Colour sphere based on normal

demo.c

@@ -59,8 +59,11 @@ static double intersect(sphere_t sphere, ray_t ray)
 static vec3_t raycol(ray_t ray)
 {
 	const double t = intersect(obj, ray);
-	if (t > 0.0)
-		return red;
+	if (t > 0.0) {
+		const vec3_t p = vec3_add(ray.orig, vec3_scale(ray.dir, t));
+		const vec3_t normal = vec3_unit(vec3_sub(p, obj.centre));
+		return vec3_scale(vec3_add(normal, white), 0.5);
+	}
 
 	const double a = (ray.dir.y + 1.0) / 2.0;
 	return vec3_add(vec3_scale(lightblue, a), vec3_scale(white, 1 - a));