#version 330 core

out vec4 FragColor;
in vec3 fragPos;

uniform samplerCube environmentMap;
uniform float sampleDelta;

const float PI = 3.14159265359;

void main() {		
    vec3 normal = normalize(fragPos);

    vec3 irradiance = vec3(0.0);   
    
    // tangent space calculation from origin point
    vec3 up = vec3(0.0, 1.0, 0.0);
    vec3 right = cross(up, normal);
    up = cross(normal, right);
       
    float nrSamples = 0.0f;

    for(float phi = 0.0; phi < 2.0 * PI; phi += sampleDelta) {
        for(float theta = 0.0; theta < 0.5 * PI; theta += sampleDelta) {
            // spherical to cartesian (in tangent space)
            vec3 tangentSample = vec3(sin(theta) * cos(phi),  sin(theta) * sin(phi), cos(theta));
            // tangent space to world
            vec3 sampleVec = tangentSample.x * right + tangentSample.y * up + tangentSample.z * normal; 

            irradiance += texture(environmentMap, sampleVec).rgb * cos(theta) * sin(theta);
            nrSamples++;
        }
    }

    irradiance = PI * irradiance * (1.0 / float(nrSamples));
    
    FragColor = vec4(irradiance, 1.0);
}
