add generic reflector

src/trace.rs

@@ -1,7 +1,7 @@
 use std::f32::consts::PI;
 
 use glam::{Mat4, Vec2, Vec3, vec3};
-use rand_distr::Distribution;
+use rand_distr::{Distribution, UnitSphere};
 
 use crate::{camera::OrbitalCamera, ray::Ray};
 
@@ -134,3 +134,27 @@ impl Scene {
 		})
 	}
 }
+
+pub trait Reflector {
+	fn brdf(&self, normal: Vec3, incident: Vec3, reflected: Vec3) -> f32 /* 1/sr */;
+	fn reflect(&self, rgen: &mut impl rand::Rng, normal: Vec3, incident: Vec3) -> Vec3;
+}
+
+pub struct Lambertian;
+
+impl Reflector for Lambertian {
+	fn brdf(&self, _normal: Vec3, _incident: Vec3, _reflected: Vec3) -> f32 {
+		1. / PI
+	}
+
+	fn reflect(&self, rgen: &mut impl rand::Rng, normal: Vec3, _incident: Vec3) -> Vec3 {
+		let sphere: Vec3 = UnitSphere.sample(rgen).into();
+		let sphere_n = normal.dot(sphere); // uniform on [-1, 1]!
+		let sphere_t = sphere - sphere_n * normal;
+
+		let out_n_len2 = sphere_n.abs();
+		let out_t = (1. + out_n_len2).recip().sqrt() * sphere_t;
+		let out_n = out_n_len2.sqrt() * normal;
+		out_t + out_n
+	}
+}