Add meshing for ConicalFrustum (bevyengine#11819)

# Objective

The `ConicalFrustum` primitive should support meshing.

## Solution

Implement meshing for the `ConicalFrustum` primitive. The implementation
is nearly identical to `Cylinder` meshing, but supports two radii.

The default conical frustum is equivalent to a cone with a height of 1
and a radius of 0.5, truncated at half-height.


![kuva](https://github.com/bevyengine/bevy/assets/57632562/b4cab136-ff55-4056-b818-1218e4f38845)

crates/bevy_math/src/primitives/dim3.rs

@@ -599,6 +599,17 @@ pub struct ConicalFrustum {
 }
 impl Primitive3d for ConicalFrustum {}
 
+impl Default for ConicalFrustum {
+    /// Returns the default [`ConicalFrustum`] with a top radius of `0.25`, bottom radius of `0.5`, and a height of `0.5`.
+    fn default() -> Self {
+        Self {
+            radius_top: 0.25,
+            radius_bottom: 0.5,
+            height: 0.5,
+        }
+    }
+}
+
 /// The type of torus determined by the minor and major radii
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum TorusKind {

crates/bevy_render/src/mesh/primitives/dim3/conical_frustum.rs

@@ -0,0 +1,190 @@
+use crate::{
+    mesh::{Indices, Mesh, Meshable},
+    render_asset::RenderAssetUsages,
+};
+use bevy_math::{primitives::ConicalFrustum, Vec3};
+use wgpu::PrimitiveTopology;
+
+/// A builder used for creating a [`Mesh`] with a [`ConicalFrustum`] shape.
+#[derive(Clone, Copy, Debug)]
+pub struct ConicalFrustumMeshBuilder {
+    /// The [`ConicalFrustum`] shape.
+    pub frustum: ConicalFrustum,
+    /// The number of vertices used for the top and bottom of the conical frustum.
+    ///
+    /// The default is `32`.
+    pub resolution: u32,
+    /// The number of horizontal lines subdividing the lateral surface of the conical frustum.
+    ///
+    /// The default is `1`.
+    pub segments: u32,
+}
+
+impl Default for ConicalFrustumMeshBuilder {
+    fn default() -> Self {
+        Self {
+            frustum: ConicalFrustum::default(),
+            resolution: 32,
+            segments: 1,
+        }
+    }
+}
+
+impl ConicalFrustumMeshBuilder {
+    /// Creates a new [`ConicalFrustumMeshBuilder`] from the given top and bottom radii, a height,
+    /// and a resolution used for the top and bottom.
+    #[inline]
+    pub const fn new(radius_top: f32, radius_bottom: f32, height: f32, resolution: u32) -> Self {
+        Self {
+            frustum: ConicalFrustum {
+                radius_top,
+                radius_bottom,
+                height,
+            },
+            resolution,
+            segments: 1,
+        }
+    }
+
+    /// Sets the number of vertices used for the top and bottom of the conical frustum.
+    #[inline]
+    pub const fn resolution(mut self, resolution: u32) -> Self {
+        self.resolution = resolution;
+        self
+    }
+
+    /// Sets the number of horizontal lines subdividing the lateral surface of the conical frustum.
+    #[inline]
+    pub const fn segments(mut self, segments: u32) -> Self {
+        self.segments = segments;
+        self
+    }
+
+    /// Builds a [`Mesh`] based on the configuration in `self`.
+    pub fn build(&self) -> Mesh {
+        debug_assert!(self.resolution > 2);
+        debug_assert!(self.segments > 0);
+
+        let ConicalFrustum {
+            radius_top,
+            radius_bottom,
+            height,
+        } = self.frustum;
+        let half_height = height / 2.0;
+
+        let num_rings = self.segments + 1;
+        let num_vertices = (self.resolution * 2 + num_rings * (self.resolution + 1)) as usize;
+        let num_faces = self.resolution * (num_rings - 2);
+        let num_indices = ((2 * num_faces + 2 * (self.resolution - 1) * 2) * 3) as usize;
+
+        let mut positions = Vec::with_capacity(num_vertices);
+        let mut normals = Vec::with_capacity(num_vertices);
+        let mut uvs = Vec::with_capacity(num_vertices);
+        let mut indices = Vec::with_capacity(num_indices);
+
+        let step_theta = std::f32::consts::TAU / self.resolution as f32;
+        let step_y = height / self.segments as f32;
+        let step_radius = (radius_top - radius_bottom) / self.segments as f32;
+
+        // Rings
+        for ring in 0..num_rings {
+            let y = -half_height + ring as f32 * step_y;
+            let radius = radius_bottom + ring as f32 * step_radius;
+
+            for segment in 0..=self.resolution {
+                let theta = segment as f32 * step_theta;
+                let (sin, cos) = theta.sin_cos();
+
+                positions.push([radius * cos, y, radius * sin]);
+                normals.push(
+                    Vec3::new(cos, (radius_bottom - radius_top) / height, sin)
+                        .normalize()
+                        .to_array(),
+                );
+                uvs.push([
+                    segment as f32 / self.resolution as f32,
+                    ring as f32 / self.segments as f32,
+                ]);
+            }
+        }
+
+        // Lateral surface
+        for i in 0..self.segments {
+            let ring = i * (self.resolution + 1);
+            let next_ring = (i + 1) * (self.resolution + 1);
+
+            for j in 0..self.resolution {
+                indices.extend_from_slice(&[
+                    ring + j,
+                    next_ring + j,
+                    ring + j + 1,
+                    next_ring + j,
+                    next_ring + j + 1,
+                    ring + j + 1,
+                ]);
+            }
+        }
+
+        // Caps
+        let mut build_cap = |top: bool, radius: f32| {
+            let offset = positions.len() as u32;
+            let (y, normal_y, winding) = if top {
+                (half_height, 1.0, (1, 0))
+            } else {
+                (-half_height, -1.0, (0, 1))
+            };
+
+            for i in 0..self.resolution {
+                let theta = i as f32 * step_theta;
+                let (sin, cos) = theta.sin_cos();
+
+                positions.push([cos * radius, y, sin * radius]);
+                normals.push([0.0, normal_y, 0.0]);
+                uvs.push([0.5 * (cos + 1.0), 1.0 - 0.5 * (sin + 1.0)]);
+            }
+
+            for i in 1..(self.resolution - 1) {
+                indices.extend_from_slice(&[
+                    offset,
+                    offset + i + winding.0,
+                    offset + i + winding.1,
+                ]);
+            }
+        };
+
+        build_cap(true, radius_top);
+        build_cap(false, radius_bottom);
+
+        Mesh::new(
+            PrimitiveTopology::TriangleList,
+            RenderAssetUsages::default(),
+        )
+        .with_inserted_indices(Indices::U32(indices))
+        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
+        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
+    }
+}
+
+impl Meshable for ConicalFrustum {
+    type Output = ConicalFrustumMeshBuilder;
+
+    fn mesh(&self) -> Self::Output {
+        ConicalFrustumMeshBuilder {
+            frustum: *self,
+            ..Default::default()
+        }
+    }
+}
+
+impl From<ConicalFrustum> for Mesh {
+    fn from(frustum: ConicalFrustum) -> Self {
+        frustum.mesh().build()
+    }
+}
+
+impl From<ConicalFrustumMeshBuilder> for Mesh {
+    fn from(frustum: ConicalFrustumMeshBuilder) -> Self {
+        frustum.build()
+    }
+}

crates/bevy_render/src/mesh/primitives/dim3/cylinder.rs

@@ -145,8 +145,6 @@ impl MeshBuilder for CylinderMeshBuilder {
             }
         };
 
-        // top
-
         build_cap(true);
         build_cap(false);
 

crates/bevy_render/src/mesh/primitives/dim3/mod.rs

@@ -1,5 +1,6 @@
 mod capsule;
 mod cone;
+mod conical_frustum;
 mod cuboid;
 mod cylinder;
 mod plane;
@@ -10,6 +11,7 @@ pub(crate) mod triangle3d;
 
 pub use capsule::*;
 pub use cone::*;
+pub use conical_frustum::*;
 pub use cylinder::*;
 pub use plane::*;
 pub use sphere::*;

examples/3d/3d_shapes.rs

@@ -24,7 +24,7 @@ fn main() {
 #[derive(Component)]
 struct Shape;
 
-const X_EXTENT: f32 = 12.0;
+const X_EXTENT: f32 = 14.0;
 
 fn setup(
     mut commands: Commands,
@@ -44,6 +44,7 @@ fn setup(
         meshes.add(Torus::default()),
         meshes.add(Cylinder::default()),
         meshes.add(Cone::default()),
+        meshes.add(ConicalFrustum::default()),
         meshes.add(Sphere::default().mesh().ico(5).unwrap()),
         meshes.add(Sphere::default().mesh().uv(32, 18)),
     ];