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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;
vec2 c = 2.0 * (gtf_Color.rg - 0.5);
vec2 acos_c = vec2(0.0);
vec2 scale = vec2(1.0);
vec2 sign = vec2(1.0);
// pow can't handle negative numbers so take advantage of symmetry
if(c.r < 0.0)
{
sign.r = -1.0;
c.r *= -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.r += scale.r * pow(c.r, float(i)) / float(i);
scale.r *= float(i) / float(i + 1);
}
acos_c.r = M_PI / 2.0 - sign.r * acos_c.r;
// pow can't handle negative numbers so take advantage of symmetry
if(c.g < 0.0)
{
sign.g = -1.0;
c.g *= -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.g += scale.g * pow(c.g, float(i)) / float(i);
scale.g *= float(i) / float(i + 1);
}
acos_c.g = M_PI / 2.0 - sign.g * acos_c.g;
color = vec4(acos_c / M_PI, 0.0, 1.0);
gl_Position = gtf_ModelViewProjectionMatrix * gtf_Vertex;
}
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