[Impeller] Move gaussian routines into the shader library (#37037)

Author: bdero

ci/licenses_golden/licenses_flutter

@@ -1107,6 +1107,7 @@ FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/blending.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/branching.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/color.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/constants.glsl
+FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/gaussian.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/texture.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/transform.glsl
 FILE: ../../../flutter/impeller/compiler/shader_lib/impeller/types.glsl

impeller/compiler/shader_lib/impeller/constants.glsl

@@ -13,4 +13,7 @@ const float k1Over2Pi = 0.1591549430918;
 // sqrt(2 * pi)
 const float kSqrtTwoPi = 2.50662827463;
 
+// sqrt(2) / 2 == 1 / sqrt(2)
+const float kHalfSqrtTwo = 0.70710678118;
+
 #endif

impeller/compiler/shader_lib/impeller/gaussian.glsl

@@ -0,0 +1,35 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef GAUSSIAN_GLSL_
+#define GAUSSIAN_GLSL_
+
+#include <impeller/constants.glsl>
+
+float IPGaussian(float x, float sigma) {
+  float variance = sigma * sigma;
+  return exp(-0.5 * x * x / variance) / (kSqrtTwoPi * sigma);
+}
+
+/// Abramowitz and Stegun erf approximation.
+float IPErf(float x) {
+  float a = abs(x);
+  // 0.278393*x + 0.230389*x^2 + 0.078108*x^4 + 1
+  float b = (0.278393 + (0.230389 + 0.078108 * a * a) * a) * a + 1.0;
+  return sign(x) * (1 - 1 / (b * b * b * b));
+}
+
+vec2 IPVec2Erf(vec2 x) {
+  return vec2(IPErf(x.x), IPErf(x.y));
+}
+
+/// Indefinite integral of the Gaussian function (with constant range 0->1).
+float IPGaussianIntegral(float x, float sigma) {
+  // ( 1 + erf( x * (sqrt(2) / (2 * sigma) ) ) / 2
+  // Because this sigmoid is always > 1, we remap it (n * 1.07 - 0.07)
+  // so that it always fades to zero before it reaches the blur radius.
+  return 0.535 * IPErf(x * (kHalfSqrtTwo / sigma)) + 0.465;
+}
+
+#endif

impeller/entity/shaders/border_mask_blur.frag

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <impeller/gaussian.glsl>
 #include <impeller/texture.glsl>
 
 // Constant time mask blur for image borders.
@@ -27,30 +28,12 @@ in float v_outer_blur_factor;
 
 out vec4 frag_color;
 
-// Abramowitz and Stegun erf approximation.
-float erf(float x) {
-  float a = abs(x);
-  // 0.278393*x + 0.230389*x^2 + 0.078108*x^4 + 1
-  float b = (0.278393 + (0.230389 + 0.078108 * a * a) * a) * a + 1.0;
-  return sign(x) * (1 - 1 / (b * b * b * b));
-}
-
-const float kHalfSqrtTwo = 0.70710678118;
-
-// Indefinite integral of the Gaussian function (with constant range 0->1).
-float GaussianIntegral(float x, float sigma) {
-  // ( 1 + erf( x * (sqrt(2) / (2 * sigma) ) ) / 2
-  // Because this sigmoid is always > 1, we remap it (n * 1.07 - 0.07)
-  // so that it always fades to zero before it reaches the blur radius.
-  return 0.535 * erf(x * (kHalfSqrtTwo / sigma)) + 0.465;
-}
-
 float BoxBlurMask(vec2 uv) {
   // LTRB
-  return GaussianIntegral(uv.x, v_sigma_uv.x) *      //
-         GaussianIntegral(uv.y, v_sigma_uv.y) *      //
-         GaussianIntegral(1 - uv.x, v_sigma_uv.x) *  //
-         GaussianIntegral(1 - uv.y, v_sigma_uv.y);
+  return IPGaussianIntegral(uv.x, v_sigma_uv.x) *      //
+         IPGaussianIntegral(uv.y, v_sigma_uv.y) *      //
+         IPGaussianIntegral(1 - uv.x, v_sigma_uv.x) *  //
+         IPGaussianIntegral(1 - uv.y, v_sigma_uv.y);
 }
 
 void main() {

impeller/entity/shaders/gaussian_blur.frag

@@ -14,6 +14,7 @@
 //     level of log2(min_radius).
 
 #include <impeller/constants.glsl>
+#include <impeller/gaussian.glsl>
 #include <impeller/texture.glsl>
 
 uniform sampler2D texture_sampler;
@@ -46,18 +47,13 @@ in vec2 v_src_texture_coords;
 
 out vec4 frag_color;
 
-float Gaussian(float x) {
-  float variance = frag_info.blur_sigma * frag_info.blur_sigma;
-  return exp(-0.5 * x * x / variance) / (kSqrtTwoPi * frag_info.blur_sigma);
-}
-
 void main() {
   vec4 total_color = vec4(0);
   float gaussian_integral = 0;
   vec2 blur_uv_offset = frag_info.blur_direction / frag_info.texture_size;
 
   for (float i = -frag_info.blur_radius; i <= frag_info.blur_radius; i++) {
-    float gaussian = Gaussian(i);
+    float gaussian = IPGaussian(i, frag_info.blur_sigma);
     gaussian_integral += gaussian;
     total_color +=
         gaussian *