Make spheres a bit fuzzy

src/trace.wgsl

@@ -60,6 +60,8 @@ fn to_sphere(center: vec3f, radius: f32, t: ptr<function, f32>) -> bool {
 }
 
 fn trace_fragment(in: Varying) -> vec4f {
+	seed(in.screen);
+
 	var result = vec4(0.0, 0.0, 0.0, 0.0);
 	var color = vec3(1.0, 1.0, 1.0);
 	pos = in.eye;
@@ -83,7 +85,9 @@ fn trace_fragment(in: Varying) -> vec4f {
 		let normal = (pos - s.center) / s.radius;
 		result += vec4(color * s.emit_color * -dot(normal, ray), 0.0);
 		color *= s.reflect_color;
-		ray = reflect(ray, normal);
+		let diffuse = normal + rand_sphere();
+		let specular = reflect(ray, normal);
+		ray = normalize(mix(diffuse, specular, 0.8));
 		if (length(color) < params.min_strength) {
 			break;
 		}
@@ -91,3 +95,38 @@ fn trace_fragment(in: Varying) -> vec4f {
 
 	return clamp(result, vec4(0.0), vec4(1.0));
 }
+
+fn hash(key : u32) -> u32 {
+        var v = key;
+        v *= 0xb384af1bu;
+        v ^= v >> 15u;
+        return v;
+}
+
+var<private> rand_state: u32;
+
+fn seed(key: vec4f) {
+	let x = bitcast<u32>(key.x);
+	let y = bitcast<u32>(key.y);
+	rand_state = hash(hash(x) ^ y);
+}
+
+fn rand_next() -> u32 {
+	rand_state = hash(rand_state);
+	return rand_state;
+}
+
+fn rand_float() -> f32 {
+	return f32(rand_next()) / 0x1p32;
+}
+
+fn rand_sphere() -> vec3f {
+	loop {
+		let v = vec3f(rand_float(), rand_float(), rand_float());
+		let l = length(v);
+		if (length(v) <= 1.0) {
+			return v / l;
+		}
+	}
+	return vec3f(0.0); // unreachable
+}