From 7ee6da0234b7c72a646ac752828721c7d958b722 Mon Sep 17 00:00:00 2001
From: Robert Garrett <robertgarrett404@gmail.com>
Date: Tue, 6 Jun 2023 18:06:59 -0500
Subject: [PATCH] Feat: Total internal reflection, Schlick's approx.

Completion of Chapter 10: Dielectrics.
---
 src/main.rs     | 13 +++++++++++--
 src/material.rs | 19 +++++++++++++++++--
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 6a590a9..4fee26d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -34,9 +34,9 @@ fn main() {
     // world
 
     let mat_ground = Material::Lambertian{ albedo: Vec3::new(0.8, 0.8, 0.0) };
-    let mat_center = Material::Dielectric { index_refraction: 1.5 };
+    let mat_center = Material::Lambertian{ albedo: Vec3::new(0.1, 0.2, 0.5) };
     let mat_left   = Material::Dielectric { index_refraction: 1.5 };
-    let mat_right  = Material::Metal{ albedo: Vec3::new(0.8, 0.6, 0.2), fuzz: 1.0 };
+    let mat_right  = Material::Metal{ albedo: Vec3::new(0.8, 0.6, 0.2), fuzz: 0.0 };
 
     let mut world = HittableList::new();
     world.add(
@@ -68,6 +68,15 @@ fn main() {
         )
     );
     
+    world.add(
+        Box::new(
+            Sphere{
+                center: Vec3::new(-1.0, 0.0, -1.0),
+                radius: -0.4,
+                material: Some(mat_left), }
+        )
+    );
+
     world.add(
         Box::new(
             Sphere{
diff --git a/src/material.rs b/src/material.rs
index 8f21757..df1abb3 100644
--- a/src/material.rs
+++ b/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);
+    }
 }