Add Meshable trait and implement meshing for 2D primitives (#11431)

# Objective

The first part of #10569, split up from #11007.

The goal is to implement meshing support for Bevy's new geometric
primitives, starting with 2D primitives. 3D meshing will be added in a
follow-up, and we can consider removing the old mesh shapes completely.

## Solution

Add a `Meshable` trait that primitives need to implement to support
meshing, as suggested by the
[RFC](https://github.com/bevyengine/rfcs/blob/main/rfcs/12-primitive-shapes.md#meshing).

```rust
/// A trait for shapes that can be turned into a [`Mesh`].
pub trait Meshable {
    /// The output of [`Self::mesh`]. This can either be a [`Mesh`]
    /// or a builder used for creating a [`Mesh`].
    type Output;

    /// Creates a [`Mesh`] for a shape.
    fn mesh(&self) -> Self::Output;
}
```

This PR implements it for the following primitives:

- `Circle`
- `Ellipse`
- `Rectangle`
- `RegularPolygon`
- `Triangle2d`

The `mesh` method typically returns a builder-like struct such as
`CircleMeshBuilder`. This is needed to support shape-specific
configuration for things like mesh resolution or UV configuration:

```rust
meshes.add(Circle { radius: 0.5 }.mesh().resolution(64));
```

Note that if no configuration is needed, you can even skip calling
`mesh` because `From<MyPrimitive>` is implemented for `Mesh`:

```rust
meshes.add(Circle { radius: 0.5 });
```

I also updated the `2d_shapes` example to use primitives, and tweaked
the colors to have better contrast against the dark background.

Before:

![Old 2D
shapes](https://github.com/bevyengine/bevy/assets/57632562/f1d8c2d5-55be-495f-8ed4-5890154b81ca)

After:

![New 2D
shapes](https://github.com/bevyengine/bevy/assets/57632562/f166c013-34b8-4752-800a-5517b284d978)

Here you can see the UVs and different facing directions: (taken from
#11007, so excuse the 3D primitives at the bottom left)

![UVs and facing
directions](https://github.com/bevyengine/bevy/assets/57632562/eaf0be4e-187d-4b6d-8fb8-c996ba295a8a)

---

## Changelog

- Added `bevy_render::mesh::primitives` module
- Added `Meshable` trait and implemented it for:
  - `Circle`
  - `Ellipse`
  - `Rectangle`
  - `RegularPolygon`
  - `Triangle2d`
- Implemented `Default` and `Copy` for several 2D primitives
- Updated `2d_shapes` example to use primitives
- Tweaked colors in `2d_shapes` example to have better contrast against
the (new-ish) dark background

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

Cargo.toml

@@ -402,7 +402,7 @@ doc-scrape-examples = true
 
 [package.metadata.example.2d_shapes]
 name = "2D Shapes"
-description = "Renders a rectangle, circle, and hexagon"
+description = "Renders simple 2D primitive shapes like circles and polygons"
 category = "2D Rendering"
 wasm = true
 

crates/bevy_math/src/primitives/dim2.rs

@@ -90,6 +90,13 @@ pub struct Circle {
 }
 impl Primitive2d for Circle {}
 
+impl Default for Circle {
+    /// Returns the default [`Circle`] with a radius of `0.5`.
+    fn default() -> Self {
+        Self { radius: 0.5 }
+    }
+}
+
 impl Circle {
     /// Create a new [`Circle`] from a `radius`
     #[inline(always)]
@@ -147,6 +154,15 @@ pub struct Ellipse {
 }
 impl Primitive2d for Ellipse {}
 
+impl Default for Ellipse {
+    /// Returns the default [`Ellipse`] with a half-width of `1.0` and a half-height of `0.5`.
+    fn default() -> Self {
+        Self {
+            half_size: Vec2::new(1.0, 0.5),
+        }
+    }
+}
+
 impl Ellipse {
     /// Create a new `Ellipse` from half of its width and height.
     ///
@@ -197,6 +213,15 @@ pub struct Plane2d {
 }
 impl Primitive2d for Plane2d {}
 
+impl Default for Plane2d {
+    /// Returns the default [`Plane2d`] with a normal pointing in the `+Y` direction.
+    fn default() -> Self {
+        Self {
+            normal: Direction2d::Y,
+        }
+    }
+}
+
 impl Plane2d {
     /// Create a new `Plane2d` from a normal
     ///
@@ -343,10 +368,19 @@ pub struct Triangle2d {
 }
 impl Primitive2d for Triangle2d {}
 
+impl Default for Triangle2d {
+    /// Returns the default [`Triangle2d`] with the vertices `[0.0, 0.5]`, `[-0.5, -0.5]`, and `[0.5, -0.5]`.
+    fn default() -> Self {
+        Self {
+            vertices: [Vec2::Y * 0.5, Vec2::new(-0.5, -0.5), Vec2::new(0.5, -0.5)],
+        }
+    }
+}
+
 impl Triangle2d {
     /// Create a new `Triangle2d` from points `a`, `b`, and `c`
     #[inline(always)]
-    pub fn new(a: Vec2, b: Vec2, c: Vec2) -> Self {
+    pub const fn new(a: Vec2, b: Vec2, c: Vec2) -> Self {
         Self {
             vertices: [a, b, c],
         }
@@ -438,6 +472,15 @@ pub struct Rectangle {
     pub half_size: Vec2,
 }
 
+impl Default for Rectangle {
+    /// Returns the default [`Rectangle`] with a half-width and half-height of `0.5`.
+    fn default() -> Self {
+        Self {
+            half_size: Vec2::splat(0.5),
+        }
+    }
+}
+
 impl Rectangle {
     /// Create a new `Rectangle` from a full width and height
     #[inline(always)]
@@ -559,9 +602,19 @@ pub struct RegularPolygon {
 }
 impl Primitive2d for RegularPolygon {}
 
+impl Default for RegularPolygon {
+    /// Returns the default [`RegularPolygon`] with six sides (a hexagon) and a circumradius of `0.5`.
+    fn default() -> Self {
+        Self {
+            circumcircle: Circle { radius: 0.5 },
+            sides: 6,
+        }
+    }
+}
+
 impl RegularPolygon {
     /// Create a new `RegularPolygon`
-    /// from the radius of the circumcircle and number of sides
+    /// from the radius of the circumcircle and a number of sides
     ///
     /// # Panics
     ///

crates/bevy_render/src/lib.rs

@@ -34,7 +34,7 @@ pub mod prelude {
             Projection,
         },
         color::Color,
-        mesh::{morph::MorphWeights, shape, Mesh},
+        mesh::{morph::MorphWeights, primitives::Meshable, shape, Mesh},
         render_resource::Shader,
         spatial_bundle::SpatialBundle,
         texture::{Image, ImagePlugin},

crates/bevy_render/src/mesh/mod.rs

@@ -1,10 +1,12 @@
 #[allow(clippy::module_inception)]
 mod mesh;
 pub mod morph;
+pub mod primitives;
 /// Generation for some primitive shape meshes.
 pub mod shape;
 
 pub use mesh::*;
+pub use primitives::*;
 
 use crate::{prelude::Image, render_asset::RenderAssetPlugin};
 use bevy_app::{App, Plugin};

crates/bevy_render/src/mesh/primitives/dim2.rs

@@ -0,0 +1,268 @@
+use crate::{
+    mesh::{Indices, Mesh},
+    render_asset::RenderAssetPersistencePolicy,
+};
+
+use super::Meshable;
+use bevy_math::{
+    primitives::{Circle, Ellipse, Rectangle, RegularPolygon, Triangle2d, WindingOrder},
+    Vec2,
+};
+use wgpu::PrimitiveTopology;
+
+/// A builder used for creating a [`Mesh`] with a [`Circle`] shape.
+#[derive(Clone, Copy, Debug)]
+pub struct CircleMeshBuilder {
+    /// The [`Circle`] shape.
+    pub circle: Circle,
+    /// The number of vertices used for the circle mesh.
+    /// The default is `32`.
+    #[doc(alias = "vertices")]
+    pub resolution: usize,
+}
+
+impl Default for CircleMeshBuilder {
+    fn default() -> Self {
+        Self {
+            circle: Circle::default(),
+            resolution: 32,
+        }
+    }
+}
+
+impl CircleMeshBuilder {
+    /// Creates a new [`CircleMeshBuilder`] from a given radius and vertex count.
+    #[inline]
+    pub const fn new(radius: f32, resolution: usize) -> Self {
+        Self {
+            circle: Circle { radius },
+            resolution,
+        }
+    }
+
+    /// Sets the number of vertices used for the circle mesh.
+    #[inline]
+    #[doc(alias = "vertices")]
+    pub const fn resolution(mut self, resolution: usize) -> Self {
+        self.resolution = resolution;
+        self
+    }
+
+    /// Builds a [`Mesh`] based on the configuration in `self`.
+    pub fn build(&self) -> Mesh {
+        RegularPolygon::new(self.circle.radius, self.resolution).mesh()
+    }
+}
+
+impl Meshable for Circle {
+    type Output = CircleMeshBuilder;
+
+    fn mesh(&self) -> Self::Output {
+        CircleMeshBuilder {
+            circle: *self,
+            ..Default::default()
+        }
+    }
+}
+
+impl From<Circle> for Mesh {
+    fn from(circle: Circle) -> Self {
+        circle.mesh().build()
+    }
+}
+
+impl From<CircleMeshBuilder> for Mesh {
+    fn from(circle: CircleMeshBuilder) -> Self {
+        circle.build()
+    }
+}
+
+impl Meshable for RegularPolygon {
+    type Output = Mesh;
+
+    fn mesh(&self) -> Self::Output {
+        // The ellipse mesh is just a regular polygon with two radii
+        Ellipse::new(self.circumcircle.radius, self.circumcircle.radius)
+            .mesh()
+            .resolution(self.sides)
+            .build()
+    }
+}
+
+impl From<RegularPolygon> for Mesh {
+    fn from(polygon: RegularPolygon) -> Self {
+        polygon.mesh()
+    }
+}
+
+/// A builder used for creating a [`Mesh`] with an [`Ellipse`] shape.
+#[derive(Clone, Copy, Debug)]
+pub struct EllipseMeshBuilder {
+    /// The [`Ellipse`] shape.
+    pub ellipse: Ellipse,
+    /// The number of vertices used for the ellipse mesh.
+    /// The default is `32`.
+    #[doc(alias = "vertices")]
+    pub resolution: usize,
+}
+
+impl Default for EllipseMeshBuilder {
+    fn default() -> Self {
+        Self {
+            ellipse: Ellipse::default(),
+            resolution: 32,
+        }
+    }
+}
+
+impl EllipseMeshBuilder {
+    /// Creates a new [`EllipseMeshBuilder`] from a given half width and half height and a vertex count.
+    #[inline]
+    pub const fn new(half_width: f32, half_height: f32, resolution: usize) -> Self {
+        Self {
+            ellipse: Ellipse::new(half_width, half_height),
+            resolution,
+        }
+    }
+
+    /// Sets the number of vertices used for the ellipse mesh.
+    #[inline]
+    #[doc(alias = "vertices")]
+    pub const fn resolution(mut self, resolution: usize) -> Self {
+        self.resolution = resolution;
+        self
+    }
+
+    /// Builds a [`Mesh`] based on the configuration in `self`.
+    pub fn build(&self) -> Mesh {
+        let mut indices = Vec::with_capacity((self.resolution - 2) * 3);
+        let mut positions = Vec::with_capacity(self.resolution);
+        let normals = vec![[0.0, 0.0, 1.0]; self.resolution];
+        let mut uvs = Vec::with_capacity(self.resolution);
+
+        // Add pi/2 so that there is a vertex at the top (sin is 1.0 and cos is 0.0)
+        let start_angle = std::f32::consts::FRAC_PI_2;
+        let step = std::f32::consts::TAU / self.resolution as f32;
+
+        for i in 0..self.resolution {
+            // Compute vertex position at angle theta
+            let theta = start_angle + i as f32 * step;
+            let (sin, cos) = theta.sin_cos();
+            let x = cos * self.ellipse.half_size.x;
+            let y = sin * self.ellipse.half_size.y;
+
+            positions.push([x, y, 0.0]);
+            uvs.push([0.5 * (cos + 1.0), 1.0 - 0.5 * (sin + 1.0)]);
+        }
+
+        for i in 1..(self.resolution as u32 - 1) {
+            indices.extend_from_slice(&[0, i, i + 1]);
+        }
+
+        Mesh::new(
+            PrimitiveTopology::TriangleList,
+            RenderAssetPersistencePolicy::Keep,
+        )
+        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
+        .with_indices(Some(Indices::U32(indices)))
+    }
+}
+
+impl Meshable for Ellipse {
+    type Output = EllipseMeshBuilder;
+
+    fn mesh(&self) -> Self::Output {
+        EllipseMeshBuilder {
+            ellipse: *self,
+            ..Default::default()
+        }
+    }
+}
+
+impl From<Ellipse> for Mesh {
+    fn from(ellipse: Ellipse) -> Self {
+        ellipse.mesh().build()
+    }
+}
+
+impl From<EllipseMeshBuilder> for Mesh {
+    fn from(ellipse: EllipseMeshBuilder) -> Self {
+        ellipse.build()
+    }
+}
+
+impl Meshable for Triangle2d {
+    type Output = Mesh;
+
+    fn mesh(&self) -> Self::Output {
+        let [a, b, c] = self.vertices;
+
+        let positions = vec![[a.x, a.y, 0.0], [b.x, b.y, 0.0], [c.x, c.y, 0.0]];
+        let normals = vec![[0.0, 0.0, 1.0]; 3];
+
+        // The extents of the bounding box of the triangle,
+        // used to compute the UV coordinates of the points.
+        let extents = a.min(b).min(c).abs().max(a.max(b).max(c)) * Vec2::new(1.0, -1.0);
+        let uvs = vec![
+            a / extents / 2.0 + 0.5,
+            b / extents / 2.0 + 0.5,
+            c / extents / 2.0 + 0.5,
+        ];
+
+        let is_ccw = self.winding_order() == WindingOrder::CounterClockwise;
+        let indices = if is_ccw {
+            Indices::U32(vec![0, 1, 2])
+        } else {
+            Indices::U32(vec![0, 2, 1])
+        };
+
+        Mesh::new(
+            PrimitiveTopology::TriangleList,
+            RenderAssetPersistencePolicy::Keep,
+        )
+        .with_indices(Some(indices))
+        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
+    }
+}
+
+impl From<Triangle2d> for Mesh {
+    fn from(triangle: Triangle2d) -> Self {
+        triangle.mesh()
+    }
+}
+
+impl Meshable for Rectangle {
+    type Output = Mesh;
+
+    fn mesh(&self) -> Self::Output {
+        let [hw, hh] = [self.half_size.x, self.half_size.y];
+        let positions = vec![
+            [hw, hh, 0.0],
+            [-hw, hh, 0.0],
+            [-hw, -hh, 0.0],
+            [hw, -hh, 0.0],
+        ];
+        let normals = vec![[0.0, 0.0, 1.0]; 4];
+        let uvs = vec![[1.0, 0.0], [0.0, 0.0], [0.0, 1.0], [1.0, 1.0]];
+        let indices = Indices::U32(vec![0, 1, 2, 0, 2, 3]);
+
+        Mesh::new(
+            PrimitiveTopology::TriangleList,
+            RenderAssetPersistencePolicy::Keep,
+        )
+        .with_indices(Some(indices))
+        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
+    }
+}
+
+impl From<Rectangle> for Mesh {
+    fn from(rectangle: Rectangle) -> Self {
+        rectangle.mesh()
+    }
+}

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

@@ -0,0 +1,35 @@
+//! Mesh generation for [primitive shapes](bevy_math::primitives).
+//!
+//! Primitives that support meshing implement the [`Meshable`] trait.
+//! Calling [`mesh`](Meshable::mesh) will return either a [`Mesh`](super::Mesh) or a builder
+//! that can be used to specify shape-specific configuration for creating the [`Mesh`](super::Mesh).
+//!
+//! ```
+//! # use bevy_asset::Assets;
+//! # use bevy_ecs::prelude::ResMut;
+//! # use bevy_math::prelude::Circle;
+//! # use bevy_render::prelude::*;
+//! #
+//! # fn setup(mut meshes: ResMut<Assets<Mesh>>) {
+//! // Create circle mesh with default configuration
+//! let circle = meshes.add(Circle { radius: 25.0 });
+//!
+//! // Specify number of vertices
+//! let circle = meshes.add(Circle { radius: 25.0 }.mesh().resolution(64));
+//! # }
+//! ```
+
+#![warn(missing_docs)]
+
+mod dim2;
+pub use dim2::{CircleMeshBuilder, EllipseMeshBuilder};
+
+/// A trait for shapes that can be turned into a [`Mesh`](super::Mesh).
+pub trait Meshable {
+    /// The output of [`Self::mesh`]. This can either be a [`Mesh`](super::Mesh)
+    /// or a builder used for creating a [`Mesh`](super::Mesh).
+    type Output;
+
+    /// Creates a [`Mesh`](super::Mesh) for a shape.
+    fn mesh(&self) -> Self::Output;
+}

examples/2d/2d_shapes.rs

@@ -18,36 +18,47 @@ fn setup(
 
     // Circle
     commands.spawn(MaterialMesh2dBundle {
-        mesh: meshes.add(shape::Circle::new(50.)).into(),
-        material: materials.add(Color::PURPLE),
-        transform: Transform::from_translation(Vec3::new(-150., 0., 0.)),
+        mesh: meshes.add(Circle { radius: 50.0 }).into(),
+        material: materials.add(Color::VIOLET),
+        transform: Transform::from_translation(Vec3::new(-225.0, 0.0, 0.0)),
+        ..default()
+    });
+
+    // Ellipse
+    commands.spawn(MaterialMesh2dBundle {
+        mesh: meshes.add(Ellipse::new(25.0, 50.0)).into(),
+        material: materials.add(Color::TURQUOISE),
+        transform: Transform::from_translation(Vec3::new(-100.0, 0.0, 0.0)),
         ..default()
     });
 
     // Rectangle
-    commands.spawn(SpriteBundle {
-        sprite: Sprite {
-            color: Color::rgb(0.25, 0.25, 0.75),
-            custom_size: Some(Vec2::new(50.0, 100.0)),
-            ..default()
-        },
-        transform: Transform::from_translation(Vec3::new(-50., 0., 0.)),
-        ..default()
-    });
-
-    // Quad
     commands.spawn(MaterialMesh2dBundle {
-        mesh: meshes.add(shape::Quad::new(Vec2::new(50., 100.))).into(),
+        mesh: meshes.add(Rectangle::new(50.0, 100.0)).into(),
         material: materials.add(Color::LIME_GREEN),
-        transform: Transform::from_translation(Vec3::new(50., 0., 0.)),
+        transform: Transform::from_translation(Vec3::new(0.0, 0.0, 0.0)),
         ..default()
     });
 
     // Hexagon
     commands.spawn(MaterialMesh2dBundle {
-        mesh: meshes.add(shape::RegularPolygon::new(50., 6)).into(),
-        material: materials.add(Color::TURQUOISE),
-        transform: Transform::from_translation(Vec3::new(150., 0., 0.)),
+        mesh: meshes.add(RegularPolygon::new(50.0, 6)).into(),
+        material: materials.add(Color::YELLOW),
+        transform: Transform::from_translation(Vec3::new(125.0, 0.0, 0.0)),
+        ..default()
+    });
+
+    // Triangle
+    commands.spawn(MaterialMesh2dBundle {
+        mesh: meshes
+            .add(Triangle2d::new(
+                Vec2::Y * 50.0,
+                Vec2::new(-50.0, -50.0),
+                Vec2::new(50.0, -50.0),
+            ))
+            .into(),
+        material: materials.add(Color::ORANGE),
+        transform: Transform::from_translation(Vec3::new(250.0, 0.0, 0.0)),
         ..default()
     });
 }

examples/README.md

@@ -95,7 +95,7 @@ Example | Description
 [2D Bloom](../examples/2d/bloom_2d.rs) | Illustrates bloom post-processing in 2d
 [2D Gizmos](../examples/2d/2d_gizmos.rs) | A scene showcasing 2D gizmos
 [2D Rotation](../examples/2d/rotation.rs) | Demonstrates rotating entities in 2D with quaternions
-[2D Shapes](../examples/2d/2d_shapes.rs) | Renders a rectangle, circle, and hexagon
+[2D Shapes](../examples/2d/2d_shapes.rs) | Renders simple 2D primitive shapes like circles and polygons
 [2D Viewport To World](../examples/2d/2d_viewport_to_world.rs) | Demonstrates how to use the `Camera::viewport_to_world_2d` method
 [Custom glTF vertex attribute 2D](../examples/2d/custom_gltf_vertex_attribute.rs) | Renders a glTF mesh in 2D with a custom vertex attribute
 [Manual Mesh 2D](../examples/2d/mesh2d_manual.rs) | Renders a custom mesh "manually" with "mid-level" renderer apis