Function d_ggx

tools/bloom-reference/src/main.rs:1077–1082

GGX (Trowbridge-Reitz) normal distribution.

(n_dot_h: f32, alpha: f32)

/// GGX (Trowbridge-Reitz) normal distribution.
fn d_ggx(n_dot_h: f32, alpha: f32) -> f32 {
    let a2 = alpha * alpha;
    let nh2 = n_dot_h * n_dot_h;
    let denom = nh2 * (a2 - 1.0) + 1.0;
    a2 / (std::f32::consts::PI * denom * denom)
}

/// Smith visibility term for GGX with height-correlated formulation.
/// Closer to the ground truth than the separable G1*G2, and only