3 files changed, 41 insertions, 3 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