Add RegularPolygon and Circle meshes (#3730)

# Objective

Bevy users often want to create circles and other simple shapes.

All the machinery is in place to accomplish this, and there are external crates that help. But when writing code for e.g. a new bevy example, it's not really possible to draw a circle without bringing in a new asset, writing a bunch of scary looking mesh code, or adding a dependency.

In particular, this PR was inspired by this interaction in another PR: #3721 (comment)

## Solution

This PR adds `shape::RegularPolygon` and `shape::Circle` (which is just a `RegularPolygon` that defaults to a large number of sides)

## Discussion

There's a lot of ongoing discussion about shapes in <bevyengine/rfcs#12> and at least one other lingering shape PR (although it seems incomplete).

That RFC currently includes `RegularPolygon` and `Circle` shapes, so I don't think that having working mesh generation code in the engine for those shapes would add much burden to an author of an implementation.

But if we'd prefer not to add additional shapes until after that's sorted out, I'm happy to close this for now.

## Alternatives for users

For any users stumbling on this issue, here are some plugins that will help if you need more shapes.

https://github.com/Nilirad/bevy_prototype_lyon
https://github.com/johanhelsing/bevy_smud
https://github.com/Weasy666/bevy_svg
https://github.com/redpandamonium/bevy_more_shapes
https://github.com/ForesightMiningSoftwareCorporation/bevy_polyline

Author: rparrett

Cargo.toml

@@ -144,8 +144,8 @@ name = "mesh2d_manual"
 path = "examples/2d/mesh2d_manual.rs"
 
 [[example]]
-name = "rect"
-path = "examples/2d/rect.rs"
+name = "shapes"
+path = "examples/2d/shapes.rs"
 
 [[example]]
 name = "sprite"

crates/bevy_render/src/mesh/shape/mod.rs

@@ -224,11 +224,13 @@ impl From<Plane> for Mesh {
 
 mod capsule;
 mod icosphere;
+mod regular_polygon;
 mod torus;
 mod uvsphere;
 
 pub use capsule::{Capsule, CapsuleUvProfile};
 pub use icosphere::Icosphere;
+pub use regular_polygon::{Circle, RegularPolygon};
 pub use torus::Torus;
 pub use uvsphere::UVSphere;
 use wgpu::PrimitiveTopology;

crates/bevy_render/src/mesh/shape/regular_polygon.rs

@@ -0,0 +1,103 @@
+use crate::mesh::{Indices, Mesh};
+use wgpu::PrimitiveTopology;
+
+/// A regular polygon in the xy plane
+#[derive(Debug, Copy, Clone)]
+pub struct RegularPolygon {
+    /// Inscribed radius in the xy plane.
+    pub radius: f32,
+    /// Number of sides.
+    pub sides: usize,
+}
+
+impl Default for RegularPolygon {
+    fn default() -> Self {
+        Self {
+            radius: 0.5,
+            sides: 6,
+        }
+    }
+}
+
+impl RegularPolygon {
+    /// Creates a regular polygon in the xy plane
+    pub fn new(radius: f32, sides: usize) -> Self {
+        Self { radius, sides }
+    }
+}
+
+impl From<RegularPolygon> for Mesh {
+    fn from(polygon: RegularPolygon) -> Self {
+        let RegularPolygon { radius, sides } = polygon;
+
+        debug_assert!(sides > 2, "RegularPolygon requires at least 3 sides.");
+
+        let mut positions = Vec::with_capacity(sides);
+        let mut normals = Vec::with_capacity(sides);
+        let mut uvs = Vec::with_capacity(sides);
+
+        let step = std::f32::consts::TAU / sides as f32;
+        for i in 0..sides {
+            let theta = std::f32::consts::FRAC_PI_2 - i as f32 * step;
+            let (sin, cos) = theta.sin_cos();
+
+            positions.push([cos * radius, sin * radius, 0.0]);
+            normals.push([0.0, 0.0, 1.0]);
+            uvs.push([0.5 * (cos + 1.0), 1.0 - 0.5 * (sin + 1.0)]);
+        }
+
+        let mut indices = Vec::with_capacity((sides - 2) * 3);
+        for i in 1..(sides as u32 - 1) {
+            indices.extend_from_slice(&[0, i + 1, i]);
+        }
+
+        let mut mesh = Mesh::new(PrimitiveTopology::TriangleList);
+        mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
+        mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
+        mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
+        mesh.set_indices(Some(Indices::U32(indices)));
+        mesh
+    }
+}
+
+/// A circle in the xy plane
+pub struct Circle {
+    /// Inscribed radius in the xy plane.
+    pub radius: f32,
+    /// The number of vertices used.
+    pub vertices: usize,
+}
+
+impl Default for Circle {
+    fn default() -> Self {
+        Self {
+            radius: 0.5,
+            vertices: 64,
+        }
+    }
+}
+
+impl Circle {
+    /// Creates a circle in the xy plane
+    pub fn new(radius: f32) -> Self {
+        Self {
+            radius,
+            ..Default::default()
+        }
+    }
+}
+
+impl From<Circle> for RegularPolygon {
+    fn from(circle: Circle) -> Self {
+        Self {
+            radius: circle.radius,
+            sides: circle.vertices,
+        }
+    }
+}
+
+impl From<Circle> for Mesh {
+    fn from(circle: Circle) -> Self {
+        Mesh::from(RegularPolygon::from(circle))
+    }
+}

examples/2d/rect.rs

@@ -0,0 +1,42 @@
+use bevy::{prelude::*, sprite::MaterialMesh2dBundle};
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .run();
+}
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<ColorMaterial>>,
+) {
+    commands.spawn_bundle(OrthographicCameraBundle::new_2d());
+
+    // Rectangle
+    commands.spawn_bundle(SpriteBundle {
+        sprite: Sprite {
+            color: Color::rgb(0.25, 0.25, 0.75),
+            custom_size: Some(Vec2::new(50.0, 100.0)),
+            ..default()
+        },
+        ..default()
+    });
+
+    // Circle
+    commands.spawn_bundle(MaterialMesh2dBundle {
+        mesh: meshes.add(shape::Circle::new(50.).into()).into(),
+        material: materials.add(ColorMaterial::from(Color::PURPLE)),
+        transform: Transform::from_translation(Vec3::new(-100., 0., 0.)),
+        ..default()
+    });
+
+    // Hexagon
+    commands.spawn_bundle(MaterialMesh2dBundle {
+        mesh: meshes.add(shape::RegularPolygon::new(50., 6).into()).into(),
+        material: materials.add(ColorMaterial::from(Color::TURQUOISE)),
+        transform: Transform::from_translation(Vec3::new(100., 0., 0.)),
+        ..default()
+    });
+}

examples/2d/shapes.rs

@@ -89,7 +89,7 @@ Example | File | Description
 `move_sprite` | [`2d/move_sprite.rs`](./2d/move_sprite.rs) | Changes the transform of a sprite.
 `mesh2d` | [`2d/mesh2d.rs`](./2d/mesh2d.rs) | Renders a 2d mesh
 `mesh2d_manual` | [`2d/mesh2d_manual.rs`](./2d/mesh2d_manual.rs) | Renders a custom mesh "manually" with "mid-level" renderer apis.
-`rect` | [`2d/rect.rs`](./2d/rect.rs) | Renders a rectangle
+`shapes` | [`2d/shapes.rs`](./2d/shapes.rs) | Renders a rectangle, circle, and hexagon
 `sprite` | [`2d/sprite.rs`](./2d/sprite.rs) | Renders a sprite
 `sprite_sheet` | [`2d/sprite_sheet.rs`](./2d/sprite_sheet.rs) | Renders an animated sprite
 `text2d` | [`2d/text2d.rs`](./2d/text2d.rs) | Generates text in 2d