From 7c8cc0d97012446d1497367066281bdc1d7cebcb Mon Sep 17 00:00:00 2001
From: Bruno BELANYI <brunobelanyi@gmail.com>
Date: Mon, 16 Mar 2020 17:27:31 +0100
Subject: [PATCH] library: shape: add Sphere implementation

---
 src/shape/sphere.rs | 165 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 165 insertions(+)
 create mode 100644 src/shape/sphere.rs

diff --git a/src/shape/sphere.rs b/src/shape/sphere.rs
new file mode 100644
index 0000000..78f07b9
--- /dev/null
+++ b/src/shape/sphere.rs
@@ -0,0 +1,165 @@
+use super::super::{Point, Point2D, Vector};
+use super::Shape;
+use bvh::aabb::{Bounded, AABB};
+use bvh::ray::Ray;
+
+/// Represent a sphere shape inside the scene.
+#[derive(Debug, PartialEq)]
+pub struct Sphere {
+    /// The sphere is inverted if it is expected to be seen from the inside.
+    inverted: bool,
+    /// The center of the sphere in space.
+    center: Point,
+    /// The radius of the sphere being rendered.
+    radius: f32,
+}
+
+impl Sphere {
+    /// Return a sphere which should be rendered as seen from the outside.
+    pub fn new(center: Point, radius: f32) -> Self {
+        Sphere {
+            center,
+            radius,
+            inverted: false,
+        }
+    }
+
+    /// Return a sphere which should be rendered as seen from the inside.
+    pub fn inverted_new(center: Point, radius: f32) -> Self {
+        Sphere {
+            center,
+            radius,
+            inverted: true,
+        }
+    }
+}
+
+impl Bounded for Sphere {
+    fn aabb(&self) -> AABB {
+        let delt = Vector::new(self.radius, self.radius, self.radius);
+        let min = self.center - delt;
+        let max = self.center + delt;
+        AABB::with_bounds(min, max)
+    }
+}
+
+impl Shape for Sphere {
+    fn intersect(&self, ray: &Ray) -> Option<f32> {
+        use std::mem;
+
+        let delt = self.center - ray.origin;
+        let tca = ray.direction.dot(&delt);
+        let d2 = delt.norm_squared() - tca * tca;
+        let r_2 = self.radius * self.radius;
+
+        if d2 > r_2 {
+            return None;
+        }
+
+        let thc = (r_2 - d2).sqrt();
+        let mut t_0 = tca - thc;
+        let mut t_1 = tca + thc;
+
+        if t_0 > t_1 {
+            mem::swap(&mut t_0, &mut t_1)
+        }
+        if t_0 < 0. {
+            t_0 = t_1
+        }
+
+        if t_0 < 0. {
+            None
+        } else {
+            Some(t_0)
+        }
+    }
+
+    fn normal(&self, point: &Point) -> Vector {
+        let delt = if self.inverted {
+            self.center - point
+        } else {
+            point - self.center
+        };
+        delt.normalize()
+    }
+
+    fn project_texel(&self, point: &Point) -> Point2D {
+        // Project the sphere on the XY-plane
+        Point2D::new(
+            0.5 + (point.x - self.center.x) / (2. * self.radius),
+            0.5 + (point.y - self.center.y) / (2. * self.radius),
+        )
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn simple_sphere() -> Sphere {
+        Sphere::new(Point::origin(), 1.)
+    }
+
+    #[test]
+    fn intersect_along_axis_works() {
+        let sphere = simple_sphere();
+        let ray = Ray::new(Point::new(-2., 0., 0.), Vector::new(1., 0., 0.));
+        assert_eq!(sphere.intersect(&ray), Some(1.))
+    }
+
+    #[test]
+    fn non_intersect_along_axis_works() {
+        let sphere = simple_sphere();
+        let ray = Ray::new(Point::new(-2., 0., 0.), Vector::new(-1., 0., 0.));
+        assert_eq!(sphere.intersect(&ray), None)
+    }
+
+    #[test]
+    fn intersect_not_on_axis() {
+        let sphere = simple_sphere();
+        let ray = Ray::new(Point::new(1., 1., 1.), Vector::new(-1., -1., -1.));
+        assert_eq!(sphere.intersect(&ray), Some(f32::sqrt(3.) - 1.))
+    }
+
+    #[test]
+    fn normal_works() {
+        let sphere = simple_sphere();
+        assert_eq!(
+            sphere.normal(&Point::new(-1., 0., 0.)),
+            Vector::new(-1., 0., 0.)
+        )
+    }
+
+    #[test]
+    fn inverted_normal_works() {
+        let sphere = Sphere::inverted_new(Point::origin(), 1.);
+        assert_eq!(
+            sphere.normal(&Point::new(-1., 0., 0.)),
+            Vector::new(1., 0., 0.)
+        )
+    }
+
+    #[test]
+    fn projection_works_1() {
+        let sphere = simple_sphere();
+        let projection = sphere.project_texel(&Point::new(-1., -1., 1.));
+        assert!(projection.x.abs() < 1e-5);
+        assert!(projection.y.abs() < 1e-5)
+    }
+
+    #[test]
+    fn projection_works_2() {
+        let sphere = simple_sphere();
+        let projection = sphere.project_texel(&Point::new(1., -1., 1.));
+        assert!((projection.x - 1.).abs() < 1e-5);
+        assert!(projection.y.abs() < 1e-5)
+    }
+
+    #[test]
+    fn projection_works_3() {
+        let sphere = simple_sphere();
+        let projection = sphere.project_texel(&Point::new(1., 0., 1.));
+        assert!((projection.x - 1.).abs() < 1e-5);
+        assert!((projection.y - 0.5).abs() < 1e-5)
+    }
+}