Correctly handle blend primitives, where the contents are clipped.

In some situations, the items within a filter stacking context will
have clips applied. In this case, we minimize the allocated size
of the intermediate surface, to avoid drawing redundant information
that won't be seen.

However, if the stacking context itself does not have the same clip
applied, then we need to ensure that when we draw the blend primitive,
we don't sample outside the smaller region of the intermediate
surface that was rendered.

Ideally, the stacking context should have a clip applied to it so
that we don't waste time drawing transparent pixels. Adding a clip
to the stacking context here would provide an optimization win in
many cases. However, even without one present, we should still
ensure correct rendering, thus this fix is required.

Also fix a case where brush primitives with segments were not setting
the correct bounding rect for a render task. This wasn't causing a
problem here, but may have caused other issues.

webrender/res/brush_blend.glsl

@@ -13,6 +13,7 @@ flat varying float vAmount;
 flat varying int vOp;
 flat varying mat4 vColorMat;
 flat varying vec4 vColorOffset;
+flat varying vec4 vUvClipBounds;
 
 #ifdef WR_VERTEX_SHADER
 
@@ -30,6 +31,10 @@ void brush_vs(
               src_task.content_origin;
     vUv = vec3(uv / texture_size, src_task.common_data.texture_layer_index);
 
+    vec2 uv0 = src_task.common_data.task_rect.p0;
+    vec2 uv1 = uv0 + src_task.common_data.task_rect.size;
+    vUvClipBounds = vec4(uv0, uv1) / texture_size.xyxy;
+
     vOp = user_data.y;
 
     float lumR = 0.2126;
@@ -142,6 +147,10 @@ vec4 brush_fs() {
             color = vColorMat * Cs + vColorOffset;
     }
 
+    // Fail-safe to ensure that we don't sample outside the rendered
+    // portion of a blend source.
+    color.a *= point_inside_rect(vUv.xy, vUvClipBounds.xy, vUvClipBounds.zw);
+
     // Pre-multiply the alpha into the output value.
     color.rgb *= color.a;
 

webrender/res/rect.glsl

@@ -46,3 +46,8 @@ bool rect_inside_rect(RectWithSize little, RectWithSize big) {
     return all(lessThanEqual(vec4(big.p0, little.p0 + little.size),
                              vec4(little.p0, big.p0 + big.size)));
 }
+
+float point_inside_rect(vec2 p, vec2 p0, vec2 p1) {
+    vec2 s = step(p0, p) - step(p1, p);
+    return s.x * s.y;
+}

webrender/src/batch.rs

@@ -301,12 +301,19 @@ impl BatchList {
         }
     }
 
+    fn add_bounding_rect(
+        &mut self,
+        task_relative_bounding_rect: &DeviceIntRect,
+    ) {
+        self.combined_bounding_rect = self.combined_bounding_rect.union(task_relative_bounding_rect);
+    }
+
     pub fn get_suitable_batch(
         &mut self,
         key: BatchKey,
         task_relative_bounding_rect: &DeviceIntRect,
     ) -> &mut Vec<PrimitiveInstance> {
-        self.combined_bounding_rect = self.combined_bounding_rect.union(task_relative_bounding_rect);
+        self.add_bounding_rect(task_relative_bounding_rect);
 
         match key.blend_mode {
             BlendMode::None => {
@@ -1242,6 +1249,8 @@ impl AlphaBatchBuilder {
             user_data,
         };
 
+        self.batch_list.add_bounding_rect(task_relative_bounding_rect);
+
         match brush.segment_desc {
             Some(ref segment_desc) => {
                 let alpha_batch_key = BatchKey {

wrench/reftests/filters/blend-clipped.yaml

@@ -0,0 +1,110 @@
+---
+root:
+  items:
+    -
+      type: "stacking-context"
+      items:
+        -
+          bounds: [0, 0, 1887, 2081]
+          "clip-rect": [0, 0, 1887, 2081]
+          "clip-and-scroll": 0
+          "backface-visible": true
+          type: clip
+          id: 1
+          "content-size": [1887, 2081]
+        -
+          bounds: [0, 111, 1887, 1970]
+          "clip-rect": [0, 111, 1887, 1970]
+          "clip-and-scroll": 0
+          "backface-visible": true
+          type: clip
+          id: 2
+          "content-size": [1887, 1970]
+        -
+          bounds: [0, 111, 1887, 1971]
+          "clip-rect": [0, 111, 1887, 1971]
+          "clip-and-scroll": 2
+          "backface-visible": true
+          type: iframe
+          id: [1, 3]
+  id: [1, 1]
+pipelines:
+  -
+    id: [1, 3]
+    items:
+      -
+        type: "stacking-context"
+        items:
+          -
+            bounds: [0, 0, 1887, 1971]
+            "clip-rect": [0, 0, 1887, 1971]
+            "clip-and-scroll": 0
+            type: clip
+            id: 1
+            "content-size": [1887, 1971]
+          -
+            "clip-rect": [0, 0, 1887, 1971]
+            "clip-and-scroll": 1
+            type: "scroll-frame"
+            id: 2
+            "content-size": [1887, 1971]
+            bounds: [0, 0, 1887, 1971]
+          -
+            bounds: [0, 0, 1887, 1971]
+            "clip-rect": [0, 0, 1887, 1971]
+            "clip-and-scroll": 2
+            type: clip
+            id: 3
+            "content-size": [1887, 1971]
+          -
+            "clip-and-scroll": 3
+            type: "stacking-context"
+            items:
+              -
+                bounds: [0, -186, 1887, 239]
+                "clip-rect": [0, -186, 1887, 239]
+                "clip-and-scroll": 3
+                "backface-visible": true
+                type: clip
+                id: 4
+                "content-size": [1887, 239]
+              -
+                bounds: [-660.45, -186, 0, 0]
+                "clip-rect": [-660.45, -186, 0, 0]
+                "clip-and-scroll": 4
+                type: "stacking-context"
+                transform: [1, 0, 0, 0, -0.57735026, 1, 0, 0, 0, 0, 1, 0, 68.849, 0, 0, 1]
+                items:
+                  -
+                    "clip-and-scroll": 4
+                    type: "stacking-context"
+                    filters:
+                      - opacity(0.8)
+                    items:
+                      -
+                        bounds: [0, 0, 1887, 239]
+                        "clip-rect": [0, 0, 1887, 239]
+                        "clip-and-scroll": 4
+                        type: clip
+                        id: 5
+                        "content-size": [1887, 239]
+                      -
+                        bounds: [0, 0, 1887, 239]
+                        "clip-rect": [0, 0, 1887, 239]
+                        "clip-and-scroll": 5
+                        "backface-visible": true
+                        type: gradient
+                        start: [943.5, -0.00000000000005684342]
+                        end: [943.5, 238.5]
+                        "tile-size": [1887, 238.5]
+                        "tile-spacing": [0, 0]
+                        stops:
+                          - 0
+                          - 0 153 204.00002 1.0000
+                          - 0.3
+                          - 0 153 204.00002 1.0000
+                          - 0.6
+                          - 7.0000005 137 194.00002 1.0000
+                          - 1
+                          - 0 91 137 1.0000
+                        repeat: false

wrench/reftests/filters/reftest.list

@@ -32,3 +32,5 @@
 == filter-long-chain.yaml filter-long-chain.png
 == filter-drop-shadow.yaml filter-drop-shadow.png
 == filter-drop-shadow-on-viewport-edge.yaml filter-drop-shadow-on-viewport-edge.png
+== blend-clipped.yaml blend-clipped.png
+