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// Copyright 2020 The Cobalt Authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
precision mediump float;
uniform vec4 u_color;
uniform vec4 u_blur_rect;
uniform vec2 u_scale_add;
varying vec2 v_offset;
// Calculate the normalized gaussian integral from (pos.x * k, pos.y * k)
// where k = sqrt(2) * sigma and pos.x <= pos.y. This is just a 1D filter --
// the pos.x and pos.y values are expected to be on the same axis.
float GaussianIntegral(vec2 pos) {
// Approximation of the error function.
// For x >= 0,
// erf(x) = 1 - 1 / (1 + k1 * x + k2 * x^2 + k3 * x^3 + k4 * x^4)^4
// where k1 = 0.278393, k2 = 0.230389, k3 = 0.000972, k4 = 0.078108.
// For y < 0,
// erf(y) = -erf(-y).
vec2 s = sign(pos);
vec2 a = abs(pos);
vec2 t = 1.0 +
(0.278393 + (0.230389 + (0.000972 + 0.078108 * a) * a) * a) * a;
vec2 t2 = t * t;
vec2 erf = s - s / (t2 * t2);
// erf(x) = the integral of the normalized gaussian from [-x * k, x * k],
// where k = sqrt(2) * sigma. Find the integral from (pos.x * k, pos.y * k).
return dot(erf, vec2(-0.5, 0.5));
}
void main() {
// Get the integral over the interval occupied by the rectangle. Both
// v_offset and u_blur_rect are already scaled for the integral function.
float integral = GaussianIntegral(u_blur_rect.xz - v_offset.xx) *
GaussianIntegral(u_blur_rect.yw - v_offset.yy);
float blur_scale = integral * u_scale_add.x + u_scale_add.y;
gl_FragColor = u_color * blur_scale;
}