From 5dbd827728c3624b943c67b5b1a63aee07f8f698 Mon Sep 17 00:00:00 2001
From: Matty <weatherleymatthew@gmail.com>
Date: Wed, 22 May 2024 09:13:04 -0400
Subject: [PATCH] Annulus sampling (#13471)

# Objective

Add random sampling for the `Annulus` primitive. This is part of ongoing
work to bring the various `bevy_math` primitives to feature parity.

## Solution

`Annulus` implements `ShapeSample`. Boundary sampling is implemented in
the obvious way, and interior sampling works exactly as in the
implementation for `Circle`, using the fact that the square of the
radius should be taken uniformly from between r^2 and R^2, where r and R
are the inner and outer radii respectively.

## Testing

I generated a bunch of random points and rendered them. Here's 1000
points on the interior of the default annulus:
<img width="1440" alt="Screenshot 2024-05-22 at 8 01 34 AM"
src="https://github.com/bevyengine/bevy/assets/2975848/19c31bb0-edba-477f-b247-2b12d854afae">

This looks kind of weird around the edges, but I verified that they're
all actually inside the annulus, so I assume it has to do with the fact
that the rendered circles have some radius.
---
 .../bevy_math/src/sampling/shape_sampling.rs  | 29 +++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/crates/bevy_math/src/sampling/shape_sampling.rs b/crates/bevy_math/src/sampling/shape_sampling.rs
index 520c1e87b7..95dfe677fd 100644
--- a/crates/bevy_math/src/sampling/shape_sampling.rs
+++ b/crates/bevy_math/src/sampling/shape_sampling.rs
@@ -191,6 +191,35 @@ impl ShapeSample for Sphere {
     }
 }
 
+impl ShapeSample for Annulus {
+    type Output = Vec2;
+
+    fn sample_interior<R: Rng + ?Sized>(&self, rng: &mut R) -> Self::Output {
+        let inner_radius = self.inner_circle.radius;
+        let outer_radius = self.outer_circle.radius;
+
+        // Like random sampling for a circle, radius is weighted by the square.
+        let r_squared = rng.gen_range((inner_radius * inner_radius)..(outer_radius * outer_radius));
+        let r = r_squared.sqrt();
+        let theta = rng.gen_range(0.0..TAU);
+
+        Vec2::new(r * theta.cos(), r * theta.sin())
+    }
+
+    fn sample_boundary<R: Rng + ?Sized>(&self, rng: &mut R) -> Self::Output {
+        let total_perimeter = self.inner_circle.perimeter() + self.outer_circle.perimeter();
+        let inner_prob = (self.inner_circle.perimeter() / total_perimeter) as f64;
+
+        // Sample from boundary circles, choosing which one by weighting by perimeter:
+        let inner = rng.gen_bool(inner_prob);
+        if inner {
+            self.inner_circle.sample_boundary(rng)
+        } else {
+            self.outer_circle.sample_boundary(rng)
+        }
+    }
+}
+
 impl ShapeSample for Rectangle {
     type Output = Vec2;