README.md

@@ -1 +1,5 @@
-# Photon Light
+# Forward ray tracing demo
+
+![Screenshot](screenshot.jpg)
+
+A simple forward ray tracing demo. “Forward” means the rays are traced starting from the light (unlike common “reverse” ray tracing where rays are traced from the camera).

src/lib.rs

@@ -14,7 +14,7 @@ use crate::{
 		DEPTH_FORMAT, OUTPUT_FORMAT,
 		lines::{LookParams, Mesh, Pipeline, Vertex},
 	},
-	trace::{Hit, Lambertian, Reflector, Scene, Source, Sphere},
+	trace::{DiscReflector, Hit, Lambertian, Reflector, Scene, Source, Sphere},
 };
 
 pub use crate::ffi::{RedrawArgs, SphericalPosition, UseNormal};
@@ -370,12 +370,12 @@ impl Core {
 				}
 			}
 		}
+		let reflector = DiscReflector { roughness: 0.25 };
 		if args.reflections > 0 {
 			let mut hits1 = hits.clone();
 			for _ in 0..args.reflections {
 				let mut hits2: Vec<LightHit> = Vec::with_capacity(hits1.len());
 				for hit in &hits1 {
-					let reflector = Lambertian;
 					let reflected = reflector.reflect(&mut prng, hit.normal, hit.incident.dir);
 					let light = hit.light * hit.object.color;
 					let ray = Ray::new(hit.incident.base, reflected);
@@ -422,7 +422,6 @@ impl Core {
 					};
 					assert!(normal.is_normalized());
 					assert!(hit.incident.dir.is_normalized());
-					let reflector = Lambertian;
 					let in_lm = light_hit.light;
 					let out_cd = in_lm
 						* reflector.brdf(normal, light_hit.incident.dir, -hit.incident.dir)

src/trace.rs

@@ -5,6 +5,10 @@ use rand_distr::{Distribution, UnitSphere};
 
 use crate::{camera::OrbitalCamera, ray::Ray};
 
+pub use reflective::DiscReflector;
+
+mod reflective;
+
 #[derive(Debug, Clone)]
 pub struct Source {
 	/// Horizontal position (angle), in radians from +X towards +Y.

src/trace/reflective.rs

@@ -0,0 +1,46 @@
+use std::f32::consts::PI;
+
+use glam::Vec3;
+use rand_distr::{Distribution, Uniform, UnitSphere};
+
+use super::Reflector;
+
+#[derive(Debug, Clone)]
+pub struct DiscReflector {
+	pub roughness: f32,
+}
+
+impl Reflector for DiscReflector {
+	fn brdf(&self, normal: Vec3, incident: Vec3, reflected: Vec3) -> f32 {
+		let specular = incident.reflect(normal);
+		let convergence = reflected.dot(specular);
+		if convergence <= 1. - self.roughness {
+			return 0.;
+		}
+		let c = -incident.dot(normal) * reflected.dot(normal);
+		if c < 0. {
+			return 0.;
+		}
+		0.5 / (self.roughness * PI * (specular.dot(normal) * reflected.dot(normal)).sqrt())
+	}
+
+	fn reflect(&self, rgen: &mut impl rand::Rng, normal: Vec3, incident: Vec3) -> Vec3 {
+		let specular = incident.reflect(normal);
+
+		let dist_theta = Uniform::new(0., (1. - self.roughness).acos()).unwrap();
+		let dist_sphere = UnitSphere;
+
+		loop {
+			let theta: f32 = dist_theta.sample(rgen);
+			let (xy, z) = theta.sin_cos();
+
+			let spherical: Vec3 = dist_sphere.sample(rgen).into();
+			let off_dir = spherical.reject_from_normalized(specular).normalize();
+
+			let reflected = xy * off_dir + z * specular;
+			if reflected.dot(normal) >= 0. {
+				return reflected;
+			}
+		}
+	}
+}