Proper dither re-enabled

webrender/res/prim_shared.glsl

@@ -763,8 +763,9 @@ vec4 dither(vec4 color) {
     const int matrix_mask = 7;
 
     ivec2 pos = ivec2(gl_FragCoord.xy) & ivec2(matrix_mask);
-    float noise_factor = 4.0 / 255.0;
-    float noise = texelFetch(sDither, pos, 0).r * noise_factor;
+    float noise_normalized = (texelFetch(sDither, pos, 0).r * 255.0 + 0.5) / 64.0;
+    float noise = (noise_normalized - 0.5) / 256.0; // scale down to the unit length
+
     return color + vec4(noise, noise, noise, 0);
 }
 #endif //WR_FRAGMENT_SHADER

webrender/res/ps_gradient.fs.glsl

@@ -12,9 +12,5 @@ void main(void) {
 #endif
 
     alpha = min(alpha, do_clip());
-
-    // TODO(gw): Re-enable the gradient dither once we get the
-    //           reftests passing.
-    //oFragColor = dither(vColor * vec4(1.0, 1.0, 1.0, alpha));
-    oFragColor = vColor * vec4(1.0, 1.0, 1.0, alpha);
+    oFragColor = dither(vColor * vec4(1.0, 1.0, 1.0, alpha));
 }

webrender/src/internal_types.rs

@@ -197,19 +197,19 @@ pub struct PackedTexel {
 
 impl PackedTexel {
     pub fn high_bytes(color: &ColorF) -> PackedTexel {
-        Self::extract_bytes(color, COLOR_FLOAT_TO_FIXED)
+        Self::extract_bytes(color, 8)
     }
 
     pub fn low_bytes(color: &ColorF) -> PackedTexel {
-        Self::extract_bytes(color, COLOR_FLOAT_TO_FIXED_WIDE)
+        Self::extract_bytes(color, 0)
     }
 
-    fn extract_bytes(color: &ColorF, multiplier: f32) -> PackedTexel {
+    fn extract_bytes(color: &ColorF, shift_by: i32) -> PackedTexel {
         PackedTexel {
-            b: ((0.5 + color.b * multiplier).floor() as u32 & 0xff) as u8,
-            g: ((0.5 + color.g * multiplier).floor() as u32 & 0xff) as u8,
-            r: ((0.5 + color.r * multiplier).floor() as u32 & 0xff) as u8,
-            a: ((0.5 + color.a * multiplier).floor() as u32 & 0xff) as u8,
+            b: ((0.5 + color.b * COLOR_FLOAT_TO_FIXED_WIDE).floor() as u32 >> shift_by & 0xff) as u8,
+            g: ((0.5 + color.g * COLOR_FLOAT_TO_FIXED_WIDE).floor() as u32 >> shift_by & 0xff) as u8,
+            r: ((0.5 + color.r * COLOR_FLOAT_TO_FIXED_WIDE).floor() as u32 >> shift_by & 0xff) as u8,
+            a: ((0.5 + color.a * COLOR_FLOAT_TO_FIXED_WIDE).floor() as u32 >> shift_by & 0xff) as u8,
         }
     }
 }

wrench/reftests/gradient/reftest.list

@@ -2,18 +2,18 @@
 == premultiplied-angle.yaml blank.yaml
 == premultiplied-radial.yaml blank.yaml
 
-fuzzy(1,20000) == linear.yaml linear-ref.png
-fuzzy(1,20000) == linear-reverse.yaml linear-ref.png
-fuzzy(1,29300) == linear-stops.yaml linear-stops-ref.png
+== linear.yaml linear-ref.png
+== linear-reverse.yaml linear-ref.png
+fuzzy(1,35000) == linear-stops.yaml linear-stops-ref.png
 
 # dithering requires us to fuzz here
 fuzzy(1,20000) == linear.yaml linear-ref.yaml
 fuzzy(1,20000) == linear-reverse.yaml linear-ref.yaml
 
 == linear-aligned-clip.yaml linear-aligned-clip-ref.yaml
 
-fuzzy(1,5) == radial-circle.yaml radial-circle-ref.png
-fuzzy(1,4) == radial-ellipse.yaml radial-ellipse-ref.png
+fuzzy(1,80000) == radial-circle.yaml radial-circle-ref.png
+fuzzy(1,80000) == radial-ellipse.yaml radial-ellipse-ref.png
 
 != radial-circle.yaml radial-ellipse.yaml