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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.
*/
#ifdef GL_ES
precision mediump float;
#endif
varying vec4 color;
void main (void)
{
const float M_PI = 3.14159265358979323846;
vec3 c = 2.0 * M_PI * ( fract(abs(color.rgb)) - 0.5 );
float sign = 1.0;
vec3 cos_c = vec3(-1.0, -1.0, -1.0);
float fact_even = 1.0;
float fact_odd = 1.0;
vec3 sum;
vec3 exp;
// At this point c is in the range [-PI, PI)
// Taylor-Maclaurin series expansion for cosine
//
// Apply the property that pow(a, b + c) = pow(a, b) * pow(a, c)
// and the property that 1.0/(a*b) = 1.0/a * 1.0/b
// to make sure no register ever overflows the range (-16384, +16384)
// mandated for mediump variables.
for(int i = 2; i <= 10; i += 2)
{
// fact_even will hold at most the value 3840.
fact_even *= float(i);
// fact_odd will always be smaller than fact_even
fact_odd *= float(i-1);
// exp is at most (5,5,5)
exp = vec3(float(i/2), float(i/2), float(i/2));
// pow(c, exp) takes at most the value pow(PI, 5), which is approx. 306
// abs(sum) is at most PI/2.0
sum = sign * pow(abs(c), exp)/fact_even;
// abs(sum/fact_odd) is at most PI/2.0
// cos_c is always bound in the range [-1.0, 1.0)
cos_c += pow(abs(c), exp)*(sum/fact_odd);
sign = -sign;
}
gl_FragColor = vec4(0.5 * cos_c + 0.5, 1.0);
}
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