Reland "Add small offset to t in GrLinearGradientLayout." am: 5540432b78 am: 4306b1d2ac

Change-Id: Ie2be82905e726808312dc791dca04ea34d92e5f5

2 files changed, 12 insertions, 3 deletions

diff --git a/src/gpu/gradients/GrLinearGradientLayout.cpp b/src/gpu/gradients/GrLinearGradientLayout.cpp
index cd53b36bcd..d809e004b0 100644
--- a/src/gpu/gradients/GrLinearGradientLayout.cpp
+++ b/src/gpu/gradients/GrLinearGradientLayout.cpp
@@ -25,8 +25,10 @@ public:
         auto gradientMatrix = _outer.gradientMatrix();
         (void)gradientMatrix;
         SkString sk_TransformedCoords2D_0 = fragBuilder->ensureCoords2D(args.fTransformedCoords[0]);
-        fragBuilder->codeAppendf("half t = half(%s.x);\n%s = half4(t, 1.0, 0.0, 0.0);\n",
-                                 sk_TransformedCoords2D_0.c_str(), args.fOutputColor);
+        fragBuilder->codeAppendf(
+                "half t = half(%s.x) + 1.0000000000000001e-05;\n%s = half4(t, 1.0, 0.0, 0.0);\n",
+                sk_TransformedCoords2D_0.c_str(),
+                args.fOutputColor);
     }
 
 private:
diff --git a/src/gpu/gradients/GrLinearGradientLayout.fp b/src/gpu/gradients/GrLinearGradientLayout.fp
index 3a187f321c..bd144316e8 100644
--- a/src/gpu/gradients/GrLinearGradientLayout.fp
+++ b/src/gpu/gradients/GrLinearGradientLayout.fp
@@ -12,7 +12,14 @@ in half4x4 gradientMatrix;
 }
 
 void main() {
-    half t = sk_TransformedCoords2D[0].x;
+    // We add a tiny delta to t. When gradient stops are set up so that a hard stop in a vertically
+    // or horizontally oriented gradient falls exactly at a column or row of pixel centers we can
+    // we can get slightly different interpolated t values along the column/row. By adding the delta
+    // we will consistently get the color to the "right" of the stop. Of course if the hard stop
+    // falls at X.5 - delta then we still could get inconsistent results, but that is much less
+    // likely. crbug.com/938592
+    // If/when we add filtering of the gradient this can be removed.
+    half t = half(sk_TransformedCoords2D[0].x) + 0.00001;
     sk_OutColor = half4(t, 1, 0, 0); // y = 1 for always valid
 }