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

use crate::{
    mesh::{Indices, Mesh},
    render_asset::RenderAssetPersistencePolicy,
};
use wgpu::PrimitiveTopology;

/// A regular polygon in the `XY` plane
#[derive(Debug, Copy, Clone)]
pub struct RegularPolygon {
    /// Circumscribed radius in the `XY` plane.
    ///
    /// In other words, the vertices of this polygon will all touch a circle of this radius.
    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) {
            // Vertices are generated in CW order above, hence the reversed indices here
            // to emit triangle vertices in CCW order.
            indices.extend_from_slice(&[0, i + 1, i]);
        }

        Mesh::new(
            PrimitiveTopology::TriangleList,
            RenderAssetPersistencePolicy::Unload,
        )
        .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)))
    }
}

/// A circle in the `XY` plane
#[derive(Debug, Copy, Clone)]
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))
    }
}