offset dependent on the config.
> 
> 2) Uses an ordered dither on GPUs that support integers in shaders.
> 
> 3) Enables dithering for all paints with dither flag when the color type of the dst is 4444
> 
> 4) Dithers r,g,b and clamps to 0,a rather than dithering all four channels (same as CPU backend).
> 
> Bug: skia:
> Change-Id: Ie22c3adc38c6d1dbbcd97e4b7d16fc843e392c2e
> Reviewed-on: https://skia-review.googlesource.com/23485
> Commit-Queue: Brian Salomon <bsalomon@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>

TBR=bsalomon@google.com,robertphillips@google.com

Change-Id: Ie82e88bd9032bf8eee745d32d9b57c335a8997c9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/24325
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

5 files changed, 22 insertions, 113 deletions

diff --git a/src/core/SkPaintPriv.cpp b/src/core/SkPaintPriv.cpp
index 7d771f88a6..dcd8dd6a5a 100644
--- a/src/core/SkPaintPriv.cpp
+++ b/src/core/SkPaintPriv.cpp
@@ -75,8 +75,8 @@ bool SkPaintPriv::ShouldDither(const SkPaint& p, SkColorType dstCT) {
         return false;
     }
 
-    // We always dither 565 or 4444 when requested.
-    if (dstCT == kRGB_565_SkColorType || dstCT == kARGB_4444_SkColorType) {
+    // We always dither 565 when requested.
+    if (dstCT == SkColorType::kRGB_565_SkColorType) {
         return true;
     }
 
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index 6d8e46ddca..b37a607052 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -535,10 +535,7 @@ static inline bool skpaint_to_grpaint_impl(GrContext* context,
     GrPixelConfigToColorType(rtc->config(), &ct);
     if (SkPaintPriv::ShouldDither(skPaint, ct) && grPaint->numColorFragmentProcessors() > 0
         && !rtc->isGammaCorrect()) {
-        auto ditherFP = GrDitherEffect::Make(rtc->config());
-        if (ditherFP) {
-            grPaint->addColorFragmentProcessor(std::move(ditherFP));
-        }
+        grPaint->addColorFragmentProcessor(GrDitherEffect::Make());
     }
 #endif
     return true;
diff --git a/src/gpu/effects/GrDitherEffect.cpp b/src/gpu/effects/GrDitherEffect.cpp
index 753bbce185..052ed4e36d 100644
--- a/src/gpu/effects/GrDitherEffect.cpp
+++ b/src/gpu/effects/GrDitherEffect.cpp
@@ -23,7 +23,7 @@ public:
         GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
         const GrDitherEffect& _outer = args.fFp.cast<GrDitherEffect>();
         (void) _outer;
-        fragBuilder->codeAppendf("float value;\nfloat range;\n@switch (%d) {\n    case 0:\n        range = 0.0039215686274509803;\n        break;\n    case 1:\n        range = 0.015873015873015872;\n        break;\n    default:\n        range = 0.0083333333333333332;\n        break;\n}\n@if (sk_Caps.integerSupport) {\n    int x = int(sk_FragCoord.x);\n    int y = int(sk_FragCoord.y);\n    uint m = uint((((((y & 1) << 5 | (x & 1) << 4) | (y & 2) << 2) | (x & 2) << 1) | (y & 4) >> 1) | (x & 4) >> 2);\n    value = float(m) / 64.0 - 0.4921875;\n} else {\n    value = fract(sin(dot(sk_FragCoord.xy, vec2(12.989800000000001, 78.233000000000004))) * 43758.545299999998) - 0.5;\n}\n%s = vec4(clamp(%s.xyz + value * range, 0.0, %s.w), %s.w);\n", _outer.rangeType(), args.fOutputColor, args.fInputColor ? args.fInputColor : "vec4(1)", args.fInputColor ? args.fInputColor : "vec4(1)", args.fInputColor ? args.fInputColor : "vec4(1)");
+        fragBuilder->codeAppendf("float r = fract(sin(dot(sk_FragCoord.xy, vec2(12.989800000000001, 78.233000000000004))) * 43758.545299999998) - 0.5;\n%s = clamp(0.0039215686274509803 * vec4(r) + %s, 0.0, 1.0);\n", args.fOutputColor, args.fInputColor ? args.fInputColor : "vec4(1)");
     }
 private:
     void onSetData(const GrGLSLProgramDataManager& pdman, const GrFragmentProcessor& _proc) override {
@@ -33,20 +33,17 @@ GrGLSLFragmentProcessor* GrDitherEffect::onCreateGLSLInstance() const {
     return new GrGLSLDitherEffect();
 }
 void GrDitherEffect::onGetGLSLProcessorKey(const GrShaderCaps& caps, GrProcessorKeyBuilder* b) const {
-    b->add32(fRangeType);
 }
 bool GrDitherEffect::onIsEqual(const GrFragmentProcessor& other) const {
     const GrDitherEffect& that = other.cast<GrDitherEffect>();
     (void) that;
-    if (fRangeType != that.fRangeType) return false;
     return true;
 }
 GR_DEFINE_FRAGMENT_PROCESSOR_TEST(GrDitherEffect);
 #if GR_TEST_UTILS
 sk_sp<GrFragmentProcessor> GrDitherEffect::TestCreate(GrProcessorTestData* testData) {
 
-    float range = testData->fRandom->nextRangeF(0.001f, 0.05f);
-    return sk_sp<GrFragmentProcessor>(new GrDitherEffect(range));
+    return GrDitherEffect::Make();
 }
 #endif
 #endif
diff --git a/src/gpu/effects/GrDitherEffect.fp b/src/gpu/effects/GrDitherEffect.fp
index 937efae264..1c35b38cd2 100644
--- a/src/gpu/effects/GrDitherEffect.fp
+++ b/src/gpu/effects/GrDitherEffect.fp
@@ -1,74 +1,17 @@
-// This controls the range of values added to color channels
-layout(key) in int rangeType;
-
-@make {
-    static sk_sp<GrFragmentProcessor> Make(GrPixelConfig dstConfig) {
-        int rangeType;
-        switch (dstConfig) {
-            case kGray_8_GrPixelConfig:
-            case kRGBA_8888_GrPixelConfig:
-            case kBGRA_8888_GrPixelConfig:
-            case kSRGBA_8888_GrPixelConfig:
-            case kSBGRA_8888_GrPixelConfig:
-                rangeType = 0;
-                break;
-            case kRGB_565_GrPixelConfig:
-                rangeType = 1;
-                break;
-            case kRGBA_4444_GrPixelConfig:
-                rangeType = 2;
-                break;
-            case kUnknown_GrPixelConfig:
-            case kAlpha_half_GrPixelConfig:
-            case kRGBA_8888_sint_GrPixelConfig:
-            case kRGBA_float_GrPixelConfig:
-            case kRG_float_GrPixelConfig:
-            case kRGBA_half_GrPixelConfig:
-            case kAlpha_8_GrPixelConfig:
-                return nullptr;
-        }
-        return sk_sp<GrFragmentProcessor>(new GrDitherEffect(rangeType));
-    }
-}
-
 void main() {
-    float value;
-    float 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 = 0.125 / 15.0;
-            break;
-    }
-    @if (sk_Caps.integerSupport) {
-        // This ordered-dither code is lifted from the cpu backend.
-        int x = int(sk_FragCoord.x);
-        int y = int(sk_FragCoord.y);
-        uint m = (y & 1) << 5 | (x & 1) << 4 |
-                 (y & 2) << 2 | (x & 2) << 1 |
-                 (y & 4) >> 1 | (x & 4) >> 2;
-        value = float(m) * 1.0 / 64.0 - 63.0 / 128.0;
-    } else {
-        // 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.
-        value = fract(sin(dot(sk_FragCoord.xy, vec2(12.9898, 78.233))) * 43758.5453) - .5;
-    }
-    // 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 = vec4(clamp(sk_InColor.rgb + value * range, 0, sk_InColor.a), sk_InColor.a);
+    // 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.
+    float r = fract(sin(dot(sk_FragCoord.xy, vec2(12.9898, 78.233))) * 43758.5453) - .5;
+    sk_OutColor = clamp(1 / 255.0 * vec4(r) + sk_InColor, 0, 1);
 }
 
 @test(testData) {
-    float range = testData->fRandom->nextRangeF(0.001f, 0.05f);
-    return sk_sp<GrFragmentProcessor>(new GrDitherEffect(range));
-}
+    return GrDitherEffect::Make();
+}
\ No newline at end of file
diff --git a/src/gpu/effects/GrDitherEffect.h b/src/gpu/effects/GrDitherEffect.h
index 63c0df7eb2..a996ad94c2 100644
--- a/src/gpu/effects/GrDitherEffect.h
+++ b/src/gpu/effects/GrDitherEffect.h
@@ -18,47 +18,19 @@
 #include "effects/GrProxyMove.h"
 class GrDitherEffect : public GrFragmentProcessor {
 public:
-    int rangeType() const { return fRangeType; }
-
-    static sk_sp<GrFragmentProcessor> Make(GrPixelConfig dstConfig) {
-        int rangeType;
-        switch (dstConfig) {
-            case kGray_8_GrPixelConfig:
-            case kRGBA_8888_GrPixelConfig:
-            case kBGRA_8888_GrPixelConfig:
-            case kSRGBA_8888_GrPixelConfig:
-            case kSBGRA_8888_GrPixelConfig:
-                rangeType = 0;
-                break;
-            case kRGB_565_GrPixelConfig:
-                rangeType = 1;
-                break;
-            case kRGBA_4444_GrPixelConfig:
-                rangeType = 2;
-                break;
-            case kUnknown_GrPixelConfig:
-            case kAlpha_half_GrPixelConfig:
-            case kRGBA_8888_sint_GrPixelConfig:
-            case kRGBA_float_GrPixelConfig:
-            case kRG_float_GrPixelConfig:
-            case kRGBA_half_GrPixelConfig:
-            case kAlpha_8_GrPixelConfig:
-                return nullptr;
-        }
-        return sk_sp<GrFragmentProcessor>(new GrDitherEffect(rangeType));
+    static sk_sp<GrFragmentProcessor> Make() {
+        return sk_sp<GrFragmentProcessor>(new GrDitherEffect());
     }
     const char* name() const override { return "DitherEffect"; }
 private:
-    GrDitherEffect(int rangeType)
-    : INHERITED(kNone_OptimizationFlags)
-    , fRangeType(rangeType) {
+    GrDitherEffect()
+    : INHERITED(kNone_OptimizationFlags) {
         this->initClassID<GrDitherEffect>();
     }
     GrGLSLFragmentProcessor* onCreateGLSLInstance() const override;
     void onGetGLSLProcessorKey(const GrShaderCaps&,GrProcessorKeyBuilder*) const override;
     bool onIsEqual(const GrFragmentProcessor&) const override;
     GR_DECLARE_FRAGMENT_PROCESSOR_TEST
-    int fRangeType;
     typedef GrFragmentProcessor INHERITED;
 };
 #endif