From f230ab264adebce9868d088ea408445d66254963 Mon Sep 17 00:00:00 2001
From: Martin Lehoux <martin@lehoux.net>
Date: Sat, 4 Apr 2020 17:53:48 -0700
Subject: feat - Add several entities

---
 color.go   |  4 ++--
 hitable.go | 50 ++++++++++++++++++++++++++++++++++++++++----------
 main.go    | 10 ++++++++--
 3 files changed, 50 insertions(+), 14 deletions(-)

diff --git a/color.go b/color.go
index 1ac8141..b8776ca 100644
--- a/color.go
+++ b/color.go
@@ -16,8 +16,8 @@ func LinearBlend(color1 Color, color2 Color, param float64) Color {
 	}
 }
 
-func RayColor(ray Ray, hitable Hitable) Color {
-	hit := hitable.Hit(ray)
+func RayColor(ray Ray, hitable_list HitableList) Color {
+	hit := hitable_list.Hit(ray, T_MIN, T_MAX)
 	if hit.time > 0.0 {
 		vec := AddVec3D(hit.normal, Vec3D{1.0, 1.0, 1.0})
 		return LinearBlend(Color{vec.x, vec.y, vec.z}, Color{0.0, 0.0, 0.0}, 0.5)
diff --git a/hitable.go b/hitable.go
index ed18a5d..bc9f9bc 100644
--- a/hitable.go
+++ b/hitable.go
@@ -9,8 +9,25 @@ type HitRecord struct {
 }
 
 type Hitable interface {
-	Hit(ray Ray) HitRecord
-	Normal(point Vec3D) Vec3D
+	Hit(ray Ray, t_min float64, t_max float64) HitRecord
+	Normal(point Vec3D, ray Ray) Vec3D
+}
+
+type HitableList []Hitable
+
+func (hitable_list HitableList) Hit(ray Ray, t_min float64, t_max float64) HitRecord {
+	closest_hit := HitRecord{T_MAX, Vec3D{}, Vec3D{}}
+	for _, hitable := range hitable_list {
+		hit := hitable.Hit(ray, t_min, closest_hit.time)
+		if hit.time > 0 {
+			closest_hit = hit
+		}
+	}
+	if closest_hit.time < T_MAX {
+		return closest_hit
+	} else {
+		return HitRecord{}
+	}
 }
 
 type Sphere struct {
@@ -18,20 +35,33 @@ type Sphere struct {
 	radius float64
 }
 
-func (sphere Sphere) Normal(point Vec3D) Vec3D {
-	return SubVec3D(point, sphere.center).Unit()
+func (sphere Sphere) Normal(point Vec3D, ray Ray) Vec3D {
+	out_normal := SubVec3D(point, sphere.center).Unit()
+	if DotProduct(ray.direction, out_normal) < 0.0 {
+		return out_normal
+	} else {
+		return out_normal.Invert()
+	}
 }
 
-func (sphere Sphere) Hit(ray Ray) HitRecord {
+func (sphere Sphere) Hit(ray Ray, t_min float64, t_max float64) HitRecord {
 	oc := SubVec3D(ray.origin, sphere.center)
 	a := DotProduct(ray.direction, ray.direction)
 	b := 2.0 * DotProduct(oc, ray.direction)
 	c := DotProduct(oc, oc) - sphere.radius*sphere.radius
 	discriminant := b*b - 4*a*c
-	if discriminant < 0 {
-		return HitRecord{}
+	if discriminant > 0 {
+		root := math.Sqrt(discriminant)
+		time := (-b - root) / (2.0 * a)
+		if t_min < time && time < t_max {
+			point := ray.PointAt(time)
+			return HitRecord{time, point, sphere.Normal(point, ray)}
+		}
+		time = (-b + root) / (2.0 * a)
+		if t_min < time && time < t_max {
+			point := ray.PointAt(time)
+			return HitRecord{time, point, sphere.Normal(point, ray)}
+		}
 	}
-	time := (-b - math.Sqrt(discriminant)) / (2.0 * a)
-	point := ray.PointAt(time)
-	return HitRecord{time, point, sphere.Normal(point)}
+	return HitRecord{}
 }
diff --git a/main.go b/main.go
index a1cb2c9..daf2718 100644
--- a/main.go
+++ b/main.go
@@ -2,19 +2,25 @@ package main
 
 const HEIGHT = 100
 const WIDTH = 200
+const T_MIN = 0.001
+const T_MAX = 1000
 
 func main() {
 	LOWER_LEFT_CORNER := Vec3D{-2.0, -1.0, -1.0}
 	ORIGIN := Vec3D{0.0, 0.0, 0.0}
 	HORIZONTAL := Vec3D{4.0, 0.0, 0.0}
 	VERTICAL := Vec3D{0.0, 2.0, 0.0}
-	SPHERE := Sphere{Vec3D{0.0, 0.0, -1.0}, .5}
+
+	SPHERE := Sphere{Vec3D{0.0, 0.0, -1.0}, 0.5}
+	EARTH := Sphere{Vec3D{0.0, -100.5, -1.0}, 100.0}
+
+	WORLD := HitableList{EARTH, SPHERE}
 	frame := Frame{}
 	for x := 0; x < HEIGHT; x++ {
 		for y := 0; y < WIDTH; y++ {
 			u, v := float64(y)/float64(WIDTH), float64(x)/float64(HEIGHT)
 			ray := Ray{ORIGIN, AddVec3D(AddVec3D(LOWER_LEFT_CORNER, ScalarProduct(u, HORIZONTAL)), ScalarProduct(v, VERTICAL))}
-			color := RayColor(ray, SPHERE)
+			color := RayColor(ray, WORLD)
 			frame[HEIGHT-1-x][y] = color.ToPixel()
 		}
 	}
-- 
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