Make sure we un-premultiply alpha before applying filters.

webrender/res/ps_blend.glsl

@@ -91,33 +91,22 @@ void main(void) {
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 vec4 Contrast(vec4 Cs, float amount) {
-    return vec4(Cs.rgb * amount - 0.5 * amount + 0.5, 1.0);
+    return vec4(Cs.rgb * amount - 0.5 * amount + 0.5, Cs.a);
 }
 
 vec4 Invert(vec4 Cs, float amount) {
-    Cs.rgb /= Cs.a;
-
-    vec3 color = mix(Cs.rgb, vec3(1.0) - Cs.rgb, amount);
-
-    // Pre-multiply the alpha into the output value.
-    return vec4(color.rgb * Cs.a, Cs.a);
+    return vec4(mix(Cs.rgb, vec3(1.0) - Cs.rgb, amount), Cs.a);
 }
 
 vec4 Brightness(vec4 Cs, float amount) {
-    // Un-premultiply the input.
-    Cs.rgb /= Cs.a;
-
     // Apply the brightness factor.
     // Resulting color needs to be clamped to output range
     // since we are pre-multiplying alpha in the shader.
-    vec3 color = clamp(Cs.rgb * amount, vec3(0.0), vec3(1.0));
-
-    // Pre-multiply the alpha into the output value.
-    return vec4(color.rgb * Cs.a, Cs.a);
+    return vec4(clamp(Cs.rgb * amount, vec3(0.0), vec3(1.0)), Cs.a);
 }
 
 vec4 Opacity(vec4 Cs, float amount) {
-    return Cs * amount;
+    return vec4(Cs.rgb, Cs.a * amount);
 }
 
 void main(void) {
@@ -128,6 +117,9 @@ void main(void) {
         discard;
     }
 
+    // Un-premultiply the input.
+    Cs.rgb /= Cs.a;
+
     switch (vOp) {
         case 0:
             oFragColor = Cs;
@@ -147,5 +139,8 @@ void main(void) {
         default:
             oFragColor = vColorMat * Cs;
     }
+
+    // Pre-multiply the alpha into the output value.
+    oFragColor.rgb *= oFragColor.a;
 }
 #endif

wrench/reftests/filters/filter-contrast-gray-alpha-1-ref.yaml

@@ -0,0 +1,13 @@
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 120, 120]
+      items:
+        - type: rect
+          bounds: [0, 0, 120, 120]
+          color: [255, 255, 255, 1]
+
+        - type: rect
+          bounds: [10, 10, 100, 100]
+          color: [223, 223, 223, 1]

wrench/reftests/filters/filter-contrast-gray-alpha-1.yaml

@@ -0,0 +1,13 @@
+---
+root:
+  items:
+    - type: stacking-context
+      bounds: [0, 0, 120, 120]
+      items:
+        - type: stacking-context
+          bounds: [10, 10, 100, 100]
+          filters: contrast(0)
+          items:
+            - type: rect
+              bounds: [0, 0, 100, 100]
+              color: [128, 128, 128, 0.25]

wrench/reftests/filters/reftest.list

@@ -9,6 +9,7 @@
 == filter-brightness-2.yaml filter-brightness-2-ref.yaml
 == filter-brightness-3.yaml filter-brightness-3-ref.yaml
 == filter-brightness-4.yaml filter-brightness-4-ref.yaml
+== filter-contrast-gray-alpha-1.yaml filter-contrast-gray-alpha-1-ref.yaml
 == filter-invert.yaml filter-invert-ref.yaml
 == filter-invert-2.yaml filter-invert-2-ref.yaml
 == filter-large-blur-radius.yaml filter-large-blur-radius.png