uniform samplerCube inputTexture;
uniform float cubeWidth;
uniform vec3 sampleDirections[NUM_SAMPLES];
uniform float weights[NUM_SAMPLES];

varying vec3 direction;

void main() {
    // Rotation by PI around y is necessary for consistency with IBLBaker
    // vec3 n = vec3(-direction.x, direction.y, -direction.z);
    vec3 n = normalize(direction);
    vec3 tangent = abs(n.z) < 0.999 ? vec3(0., 0., 1.) : vec3(1., 0., 0.);
    tangent = normalize(cross(tangent, n));
    vec3 bitangent = cross(n, tangent);
    mat3 tbn = mat3(tangent, bitangent, n);

    vec3 color = vec3(0.);
    vec3 h;
    vec3 l;
    float NoH;
    float NoL;
    float totalWeight = 0.;
    for (int i = 0; i < NUM_SAMPLES; i++) {
        h = tbn * sampleDirections[i];
        l = 2. * dot(h, n) * h - n;
        NoH = clamp(dot(h, n), 0.0, 1.0);
        NoL = clamp(dot(l, n), 0.0, 1.0);
        if (NoL > 0.) {
            float solidAngleTexel = 4. * 3.14159 / (6. * cubeWidth * cubeWidth);
            float solidAngleSample = 1.5 * 4.0 / (float(NUM_SAMPLES) * weights[i]); // Multiplying by 1.5 gives better results
            float lod = 0.5 * log2(solidAngleSample / solidAngleTexel);
            // gamma correction needed ?
            color += textureCubeLodEXT(inputTexture, l, lod).xyz * NoL;
            totalWeight += NoL;            
        }
    }

    if (totalWeight != 0.) {
        color /= totalWeight;
    }

    gl_FragColor = vec4(color, 1.0);
    // gl_FragColor = vec4(textureCube(inputTexture, normalize(direction)).xyz, 1.0);
}