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/*
Copyright (c) 2019 The Khronos Group Inc.
Use of this source code is governed by an MIT-style license that can be
found in the LICENSE.txt file.
*/
attribute vec4 gtf_Vertex;
attribute vec4 gtf_Color;
uniform mat4 gtf_ModelViewProjectionMatrix;
varying vec4 color;
void main (void)
{
const float M_PI = 3.14159265358979323846;
float c = 2.0 * (gtf_Color.r - 0.5);
float acos_c = 0.0;
float scale = 1.0;
float sign = 1.0;
// pow can't handle negative numbers so take advantage of symmetry
if(c < 0.0)
{
sign = -1.0;
c *= -1.0;
}
// Taylors series expansion for acos
// 1000/2 iterations necessary to get the accuracy with this method
for(int i = 1; i < 1000; i += 2)
{
acos_c += scale * pow(c, float(i)) / float(i);
scale *= float(i) / float(i + 1);
}
acos_c = M_PI / 2.0 - sign * acos_c;
color = vec4(acos_c / M_PI, 0.0, 0.0, 1.0);
gl_Position = gtf_ModelViewProjectionMatrix * gtf_Vertex;
}
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