From 19124e8c21b86dccb9e9d874c614aacb05c0fd4a Mon Sep 17 00:00:00 2001
From: Martin Lehoux <martin@lehoux.net>
Date: Sat, 4 Apr 2020 15:30:25 -0700
Subject: feat - Display sphere

---
 color.go   |  5 ++++-
 hitable.go | 19 +++++++++++++++++++
 main.go    |  3 ++-
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 hitable.go

diff --git a/color.go b/color.go
index 2bee0af..3514be6 100644
--- a/color.go
+++ b/color.go
@@ -16,7 +16,10 @@ func LinearBlend(color1 Color, color2 Color, param float64) Color {
 	}
 }
 
-func RayColor(ray Ray) Color {
+func RayColor(ray Ray, hitable Hitable) Color {
+	if hitable.Hit(ray) {
+		return Color{1.0, 0.0, 0.0}
+	}
 	param := 0.5 * (ray.direction.Unit().y + 1)
 	return LinearBlend(Color{0.5, 0.7, 1.0}, Color{1.0, 1.0, 1.0}, param)
 }
diff --git a/hitable.go b/hitable.go
new file mode 100644
index 0000000..a44026d
--- /dev/null
+++ b/hitable.go
@@ -0,0 +1,19 @@
+package main
+
+type Hitable interface {
+	Hit(ray Ray) bool
+}
+
+type Sphere struct {
+	center Vec3D
+	radius float64
+}
+
+func (sphere Sphere) Hit(ray Ray) bool {
+	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
+	return discriminant > 0
+}
diff --git a/main.go b/main.go
index 981a324..a1cb2c9 100644
--- a/main.go
+++ b/main.go
@@ -8,12 +8,13 @@ func main() {
 	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}
 	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)
+			color := RayColor(ray, SPHERE)
 			frame[HEIGHT-1-x][y] = color.ToPixel()
 		}
 	}
-- 
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