#version 330

in vec3 localPos;
out vec4 fragColor;

uniform samplerCube environmentMap;

const float PI = 3.14159265359;

void main() {
	// sample dir = hemisphere's orientation
	vec3 normal = normalize(localPos);
	vec3 irradiance = vec3(0);

	vec3 up = vec3(0, 1, 0);
	vec3 right = cross(up, normal);
	up = cross(normal, right);

	const float sampleDelta = 0.025;
	float numSamples = 0;
	for (float phi = 0; phi < 2 * PI; phi += sampleDelta) {
		for (float theta = 0; theta < 0.5 * PI; theta += sampleDelta) {
			vec3 tangentSample = vec3(sin(theta) * cos(phi), sin(theta) * sin(phi), cos(theta));
			// tangent space -> world space
			vec3 sampleVec = tangentSample.x * right + tangentSample.y * up + tangentSample.z * normal;
			irradiance += texture(environmentMap, sampleVec).rgb * cos(theta) * sin(theta);
			numSamples++;
		}
	}

	irradiance = (PI * irradiance) / numSamples;

	fragColor = vec4(irradiance, 1);
}