From a1de85036d09821a21d8f7b21bb85f83a8ff4cb2 Mon Sep 17 00:00:00 2001
From: Bill Thiede <git@xinu.tv>
Date: Sun, 12 Aug 2018 20:58:28 -0700
Subject: [PATCH] Support multiple spheres.

---
 bheisler/src/lib.rs       | 68 +++++++++++++++++++++++++++++++--------
 bheisler/src/rendering.rs | 26 +++++++++++----
 bheisler/src/scene.rs     | 24 +++++++++++++-
 3 files changed, 96 insertions(+), 22 deletions(-)

diff --git a/bheisler/src/lib.rs b/bheisler/src/lib.rs
index 09a8722..4c937ab 100644
--- a/bheisler/src/lib.rs
+++ b/bheisler/src/lib.rs
@@ -7,7 +7,6 @@ pub mod vector;
 
 use image::GenericImage;
 use image::Pixel;
-use rendering::Intersectable;
 use scene::Scene;
 
 pub fn render(scene: &Scene) -> image::DynamicImage {
@@ -17,8 +16,8 @@ pub fn render(scene: &Scene) -> image::DynamicImage {
         for y in 0..scene.height {
             let ray = rendering::Ray::create_prime(x, y, scene);
 
-            if scene.sphere.intersect(&ray) {
-                let c = &scene.sphere.color;
+            if let Some(intersection) = scene.trace(&ray) {
+                let c = &intersection.object.color;
                 image.put_pixel(x, y, image::Rgba(c.to_rgba()))
             } else {
                 image.put_pixel(x, y, black);
@@ -38,19 +37,60 @@ fn test_can_render_scene() {
         width: 800,
         height: 600,
         fov: 90.0,
-        sphere: Sphere {
-            center: Point {
-                x: 0.0,
-                y: 0.0,
-                z: -5.0,
+        spheres: vec![
+            Sphere {
+                center: Point {
+                    x: 2.0,
+                    y: 1.0,
+                    z: -4.0,
+                },
+                radius: 1.5,
+                color: Color {
+                    red: 1.0,
+                    green: 0.2,
+                    blue: 0.2,
+                },
             },
-            radius: 1.0,
-            color: Color {
-                red: 0.4,
-                green: 1.0,
-                blue: 0.4,
+            Sphere {
+                center: Point {
+                    x: 0.0,
+                    y: 0.0,
+                    z: -5.0,
+                },
+                radius: 1.0,
+                color: Color {
+                    red: 0.2,
+                    green: 1.0,
+                    blue: 0.2,
+                },
             },
-        },
+            Sphere {
+                center: Point {
+                    x: -3.0,
+                    y: 1.0,
+                    z: -6.0,
+                },
+                radius: 2.0,
+                color: Color {
+                    red: 0.2,
+                    green: 0.2,
+                    blue: 1.0,
+                },
+            },
+            Sphere {
+                center: Point {
+                    x: 2.0,
+                    y: 1.0,
+                    z: -10.0,
+                },
+                radius: 5.0,
+                color: Color {
+                    red: 1.0,
+                    green: 1.0,
+                    blue: 1.0,
+                },
+            },
+        ],
     };
 
     let img: image::DynamicImage = render(&scene);
diff --git a/bheisler/src/rendering.rs b/bheisler/src/rendering.rs
index 8c443e6..29460a6 100644
--- a/bheisler/src/rendering.rs
+++ b/bheisler/src/rendering.rs
@@ -4,7 +4,7 @@ use scene::Scene;
 use vector::Vector3;
 
 pub trait Intersectable {
-    fn intersect(&self, ray: &Ray) -> bool;
+    fn intersect(&self, ray: &Ray) -> Option<f32>;
 }
 
 pub struct Sphere {
@@ -14,16 +14,28 @@ pub struct Sphere {
 }
 
 impl Intersectable for Sphere {
-    fn intersect(&self, ray: &Ray) -> bool {
+    fn intersect(&self, ray: &Ray) -> Option<f32> {
         //Create a line segment between the ray origin and the center of the sphere
         let l: Vector3 = (self.center - ray.origin).into();
         //Use l as a hypotenuse and find the length of the adjacent side
-        let adj2 = l.dot(&ray.direction);
+        let adj = l.dot(&ray.direction);
         //Find the length-squared of the opposite side
-        //This is equivalent to (but faster than) (l.length() * l.length()) - (adj2 * adj2)
-        let d2 = l.dot(&l) - (adj2 * adj2);
-        //If that length-squared is less than radius squared, the ray intersects the sphere
-        d2 < (self.radius * self.radius)
+        //This is equivalent to (but faster than) (l.length() * l.length()) - (adj * adj)
+        let d2 = l.dot(&l) - (adj * adj);
+        let radius2 = self.radius * self.radius;
+        if d2 > radius2 {
+            return None;
+        }
+        let thc = (radius2 - d2).sqrt();
+        let t0 = adj - thc;
+        let t1 = adj + thc;
+
+        if t0 < 0.0 && t1 < 0.0 {
+            return None;
+        }
+
+        let distance = if t0 < t1 { t0 } else { t1 };
+        Some(distance)
     }
 }
 
diff --git a/bheisler/src/scene.rs b/bheisler/src/scene.rs
index c89562c..62c98c1 100644
--- a/bheisler/src/scene.rs
+++ b/bheisler/src/scene.rs
@@ -1,5 +1,18 @@
+use rendering::Intersectable;
+use rendering::Ray;
 use rendering::Sphere;
 
+pub struct Intersection<'a> {
+    pub distance: f32,
+    pub object: &'a Sphere,
+}
+
+impl<'a> Intersection<'a> {
+    pub fn new(distance: f32, object: &Sphere) -> Intersection {
+        Intersection { distance, object }
+    }
+}
+
 pub struct Color {
     pub red: f32,
     pub green: f32,
@@ -22,5 +35,14 @@ pub struct Scene {
     pub width: u32,
     pub height: u32,
     pub fov: f32,
-    pub sphere: Sphere,
+    pub spheres: Vec<Sphere>,
+}
+
+impl Scene {
+    pub fn trace(&self, ray: &Ray) -> Option<Intersection> {
+        self.spheres
+            .iter()
+            .filter_map(|s| s.intersect(ray).map(|d| Intersection::new(d, s)))
+            .min_by(|i1, i2| i1.distance.partial_cmp(&i2.distance).unwrap())
+    }
 }