
/*
** Copyright (c) 2012 The Khronos Group Inc.
**
** Permission is hereby granted, free of charge, to any person obtaining a
** copy of this software and/or associated documentation files (the
** "Materials"), to deal in the Materials without restriction, including
** without limitation the rights to use, copy, modify, merge, publish,
** distribute, sublicense, and/or sell copies of the Materials, and to
** permit persons to whom the Materials are furnished to do so, subject to
** the following conditions:
**
** The above copyright notice and this permission notice shall be included
** in all copies or substantial portions of the Materials.
**
** THE MATERIALS ARE PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
** EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
** MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
** IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
** CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
** TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
** MATERIALS OR THE USE OR OTHER DEALINGS IN THE MATERIALS.
*/


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;
}
