
/*
** 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.
*/


#ifdef GL_ES
precision mediump float;
#endif
varying vec4 color;

void main (void)
{
    const float M_PI = 3.14159265358979323846;
    vec2 x = 2.0 * (color.gg - 0.5);
    vec2 y = 2.0 * (color.bb - 0.5);
    vec2 c;
    vec2 atan_c = vec2(0.0);
    vec2 scale = vec2(1.0);
    vec2 sign = vec2(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[0] > epsilon || abs(y[0]) > epsilon)
    {
        if(x[0] < 0.0 ^^ y[0] < 0.0)
        {
            sign[0] = -1.0;
        }

        if(abs(y[0]) <= abs(x[0]))
        {
            c[0] = abs(y[0] / x[0]);
            // Taylors series expansion for atan
            atan_c[0] += scale[0] * pow(c[0], float(1)) / float(1);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(3)) / float(3);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(5)) / float(5);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(7)) / float(7);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(9)) / float(9);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(11)) / float(11);
            scale[0] *= -1.0;

            result[0] = sign[0] * atan_c[0] / (2.0 * M_PI) + 0.5;
        }
        else
        {
            c[0] = abs(x[0] / y[0]);

            // Taylors series expansion for atan
            atan_c[0] += scale[0] * pow(c[0], float(1)) / float(1);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(3)) / float(3);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(5)) / float(5);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(7)) / float(7);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(9)) / float(9);
            scale[0] *= -1.0;
            atan_c[0] += scale[0] * pow(c[0], float(11)) / float(11);
            scale[0] *= -1.0;

            result[0] = sign[0] * (M_PI / 2.0 - atan_c[0]) / (2.0 * M_PI) + 0.5;
        }

        if(x[0] < 0.0)
            if(y[0] < 0.0) result[0] -= 0.5;
            else if(y[0] > 0.0) result[0] += 0.5;
    }

    if(x[1] > epsilon || abs(y[1]) > epsilon)
    {

        if(x[1] < 0.0 ^^ y[1] < 0.0)
        {
            sign[1] = -1.0;
        }

        if(abs(y[1]) <= abs(x[1]))
        {
            c[1] = abs(y[1] / x[1]);
            // Taylors series expansion for atan
            atan_c[1] += scale[1] * pow(c[1], float(1)) / float(1);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(3)) / float(3);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(5)) / float(5);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(7)) / float(7);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(9)) / float(9);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(11)) / float(11);
            scale[1] *= -1.0;

            result[1] = sign[1] * atan_c[1] / (2.0 * M_PI) + 0.5;
        }
        else
        {
            c[1] = abs(x[1] / y[1]);

            // Taylors series expansion for atan
            atan_c[1] += scale[1] * pow(c[1], float(1)) / float(1);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(3)) / float(3);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(5)) / float(5);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(7)) / float(7);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(9)) / float(9);
            scale[1] *= -1.0;
            atan_c[1] += scale[1] * pow(c[1], float(11)) / float(11);
            scale[1] *= -1.0;

            result[1] = sign[1] * (M_PI / 2.0 - atan_c[1]) / (2.0 * M_PI) + 0.5;
        }

        if(x[1] < 0.0)
            if(y[1] < 0.0) result[1] -= 0.5;
            else if(y[1] > 0.0) result[1] += 0.5;
    }

    gl_FragColor = result;
}
