Bug 1600141 - Fix rounded clip rects with radii < 1px. r=nical

Differential Revision: https://phabricator.services.mozilla.com/D55203

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/res/cs_clip_rectangle.glsl

@@ -118,8 +118,13 @@ void main(void) {
 
 #ifdef WR_FEATURE_FAST_PATH
 // See http://www.iquilezles.org/www/articles/distfunctions2d/distfunctions2d.htm
-float sdf_rounded_rect(vec2 pos, vec3 clip_params) {
-    return length(max(abs(pos) - clip_params.xy, 0.0)) - clip_params.z;
+float sd_box(in vec2 pos, in vec2 box_size) {
+    vec2 d = abs(pos) - box_size;
+    return length(max(d, vec2(0.0))) + min(max(d.x,d.y), 0.0);
+}
+
+float sd_rounded_box(in vec2 pos, in vec2 box_size, in float radius) {
+    return sd_box(pos, box_size) - radius;
 }
 #endif
 
@@ -128,7 +133,7 @@ void main(void) {
     float aa_range = compute_aa_range(local_pos);
 
 #ifdef WR_FEATURE_FAST_PATH
-    float d = sdf_rounded_rect(local_pos, vClipParams);
+    float d = sd_rounded_box(local_pos, vClipParams.xy, vClipParams.z);
     float f = distance_aa(aa_range, d);
     float final_alpha = mix(f, 1.0 - f, vClipMode);
 #else

gfx/wr/wrench/reftests/aa/fractional-radii-ref.yaml

@@ -0,0 +1,18 @@
+---
+root:
+  items:
+    - type: rect
+      bounds: 20 50 100 100
+      color: blue
+
+    - type: rect
+      bounds: 140 50 100 100
+      color: blue
+
+    - type: rect
+      bounds: 260 50 100 100
+      color: blue
+
+    - type: rect
+      bounds: 380 50 100 100
+      color: blue

gfx/wr/wrench/reftests/aa/fractional-radii.yaml

@@ -0,0 +1,42 @@
+---
+root:
+  items:
+    - type: clip
+      bounds: [20, 50, 100, 100]
+      complex:
+        - rect: [20, 50, 100, 100]
+          radius: 0
+      items:
+      - type: rect
+        bounds: 20 50 100 100
+        color: blue
+
+    - type: clip
+      bounds: [140, 50, 100, 100]
+      complex:
+        - rect: [140, 50, 100, 100]
+          radius: 0.001
+      items:
+      - type: rect
+        bounds: 140 50 100 100
+        color: blue
+
+    - type: clip
+      bounds: [260, 50, 100, 100]
+      complex:
+        - rect: [260, 50, 100, 100]
+          radius: 0.01
+      items:
+      - type: rect
+        bounds: 260 50 100 100
+        color: blue
+
+    - type: clip
+      bounds: [380, 50, 100, 100]
+      complex:
+        - rect: [380, 50, 100, 100]
+          radius: 0.1
+      items:
+      - type: rect
+        bounds: 380 50 100 100
+        color: blue

gfx/wr/wrench/reftests/aa/reftest.list

@@ -1,2 +1,3 @@
 skip_on(android) == rounded-rects.yaml rounded-rects-ref.png  # Too wide for Android
 == aa-dist-bug.yaml aa-dist-bug-ref.yaml
+== fractional-radii.yaml fractional-radii-ref.yaml