From 69e0357135a49157ed7a8c2e57c331e960422e15 Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Thu, 13 Jun 2024 14:38:50 +0100
Subject: Chapter 11

---
 lib/hittable.rb |  6 +++---
 lib/material.rb | 31 +++++++++++++++++++++++++++++++
 lib/vec3.rb     |  7 +++++++
 rtiaw           | 12 +++++++-----
 4 files changed, 48 insertions(+), 8 deletions(-)

diff --git a/lib/hittable.rb b/lib/hittable.rb
index facdd14..2a433d8 100644
--- a/lib/hittable.rb
+++ b/lib/hittable.rb
@@ -2,12 +2,12 @@ class HitRecord
   def initialize(point, t, ray, out_normal, material)
     @point = point
     @t = t
-    front_face = ray.direction.dot(out_normal) < 0
-    @normal = front_face ? out_normal : -out_normal
+    @front_face = ray.direction.dot(out_normal) < 0
+    @normal = @front_face ? out_normal : -out_normal
     @material = material
   end
 
-  attr_accessor :point, :normal, :t, :material
+  attr_reader :point, :normal, :t, :front_face, :material
 end
 
 class Hittable
diff --git a/lib/material.rb b/lib/material.rb
index c935147..752b85f 100644
--- a/lib/material.rb
+++ b/lib/material.rb
@@ -36,3 +36,34 @@ class Metal < Material
     end
   end
 end
+
+class Dielectric < Material
+  def initialize(ref_index)
+    @ref_index = ref_index
+  end
+
+  def attenuation
+    Colour.new(1.0, 1.0, 1.0)
+  end
+
+  def scatter(ray, record)
+    ri = record.front_face ? (1.0 / @ref_index) : @ref_index
+    unit_dir = ray.direction.unit
+    costheta = [(-unit_dir).dot(record.normal), 1.0].min
+    sintheta = (1.0 - costheta ** 2) ** 0.5
+
+    cannot_refract = ri * sintheta > 1.0
+    maybe_reflect_anyway = Dielectric.reflectance(costheta, ri) > rand
+
+    refr = cannot_refract || maybe_reflect_anyway ?
+      unit_dir.reflect(record.normal) :
+      unit_dir.refract(record.normal, ri)
+
+    Ray.new(record.point, refr)
+  end
+
+  def self.reflectance(costheta, ri)
+    r0 = ((1.0 - ri) / (1.0 + ri)) ** 2
+    r0 + (1.0 - r0) * (1.0 - costheta) ** 5
+  end
+end
diff --git a/lib/vec3.rb b/lib/vec3.rb
index 3069b65..478fe4c 100644
--- a/lib/vec3.rb
+++ b/lib/vec3.rb
@@ -60,6 +60,13 @@ class Vec3
     self - normal * dot(normal) * 2
   end
 
+  def refract(normal, etaratio)
+    costheta = [(-self).dot(normal), 1.0].min
+    rout_perp = (self + normal * costheta) * etaratio
+    rout_parr = normal * -((1.0 - rout_perp.mag_sqr).abs ** 0.5)
+    rout_perp + rout_parr
+  end
+
   def in_unit_sphere?
     mag_sqr < 1
   end
diff --git a/rtiaw b/rtiaw
index 71b4fbd..45487b9 100755
--- a/rtiaw
+++ b/rtiaw
@@ -10,14 +10,16 @@ require 'material'
 
 mat_ground = Lambertian.new(0.8, 0.8, 0.0)
 mat_centre = Lambertian.new(0.1, 0.2, 0.5)
-mat_left = Metal.new(0.8, 0.8, 0.8, 0.3)
+mat_left = Dielectric.new(1.5)
+mat_bubble = Dielectric.new(1 / 1.5)
 mat_right = Metal.new(0.8, 0.6, 0.2, 1.0)
 
 world = Hittables.new
-world << Sphere.new(0, -100.5, -1,    100,    mat_ground)
-world << Sphere.new(0,      0, -1,    0.5,    mat_centre)
-world << Sphere.new(-1,     0, -1,    0.5,    mat_left)
-world << Sphere.new(1,      0, -1,    0.5,    mat_right)
+world << Sphere.new( 0, -100.5,   -1,    100,    mat_ground)
+world << Sphere.new( 0,      0, -1.2,    0.5,    mat_centre)
+world << Sphere.new(-1,      0,   -1,    0.5,    mat_left)
+world << Sphere.new(-1,      0,   -1,    0.4,    mat_bubble)
+world << Sphere.new( 1,      0,   -1,    0.5,    mat_right)
 
 camera = Camera.new(400, 16.0 / 9)
 camera.render(world)
-- 
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