library: light: point_light: add sample_ray method

pathtracer/src/light/point_light.rs

@@ -1,7 +1,9 @@
 use super::{Light, SpatialLight};
 use crate::core::LinearColor;
 use crate::{Point, Vector};
+use beevee::ray::Ray;
 use nalgebra::Unit;
+use rand::{distributions::Uniform, Rng};
 use serde::Deserialize;
 
 /// Represent a light emanating from a point in space, following the square distance law.
@@ -29,6 +31,36 @@ impl PointLight {
     pub fn new(position: Point, color: LinearColor) -> Self {
         PointLight { position, color }
     }
+
+    /// Uniformly sample a ray from the point-light in a random direction.
+    ///
+    /// # Examles
+    ///
+    ///```
+    /// # use pathtracer::light::PointLight;
+    /// # use pathtracer::core::color::LinearColor;
+    /// # use pathtracer::Point;
+    /// #
+    /// let dir_light = PointLight::new(
+    ///     Point::origin(),
+    ///     LinearColor::new(1.0, 0.0, 1.0),
+    /// );
+    /// let sampled = dir_light.sample_ray();
+    /// ```
+    pub fn sample_ray(&self) -> Ray {
+        let mut rng = rand::thread_rng();
+        // Sample sphere uniformly
+        // See <https://mathworld.wolfram.com/SpherePointPicking.html>
+        let theta = rng.gen_range(0., std::f32::consts::PI * 2.);
+        let y = rng.sample(Uniform::new(-1., 1.)); // Inclusive for the poles
+        let dir = Unit::new_unchecked(Vector::new(
+            // this vector is already of unit length
+            f32::sqrt(1. - y * y) * f32::cos(theta),
+            y,
+            f32::sqrt(1. - y * y) * f32::sin(theta),
+        ));
+        Ray::new(self.position, dir)
+    }
 }
 
 impl Light for PointLight {