From 3deef4d969dd50ddf540b80fe43ef16af5c17169 Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Mon, 24 Jun 2024 22:51:51 +0100
Subject: Chapter 11

---
 material.go | 46 ++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 44 insertions(+), 2 deletions(-)

(limited to 'material.go')

diff --git a/material.go b/material.go
index 66ea256..62a0467 100644
--- a/material.go
+++ b/material.go
@@ -1,5 +1,9 @@
 package main
 
+import (
+  maths "math"
+)
+
 //Material
 
 type Material interface {
@@ -7,7 +11,7 @@ type Material interface {
   Scatter(r Ray, hit HitRecord) Ray
 }
 
-//Lambertian
+//Lambertian < Material
 
 type Lambertian struct {
   Albedo Vec3
@@ -25,7 +29,7 @@ func (l Lambertian) Scatter(r Ray, hit HitRecord) Ray {
   return Ray{hit.P, scat_dir, true};
 }
 
-//Metal
+//Metal < Material
 
 type Metal struct {
   Albedo Vec3
@@ -42,3 +46,41 @@ func (m Metal) Scatter(r Ray, hit HitRecord) Ray {
   valid := refl.Dot(hit.N) > 0;
   return Ray{hit.P, refl, valid};
 }
+
+//Dielectric < Material
+
+type Dielectric struct {
+  RefractionIndex float64
+}
+
+func (d Dielectric) Attenuation() Vec3 {
+  return SplatVec3(1);
+}
+
+func (d Dielectric) Scatter(r Ray, hit HitRecord) Ray {
+  ri := d.RefractionIndex;
+  if hit.F {
+    ri = 1.0 / ri;
+  }
+
+  unit_dir := r.Direction.Unit();
+  cos_theta := maths.Min(unit_dir.Neg().Dot(hit.N), 1.0);
+  sin_theta := maths.Sqrt(1.0 - maths.Pow(cos_theta, 2));
+
+  cannot_refract := ri * sin_theta > 1.0;
+
+  var dir Vec3;
+  if cannot_refract || schlick_reflect(cos_theta, ri) {
+    dir = unit_dir.Reflect(hit.N);
+  } else {
+    dir = unit_dir.Refract(hit.N, ri);
+  }
+
+  return Ray{hit.P, dir, true};
+}
+
+func schlick_reflect(cos_theta float64, ri float64) bool {
+  r0 := maths.Pow((1 - ri) / (1 + ri), 2);
+  approx := r0 + (1 - r0) * maths.Pow(1 - cos_theta, 5);
+  return approx > Interval{0, 1}.Sample();
+}
-- 
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