/*******************************************************************************************
*
*   rPBR [shader] - Irradiance cubemap fragment shader
*
*   Copyright (c) 2017 Victor Fisac
*
**********************************************************************************************/

#version 330

# Input vertex attributes (from vertex shader)
in vec3 fragPos

# Input uniform values
uniform samplerCube environmentMap

# Constant values
const float PI = 3.14159265359f

# Output fragment color
out vec4 finalColor

void main()
[
    # The sample direction equals the hemisphere's orientation
    vec3 normal = normalize(fragPos)

    vec3 irradiance = vec3(0.0)

    vec3 up = vec3(0.0, 1.0, 0.0)
    vec3 right = cross(up, normal)
    up = cross(normal, right)

    float sampleDelta = 0.025f
    float nrSamples = 0.0

    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

            # Fetch color from environment cubemap
            irradiance += texture(environmentMap, sampleVec).rgb*cos(theta)*sin(theta)
            nrSamples++
        ]
    ]

    # Calculate irradiance average value from samples
    irradiance = PI*irradiance*(1.0/float(nrSamples))

    # Calculate final fragment color
    finalColor = vec4(irradiance, 1.0)
]