conical_frustum.rs

use crate::{Indices, Mesh, MeshBuilder, Meshable, PrimitiveTopology};
use bevy_asset::RenderAssetUsages;
use bevy_math::{ops, primitives::ConicalFrustum, Vec3};
use bevy_reflect::prelude::*;

/// A builder used for creating a [`Mesh`] with a [`ConicalFrustum`] shape.
#[derive(Clone, Copy, Debug, Reflect)]
#[reflect(Default, Debug, Clone)]
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
    }
}

impl MeshBuilder for ConicalFrustumMeshBuilder {
    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 = core::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) = ops::sin_cos(theta);

                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) = ops::sin_cos(theta);

                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()
    }
}