Use the texture filtering hardware to reduce the number of sample ope…

…rations in

the blur shader.

Linear filtering allows us to evaluate the formula

    k₀c₀ + k₁c₁                 (Formula 1)

where c₀ and c₁ are the colors of adjacent texels and k₀ and k₁ are arbitrary
factors (in this case, the results of evaluating the Gaussian function) with a
single lookup. Linear filtering evaluates the following expression for some t
between 0 and 1:

    lerp(c₀, c₁, t)

It can be shown algebraically that Formula 1 is equivalent to:

                 ⎛           k₁  ⎞
    (k₀ + k₁)lerp⎜c₀, c₁, ───────⎟
                 ⎝        k₀ + k₁⎠

Which can be readily evaluated by letting `t = k₁/(k₀ + k₁)` and performing a
texture lookup.

webrender/res/cs_blur.glsl

@@ -99,9 +99,6 @@ void main(void) {
 //           with a offset / weight uniform table and a constant
 //           loop iteration count!
 
-// TODO(gw): Make use of the bilinear sampling trick to reduce
-//           the number of texture fetches needed for a gaussian blur.
-
 void main(void) {
     SAMPLE_TYPE original_color = SAMPLE_TEXTURE(vUv);
 
@@ -119,24 +116,47 @@ void main(void) {
     gauss_coefficient.y = exp(-0.5 / (vSigma * vSigma));
     gauss_coefficient.z = gauss_coefficient.y * gauss_coefficient.y;
 
-    float gauss_coefficient_sum = 0.0;
+    float gauss_coefficient_total = gauss_coefficient.x;
     SAMPLE_TYPE avg_color = original_color * gauss_coefficient.x;
-    gauss_coefficient_sum += gauss_coefficient.x;
     gauss_coefficient.xy *= gauss_coefficient.yz;
 
-    for (int i = 1; i <= vSupport; i++) {
-        vec2 offset = vOffsetScale * float(i);
+    // Evaluate two adjacent texels at a time. We can do this because, if c0
+    // and c1 are colors of adjacent texels and k0 and k1 are arbitrary
+    // factors, this formula:
+    //
+    //     k0 * c0 + k1 * c1          (Equation 1)
+    //
+    // is equivalent to:
+    //
+    //                                 k1
+    //     (k0 + k1) * lerp(c0, c1, -------)
+    //                              k0 + k1
+    //
+    // A texture lookup of adjacent texels evaluates this formula:
+    //
+    //     lerp(c0, c1, t)
+    //
+    // for some t. So we can let `t = k1/(k0 + k1)` and effectively evaluate
+    // Equation 1 with a single texture lookup.
+
+    for (int i = 1; i <= vSupport; i += 2) {
+        float gauss_coefficient_subtotal = gauss_coefficient.x;
+        gauss_coefficient.xy *= gauss_coefficient.yz;
+        gauss_coefficient_subtotal += gauss_coefficient.x;
+        float gauss_ratio = gauss_coefficient.x / gauss_coefficient_subtotal;
+
+        vec2 offset = vOffsetScale * (float(i) + gauss_ratio);
 
         vec2 st0 = clamp(vUv.xy - offset, vUvRect.xy, vUvRect.zw);
-        avg_color += SAMPLE_TEXTURE(vec3(st0, vUv.z)) * gauss_coefficient.x;
+        avg_color += SAMPLE_TEXTURE(vec3(st0, vUv.z)) * gauss_coefficient_subtotal;
 
         vec2 st1 = clamp(vUv.xy + offset, vUvRect.xy, vUvRect.zw);
-        avg_color += SAMPLE_TEXTURE(vec3(st1, vUv.z)) * gauss_coefficient.x;
+        avg_color += SAMPLE_TEXTURE(vec3(st1, vUv.z)) * gauss_coefficient_subtotal;
 
-        gauss_coefficient_sum += 2.0 * gauss_coefficient.x;
+        gauss_coefficient_total += 2.0 * gauss_coefficient_subtotal;
         gauss_coefficient.xy *= gauss_coefficient.yz;
     }
 
-    oFragColor = vec4(avg_color) / gauss_coefficient_sum;
+    oFragColor = vec4(avg_color) / gauss_coefficient_total;
 }
 #endif

wrench/reftests/boxshadow/reftest.list

@@ -23,7 +23,7 @@ platform(linux,mac) == inset-subpx.yaml inset-subpx.png
 platform(linux,mac) fuzzy(1,4) == inset-downscale.yaml inset-downscale.png
 platform(linux,mac) fuzzy(1,50) == box-shadow-cache.yaml box-shadow-cache.png
 platform(linux,mac) fuzzy(1,685) == overlap1.yaml overlap1.png
-== overlap2.yaml overlap2.png
+fuzzy(1,61) == overlap2.yaml overlap2.png
 platform(linux,mac) fuzzy(1,48) == no-stretch.yaml no-stretch.png
 platform(linux,mac) fuzzy(1,9) == box-shadow-stretch-mode-x.yaml box-shadow-stretch-mode-x.png
 platform(linux,mac) fuzzy(1,41) == box-shadow-stretch-mode-y.yaml box-shadow-stretch-mode-y.png

wrench/reftests/filters/reftest.list

@@ -29,7 +29,7 @@ platform(linux,mac) fuzzy(1,133) == filter-large-blur-radius.yaml filter-large-b
 == filter-saturate-blue-alpha-1.yaml filter-saturate-blue-alpha-1-ref.yaml
 == filter-hue-rotate-1.yaml filter-hue-rotate-1-ref.yaml
 == filter-hue-rotate-alpha-1.yaml filter-hue-rotate-alpha-1-ref.yaml
-== filter-long-chain.yaml filter-long-chain.png
+fuzzy(1,14) == filter-long-chain.yaml filter-long-chain.png
 platform(linux,mac) == filter-drop-shadow.yaml filter-drop-shadow.png
 platform(linux,mac) == filter-drop-shadow-on-viewport-edge.yaml filter-drop-shadow-on-viewport-edge.png
 platform(linux,mac) == blend-clipped.yaml blend-clipped.png

wrench/reftests/text/reftest.list

@@ -14,7 +14,7 @@
 != shadow-clipped-text.yaml blank.yaml
 != non-opaque.yaml non-opaque-notref.yaml
 == decorations.yaml decorations-ref.yaml
-fuzzy(1,100) == decorations-suite.yaml decorations-suite.png
+fuzzy(1,173) == decorations-suite.yaml decorations-suite.png
 == 1658.yaml 1658-ref.yaml
 == split-batch.yaml split-batch-ref.yaml
 == shadow-red.yaml shadow-red-ref.yaml
@@ -54,7 +54,7 @@ platform(linux) == clipped-transform.yaml clipped-transform.png
 platform(mac) == color-bitmap-shadow.yaml color-bitmap-shadow-ref.yaml
 platform(linux) == writing-modes.yaml writing-modes-ref.yaml
 platform(linux) == blurred-shadow-local-clip-rect.yaml blurred-shadow-local-clip-rect-ref.png
-platform(linux) == two-shadows.yaml two-shadows.png
+fuzzy(1,1) platform(linux) == two-shadows.yaml two-shadows.png
 == shadow-clip.yaml shadow-clip-ref.yaml
 == shadow-fast-clip.yaml shadow-fast-clip-ref.yaml
 == shadow-partial-glyph.yaml shadow-partial-glyph-ref.yaml