uniform float checker_size_inv;
uniform vec4 checker_color_1, checker_color_2;
varying vec3 object_position;

bool
checker_color(vec3 p)
{
    vec3 pprime = checker_size_inv * object_position;
    return fract((floor(pprime.x) + floor(pprime.z)) * 0.5) == 0.0;
}

void
main()
{
    float distance_factor = (gl_FragCoord.z * 0.5 + 0.5);
    distance_factor = pow(distance_factor, 500.0)*0.5;
    
    gl_FragColor = checker_color(object_position)
        ? mix(checker_color_1, checker_color_2, distance_factor)
        : mix(checker_color_2, checker_color_1, distance_factor);
}