// PBR HELPER METHODS
float Square(float value)
{
    return value * value;
}

float getLuminance(vec3 color)
{
    return clamp(dot(color, vec3(0.2126, 0.7152, 0.0722)), 0., 1.);
}

float convertRoughnessToAverageSlope(float roughness)
{
    // Calculate AlphaG as square of roughness; add epsilon to avoid numerical issues
    const float kMinimumVariance = 0.0005;
    float alphaG = Square(roughness) + kMinimumVariance;
    return alphaG;
}