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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 x = 2.0 * (gtf_Color.g - 0.5);
float y = 2.0 * (gtf_Color.b - 0.5);
float atan_c = 0.0;
float scale = 1.0;
float sign = 1.0;
vec4 result = vec4(0.0, 0.0, 0.0, 1.0);
const float epsilon = 1.0e-4;
// Avoid evaluating atan(0, x) for x < epsilon because it's implementation-dependent
if(x > epsilon || abs(y) > epsilon)
{
if(x < 0.0 ^^ y < 0.0)
{
sign = -1.0;
}
if(abs(y) <= abs(x))
{
float c = abs(y / x);
// Taylors series expansion for atan
for(int i = 1; i < 12; i += 2)
{
atan_c += scale * pow(c, float(i)) / float(i);
scale *= -1.0;
}
result = vec4(sign * atan_c / (2.0 * M_PI) + 0.5, 0.0, 0.0, 1.0);
}
else
{
float c = abs(x / y);
// Taylors series expansion for atan
for(int i = 1; i < 12; i += 2)
{
atan_c += scale * pow(c, float(i)) / float(i);
scale *= -1.0;
}
result = vec4(sign * (M_PI / 2.0 - atan_c) / (2.0 * M_PI) + 0.5, 0.0, 0.0, 1.0);
}
if(x < 0.0)
if(y < 0.0) result.r -= 0.5;
else if(y > 0.0) result.r += 0.5;
}
color = result;
gl_Position = gtf_ModelViewProjectionMatrix * gtf_Vertex;
}
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