Function importance_sample_ggx

native/shared/src/renderer/brdf_lut.rs:42–58

(xi: (f32, f32), n: [f32; 3], roughness: f32)

}

fn importance_sample_ggx(xi: (f32, f32), n: [f32; 3], roughness: f32) -> [f32; 3] {
    let a = roughness * roughness;
    let phi = 2.0 * std::f32::consts::PI * xi.0;
    let cos_theta = ((1.0 - xi.1) / (1.0 + (a * a - 1.0) * xi.1)).sqrt();
    let sin_theta = (1.0 - cos_theta * cos_theta).max(0.0).sqrt();
    let h_local = [sin_theta * phi.cos(), sin_theta * phi.sin(), cos_theta];

    // Build TBN around N.
    let up = if n[2].abs() < 0.999 { [0.0, 0.0, 1.0] } else { [1.0, 0.0, 0.0] };
    let t = normalize3(cross3(up, n));
    let b = cross3(n, t);
    [
        t[0] * h_local[0] + b[0] * h_local[1] + n[0] * h_local[2],
        t[1] * h_local[0] + b[1] * h_local[1] + n[1] * h_local[2],
        t[2] * h_local[0] + b[2] * h_local[1] + n[2] * h_local[2],
    ]
}

fn cross3(a: [f32; 3], b: [f32; 3]) -> [f32; 3] {
    [a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0]]