Add cylinder shape (bevyengine#6809)

# Objective

Adds a cylinder shape. Fixes bevyengine#2282.

## Solution

- I added a custom cylinder shape, taken from [here](https://github.com/rparrett/typey_birb/blob/main/src/cylinder.rs) with permission from @rparrett.
- I also added the cylinder shape to the `3d_shapes` example scene.

---

## Changelog

- Added cylinder shape

Co-Authored-By: Rob Parrett <robparrett@gmail.com>
Co-Authored-By: davidhof <7483215+davidhof@users.noreply.github.com>

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

@@ -0,0 +1,127 @@
+use crate::mesh::{Indices, Mesh};
+use wgpu::PrimitiveTopology;
+
+/// A cylinder which stands on the XZ plane
+pub struct Cylinder {
+    /// Radius in the XZ plane.
+    pub radius: f32,
+    /// Height of the cylinder in the Y axis.
+    pub height: f32,
+    /// The number of vertices around each horizontal slice of the cylinder. If you are looking at the cylinder from
+    /// above, this is the number of points you will see on the circle.
+    /// A higher number will make it appear more circular.
+    pub resolution: u32,
+    /// The number of segments between the two ends. Setting this to 1 will have triangles spanning the full
+    /// height of the cylinder. Setting it to 2 will have two sets of triangles with a horizontal slice in the middle of
+    /// cylinder. Greater numbers increase triangles/slices in the same way.
+    pub segments: u32,
+}
+
+impl Default for Cylinder {
+    fn default() -> Self {
+        Self {
+            radius: 0.5,
+            height: 1.0,
+            resolution: 16,
+            segments: 1,
+        }
+    }
+}
+
+impl From<Cylinder> for Mesh {
+    fn from(c: Cylinder) -> Self {
+        debug_assert!(c.radius > 0.0);
+        debug_assert!(c.height > 0.0);
+        debug_assert!(c.resolution > 2);
+        debug_assert!(c.segments > 0);
+
+        let num_rings = c.segments + 1;
+        let num_vertices = c.resolution * 2 + num_rings * (c.resolution + 1);
+        let num_faces = c.resolution * (num_rings - 2);
+        let num_indices = (2 * num_faces + 2 * (c.resolution - 1) * 2) * 3;
+
+        let mut positions = Vec::with_capacity(num_vertices as usize);
+        let mut normals = Vec::with_capacity(num_vertices as usize);
+        let mut uvs = Vec::with_capacity(num_vertices as usize);
+        let mut indices = Vec::with_capacity(num_indices as usize);
+
+        let step_theta = std::f32::consts::TAU / c.resolution as f32;
+        let step_y = c.height / c.segments as f32;
+
+        // rings
+
+        for ring in 0..num_rings {
+            let y = -c.height / 2.0 + ring as f32 * step_y;
+
+            for segment in 0..=c.resolution {
+                let theta = segment as f32 * step_theta;
+                let (sin, cos) = theta.sin_cos();
+
+                positions.push([c.radius * cos, y, c.radius * sin]);
+                normals.push([cos, 0., sin]);
+                uvs.push([
+                    segment as f32 / c.resolution as f32,
+                    ring as f32 / c.segments as f32,
+                ]);
+            }
+        }
+
+        // barrel skin
+
+        for i in 0..c.segments {
+            let ring = i * (c.resolution + 1);
+            let next_ring = (i + 1) * (c.resolution + 1);
+
+            for j in 0..c.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| {
+            let offset = positions.len() as u32;
+            let (y, normal_y, winding) = if top {
+                (c.height / 2., 1., (1, 0))
+            } else {
+                (c.height / -2., -1., (0, 1))
+            };
+
+            for i in 0..c.resolution {
+                let theta = i as f32 * step_theta;
+                let (sin, cos) = theta.sin_cos();
+
+                positions.push([cos * c.radius, y, sin * c.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..(c.resolution as u32 - 1) {
+                indices.extend_from_slice(&[
+                    offset,
+                    offset + i + winding.0,
+                    offset + i + winding.1,
+                ]);
+            }
+        };
+
+        // top
+
+        build_cap(true);
+        build_cap(false);
+
+        let mut mesh = Mesh::new(PrimitiveTopology::TriangleList);
+        mesh.set_indices(Some(Indices::U32(indices)));
+        mesh.insert_attribute(Mesh::ATTRIBUTE_POSITION, positions);
+        mesh.insert_attribute(Mesh::ATTRIBUTE_NORMAL, normals);
+        mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
+        mesh
+    }
+}

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

@@ -221,12 +221,14 @@ impl From<Plane> for Mesh {
 }
 
 mod capsule;
+mod cylinder;
 mod icosphere;
 mod regular_polygon;
 mod torus;
 mod uvsphere;
 
 pub use capsule::{Capsule, CapsuleUvProfile};
+pub use cylinder::Cylinder;
 pub use icosphere::Icosphere;
 pub use regular_polygon::{Circle, RegularPolygon};
 pub use torus::Torus;

examples/3d/3d_shapes.rs

@@ -20,7 +20,7 @@ fn main() {
 #[derive(Component)]
 struct Shape;
 
-const X_EXTENT: f32 = 14.;
+const X_EXTENT: f32 = 14.5;
 
 fn setup(
     mut commands: Commands,
@@ -38,6 +38,7 @@ fn setup(
         meshes.add(shape::Box::default().into()),
         meshes.add(shape::Capsule::default().into()),
         meshes.add(shape::Torus::default().into()),
+        meshes.add(shape::Cylinder::default().into()),
         meshes.add(shape::Icosphere::default().try_into().unwrap()),
         meshes.add(shape::UVSphere::default().into()),
     ];