Bevy 0.12 hollow circle mesh

hollow_circle.rs

// This sort of thing will be built into Bevy in version 0.14 by meshing the new `Annulus` primitive.

pub fn hollow_circle(inner_radius: f32, thickness: f32, sides: usize) -> Mesh {
    let outer_radius = inner_radius + thickness;

    debug_assert!(sides > 2, "hollow_circle requires at least 3 sides.");

    let vertices = sides * 2;

    let mut positions = Vec::with_capacity(vertices);
    let mut normals = Vec::with_capacity(vertices);
    let mut uvs = Vec::with_capacity(vertices);

    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 * outer_radius, sin * outer_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)]);

        positions.push([cos * inner_radius, sin * inner_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((vertices + 2) * 3);

    let vertices = vertices as u32;

    for i in (0..vertices).step_by(2) {
        indices.extend_from_slice(&[i, i + 2, i + 1, i + 1, i + 2, i + 3]);
    }
    indices.extend_from_slice(&[0, vertices - 2, vertices - 1, 0, vertices - 1, 1]);

    Mesh::new(PrimitiveTopology::TriangleList)
        .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)))
}