Bug 1664459 - simplify WebRender radial gradient shader. r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D89915

gfx/wr/webrender/res/brush_radial_gradient.glsl

@@ -14,7 +14,7 @@
 
 #define V_CENTER            flat_varying_vec4_0.xy
 #define V_START_RADIUS      flat_varying_vec4_0.z
-#define V_END_RADIUS        flat_varying_vec4_0.w
+#define V_RADIUS_SCALE      flat_varying_vec4_0.w
 
 #define V_REPEATED_SIZE     flat_varying_vec4_1.xy
 #define V_GRADIENT_REPEAT   flat_varying_vec4_1.z
@@ -69,7 +69,13 @@ void radial_gradient_brush_vs(
 
     V_CENTER = gradient.center_start_end_radius.xy;
     V_START_RADIUS = gradient.center_start_end_radius.z;
-    V_END_RADIUS = gradient.center_start_end_radius.w;
+    if (gradient.center_start_end_radius.z != gradient.center_start_end_radius.w) {
+      // Store 1/rd where rd = end_radius - start_start
+      V_RADIUS_SCALE = 1.0 / (gradient.center_start_end_radius.w - gradient.center_start_end_radius.z);
+    } else {
+      // If rd = 0, we can't get its reciprocal. Instead, just use a zero scale.
+      V_RADIUS_SCALE = 0.0;
+    }
 
     // Transform all coordinates by the y scale so the
     // fragment shader can work with circles
@@ -114,43 +120,9 @@ Fragment radial_gradient_brush_fs() {
     vec2 pos = mod(V_POS, V_REPEATED_SIZE);
 #endif
 
+    // Solve for t in length(pd) = V_START_RADIUS + t * rd
     vec2 pd = pos - V_CENTER;
-    float rd = V_END_RADIUS - V_START_RADIUS;
-
-    // Solve for t in length(t - pd) = V_START_RADIUS + t * rd
-    // using a quadratic equation in form of At^2 - 2Bt + C = 0
-    float A = -(rd * rd);
-    float B = V_START_RADIUS * rd;
-    float C = dot(pd, pd) - V_START_RADIUS * V_START_RADIUS;
-
-    float offset;
-    if (A == 0.0) {
-        // Since A is 0, just solve for -2Bt + C = 0
-        if (B == 0.0) {
-            discard;
-        }
-        float t = 0.5 * C / B;
-        if (V_START_RADIUS + rd * t >= 0.0) {
-            offset = t;
-        } else {
-            discard;
-        }
-    } else {
-        float discr = B * B - A * C;
-        if (discr < 0.0) {
-            discard;
-        }
-        discr = sqrt(discr);
-        float t0 = (B + discr) / A;
-        float t1 = (B - discr) / A;
-        if (V_START_RADIUS + rd * t0 >= 0.0) {
-            offset = t0;
-        } else if (V_START_RADIUS + rd * t1 >= 0.0) {
-            offset = t1;
-        } else {
-            discard;
-        }
-    }
+    float offset = (length(pd) - V_START_RADIUS) * V_RADIUS_SCALE;
 
     vec4 color = sample_gradient(V_GRADIENT_ADDRESS,
                                  offset,
@@ -168,7 +140,7 @@ Fragment radial_gradient_brush_fs() {
 #undef V_GRADIENT_ADDRESS
 #undef V_CENTER
 #undef V_START_RADIUS
-#undef V_END_RADIUS
+#undef V_RADIUS_SCALE
 #undef V_REPEATED_SIZE
 #undef V_GRADIENT_REPEAT
 #undef V_POS

layout/reftests/border-image/reftest.list

@@ -57,7 +57,7 @@ fuzzy(0-1,0-2000) fuzzy-if(webrender&&!geckoview,1-2,3249-9500) == border-image-
 fuzzy(0-1,0-8533) fuzzy-if(webrender&&!geckoview,1-3,3188-9500) == border-image-linear-gradient-repeat-repeat-3.html border-image-linear-gradient-repeat-repeat-3-ref.html
 fuzzy(0-3,0-107563) fuzzy-if(webrender&&!geckoview,1-3,43500-107563) == border-image-linear-gradient-repeat-round-3.html border-image-linear-gradient-repeat-round-3-ref.html
 
-fuzzy-if(webrender,0-1,0-2096) == border-image-radial-gradient.html border-image-radial-gradient-ref.html
+fuzzy-if(webrender,0-2,0-2096) == border-image-radial-gradient.html border-image-radial-gradient-ref.html
 fuzzy(0-1,0-42) fuzzy-if(skiaContent,0-2,0-20) fuzzy-if(webrender,0-1,0-37818) == border-image-radial-gradient-slice-1.html border-image-radial-gradient-slice-1-ref.html
 fuzzy(0-1,0-46) fuzzy-if(OSX,0-2,0-4472) fuzzy-if(webrender,0-1,0-26363) == border-image-radial-gradient-slice-2.html border-image-radial-gradient-slice-2-ref.html
 fuzzy(0-1,0-85970) fuzzy-if(webrender,0-1,0-90873) == border-image-radial-gradient-slice-fill-1.html border-image-radial-gradient-slice-fill-1-ref.html