Disable dithering of const paints

... except for 565.

Change-Id: I8ab633661c54583478234a46942ef804eb74a619
Reviewed-on: https://skia-review.googlesource.com/19880
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Herb Derby <herb@google.com>

4 files changed, 34 insertions, 2 deletions

diff --git a/src/core/SkBlitter.cpp b/src/core/SkBlitter.cpp
index 81e261d9a4..b1caaf585d 100644
--- a/src/core/SkBlitter.cpp
+++ b/src/core/SkBlitter.cpp
@@ -14,6 +14,7 @@
 #include "SkWriteBuffer.h"
 #include "SkMask.h"
 #include "SkMaskFilter.h"
+#include "SkPaintPriv.h"
 #include "SkShaderBase.h"
 #include "SkString.h"
 #include "SkTLazy.h"
@@ -867,6 +868,11 @@ SkBlitter* SkBlitter::Choose(const SkPixmap& device,
         return alloc->make<SkA8_Coverage_Blitter>(device, *paint);
     }
 
+    if (paint->isDither() && !SkPaintPriv::ShouldDither(*paint, device.colorType())) {
+        // Disable dithering when not needed.
+        paint.writable()->setDither(false);
+    }
+
     if (UseRasterPipelineBlitter(device, *paint)) {
         auto blitter = SkCreateRasterPipelineBlitter(device, *paint, matrix, alloc);
         SkASSERT(blitter);
diff --git a/src/core/SkPaintPriv.cpp b/src/core/SkPaintPriv.cpp
index 7dfb2e30ff..b37c28655d 100644
--- a/src/core/SkPaintPriv.cpp
+++ b/src/core/SkPaintPriv.cpp
@@ -10,7 +10,7 @@
 #include "SkPaintPriv.h"
 #include "SkImage.h"
 #include "SkPaint.h"
-#include "SkShader.h"
+#include "SkShaderBase.h"
 #include "SkXfermodePriv.h"
 
 static bool changes_alpha(const SkPaint& paint) {
@@ -69,3 +69,22 @@ void SkPaintPriv::ScaleFontMetrics(SkPaint::FontMetrics* metrics, SkScalar scale
     metrics->fUnderlinePosition *= scale;
 }
 
+bool SkPaintPriv::ShouldDither(const SkPaint& p, SkColorType dstCT) {
+    // The paint dither flag can veto.
+    if (!p.isDither()) {
+        return false;
+    }
+
+#ifndef SK_SUPPORT_LEGACY_DITHERING
+    // We always dither 565 when requested.
+    if (dstCT == SkColorType::kRGB_565_SkColorType) {
+        return true;
+    }
+
+    // Otherwise, dither is only needed for non-const paints.
+    return p.getImageFilter() || p.getMaskFilter()
+        || !p.getShader() || !as_SB(p.getShader())->isConstant();
+#else
+    return true;
+#endif
+}
diff --git a/src/core/SkPaintPriv.h b/src/core/SkPaintPriv.h
index e5c98f5e1a..663ed2d836 100644
--- a/src/core/SkPaintPriv.h
+++ b/src/core/SkPaintPriv.h
@@ -8,6 +8,7 @@
 #ifndef SkPaintPriv_DEFINED
 #define SkPaintPriv_DEFINED
 
+#include "SkImageInfo.h"
 #include "SkPaint.h"
 #include "SkMatrix.h"
 
@@ -62,6 +63,7 @@ public:
         MakeTextMatrix(matrix, paint.getTextSize(), paint.getTextScaleX(), paint.getTextSkewX());
     }
     
+    static bool ShouldDither(const SkPaint&, SkColorType);
 };
 
 #endif
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index 8b54aaa780..9adcb1a9ec 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -28,6 +28,7 @@
 #include "SkMessageBus.h"
 #include "SkMipMap.h"
 #include "SkPM4fPriv.h"
+#include "SkPaintPriv.h"
 #include "SkPixelRef.h"
 #include "SkResourceCache.h"
 #include "SkShaderBase.h"
@@ -542,7 +543,11 @@ static inline bool skpaint_to_grpaint_impl(GrContext* context,
     }
 
 #ifndef SK_IGNORE_GPU_DITHER
-    if (skPaint.isDither() && grPaint->numColorFragmentProcessors() > 0 && !rtc->isGammaCorrect()) {
+    // Conservative default, in case GrPixelConfigToColorType() fails.
+    SkColorType ct = SkColorType::kRGB_565_SkColorType;
+    GrPixelConfigToColorType(rtc->config(), &ct);
+    if (SkPaintPriv::ShouldDither(skPaint, ct) && grPaint->numColorFragmentProcessors() > 0
+        && !rtc->isGammaCorrect()) {
         grPaint->addColorFragmentProcessor(GrDitherEffect::Make());
     }
 #endif