1 files changed, 61 insertions, 2 deletions
diff --git a/material.go b/material.go
index a36fb52..4667bd6 100644
--- a/material.go
+++ b/material.go
@@ -1,27 +1,86 @@
 package main
 
+import (
+	"math"
+	"math/rand"
+)
+
+// Material generate color when hitable is hit
 type Material interface {
 	Scatter(ray Ray, hit HitRecord) (Ray, Color)
 }
 
+// Lambertian is diffuse material
 type Lambertian struct {
 	albedo Color
 }
 
+// Scatter for Lambertians
 func (lambertian Lambertian) Scatter(ray Ray, hit HitRecord) (Ray, Color) {
 	// TODO: Use the correct distribution
-	direction := AddVec3D(hit.normal, RandomInUnitSphere())
+	var normal Vec3D
+	if DotProduct(ray.direction, hit.normal) < 0.0 {
+		normal = hit.normal
+	} else {
+		normal = hit.normal.Invert()
+	}
+	direction := AddVec3D(normal, RandomInUnitSphere())
 	scattered := Ray{hit.point, direction}
 	return scattered, lambertian.albedo
 }
 
+// Metal material
 type Metal struct {
 	albedo Color
 	fuzz   float64
 }
 
+// Scatter for Metal
 func (metal Metal) Scatter(ray Ray, hit HitRecord) (Ray, Color) {
-	reflected := AddVec3D(ray.direction.Unit().Reflect(hit.normal), ScalarProduct(metal.fuzz, RandomInUnitSphere()))
+	var normal Vec3D
+	if DotProduct(ray.direction, hit.normal) < 0.0 {
+		normal = hit.normal
+	} else {
+		normal = hit.normal.Invert()
+	}
+	reflected := AddVec3D(ray.direction.Unit().Reflect(normal), ScalarProduct(metal.fuzz, RandomInUnitSphere()))
 	scattered := Ray{hit.point, reflected}
 	return scattered, metal.albedo
 }
+
+// Dielectric is glass for instance
+type Dielectric struct {
+	eta float64
+}
+
+// Scatter for Dielectric
+func (dielectric Dielectric) Scatter(ray Ray, hit HitRecord) (Ray, Color) {
+	var etaI, etaT float64
+	var normal Vec3D
+	attenuation := Color{1.0, 1.0, 1.0}
+	if DotProduct(ray.direction, hit.normal) < 0.0 {
+		etaI, etaT = 1.0, dielectric.eta
+		normal = hit.normal
+	} else {
+		etaI, etaT = 1.0, dielectric.eta
+		normal = hit.normal.Invert()
+	}
+	vecI := ray.direction.Unit()
+	cosTheta := FMin(DotProduct(vecI.Invert(), normal), 1.0)
+	sinTheta := math.Sqrt(1.0 - cosTheta*cosTheta)
+	if etaI/etaT*sinTheta > 1.0 || rand.Float64() < Schlick(cosTheta, etaI, etaT) {
+		reflected := ray.direction.Unit().Reflect(normal)
+		scattered := Ray{hit.point, reflected}
+		return scattered, attenuation
+	}
+	refracted := Refract(vecI, normal, etaI, etaT)
+	scattered := Ray{hit.point, refracted}
+	return scattered, attenuation
+}
+
+// Schlick glass refraction probability
+func Schlick(cosine float64, etaI float64, etaT float64) float64 {
+	r0 := (1.0 - etaI/etaT) / (1.0 + etaI/etaT)
+	r0 *= r0
+	return r0 + (1.0-r0)*math.Pow(1.0-cosine, 5)
+}