Feat: Total internal reflection, Schlick's approx.

Completion of Chapter 10: Dielectrics.

src/material.rs

@@ -4,6 +4,7 @@ use crate::hittable::HitRecord;
 use crate::vec3;
 use crate::vec3::Vec3;
 
+use rand::Rng;
 use rand::rngs::SmallRng;
 use rand::distributions::Uniform;
 
@@ -64,14 +65,28 @@ impl Material {
                 let refraction_ratio = if rec.front_face { 1.0 / index_refraction } else { *index_refraction };
                 
                 let unit_direction = Vec3::as_unit(ray_in.dir);
-                let refracted = Vec3::refract(unit_direction, rec.normal, refraction_ratio);
+                let cos_theta = vec3::min(Vec3::dot(-unit_direction, rec.normal), 1.0);
+                let sin_theta = (1.0 - cos_theta * cos_theta).sqrt();
 
+                let cannot_refract = refraction_ratio * sin_theta > 1.0;
+                let direction = if cannot_refract || Material::reflectance(cos_theta, refraction_ratio) > srng.sample(distrib) {
+                    Vec3::reflect(unit_direction, rec.normal)
+                } else {
+                    Vec3::refract(unit_direction, rec.normal, refraction_ratio)
+                };
                 *scattered = Ray {
                     orig: rec.p,
-                    dir: refracted
+                    dir: direction 
                 };
                 return true;
             },
         }
     }
+
+    fn reflectance(cosine: f32, ref_idx: f32) -> f32 {
+        // Schlick's approximation for reflectance.
+        let r0 = (1.0 - ref_idx) / (1.0 + ref_idx);
+        let r0 = r0 * r0;
+        return r0 + (1.0 - r0) * (1.0 - cosine).powf(5.0);
+    }
 }