bevy_mesh/primitives/dim3/
triangle3d.rs

1use crate::{Indices, Mesh, MeshBuilder, Meshable, PrimitiveTopology};
2use bevy_asset::RenderAssetUsages;
3use bevy_math::{primitives::Triangle3d, Vec3};
4use bevy_reflect::prelude::*;
5
6/// A builder used for creating a [`Mesh`] with a [`Triangle3d`] shape.
7#[derive(Clone, Copy, Debug, Default, Reflect)]
8#[reflect(Default, Debug, Clone)]
9pub struct Triangle3dMeshBuilder {
10    triangle: Triangle3d,
11}
12
13impl MeshBuilder for Triangle3dMeshBuilder {
14    fn build(&self) -> Mesh {
15        let positions: Vec<_> = self.triangle.vertices.into();
16        let uvs: Vec<_> = uv_coords(&self.triangle).into();
17
18        // Every vertex has the normal of the face of the triangle (or zero if the triangle is degenerate).
19        let normal: Vec3 = normal_vec(&self.triangle);
20        let normals = vec![normal; 3];
21
22        let indices = Indices::U32(vec![0, 1, 2]);
23
24        Mesh::new(
25            PrimitiveTopology::TriangleList,
26            RenderAssetUsages::default(),
27        )
28        .with_inserted_indices(indices)
29        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
30        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
31        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
32    }
33}
34
35impl Meshable for Triangle3d {
36    type Output = Triangle3dMeshBuilder;
37
38    fn mesh(&self) -> Self::Output {
39        Triangle3dMeshBuilder { triangle: *self }
40    }
41}
42
43/// The normal of a [`Triangle3d`] with zeroing so that a [`Vec3`] is always obtained for meshing.
44#[inline]
45pub(crate) fn normal_vec(triangle: &Triangle3d) -> Vec3 {
46    triangle.normal().map_or(Vec3::ZERO, Into::into)
47}
48
49/// Unskewed uv-coordinates for a [`Triangle3d`].
50#[inline]
51pub(crate) fn uv_coords(triangle: &Triangle3d) -> [[f32; 2]; 3] {
52    let [a, b, c] = triangle.vertices;
53
54    let main_length = a.distance(b);
55    let Some(x) = (b - a).try_normalize() else {
56        return [[0., 0.], [1., 0.], [0., 1.]];
57    };
58    let y = c - a;
59
60    // `x` corresponds to one of the axes in uv-coordinates;
61    // to uv-map the triangle without skewing, we use the orthogonalization
62    // of `y` with respect to `x` as the second direction and construct a rectangle that
63    // contains `triangle`.
64    let y_proj = y.project_onto_normalized(x);
65
66    // `offset` represents the x-coordinate of the point `c`; note that x has been shrunk by a
67    // factor of `main_length`, so `offset` follows it.
68    let offset = y_proj.dot(x) / main_length;
69
70    // Obtuse triangle leaning to the left => x direction extends to the left, shifting a from 0.
71    if offset < 0. {
72        let total_length = 1. - offset;
73        let a_uv = [offset.abs() / total_length, 0.];
74        let b_uv = [1., 0.];
75        let c_uv = [0., 1.];
76
77        [a_uv, b_uv, c_uv]
78    }
79    // Obtuse triangle leaning to the right => x direction extends to the right, shifting b from 1.
80    else if offset > 1. {
81        let a_uv = [0., 0.];
82        let b_uv = [1. / offset, 0.];
83        let c_uv = [1., 1.];
84
85        [a_uv, b_uv, c_uv]
86    }
87    // Acute triangle => no extending necessary; a remains at 0 and b remains at 1.
88    else {
89        let a_uv = [0., 0.];
90        let b_uv = [1., 0.];
91        let c_uv = [offset, 1.];
92
93        [a_uv, b_uv, c_uv]
94    }
95}
96
97impl From<Triangle3d> for Mesh {
98    fn from(triangle: Triangle3d) -> Self {
99        triangle.mesh().build()
100    }
101}
102
103#[cfg(test)]
104mod tests {
105    use super::uv_coords;
106    use bevy_math::primitives::Triangle3d;
107
108    #[test]
109    fn uv_test() {
110        use bevy_math::vec3;
111        let mut triangle = Triangle3d::new(vec3(0., 0., 0.), vec3(2., 0., 0.), vec3(-1., 1., 0.));
112
113        let [a_uv, b_uv, c_uv] = uv_coords(&triangle);
114        assert_eq!(a_uv, [1. / 3., 0.]);
115        assert_eq!(b_uv, [1., 0.]);
116        assert_eq!(c_uv, [0., 1.]);
117
118        triangle.vertices[2] = vec3(3., 1., 0.);
119        let [a_uv, b_uv, c_uv] = uv_coords(&triangle);
120        assert_eq!(a_uv, [0., 0.]);
121        assert_eq!(b_uv, [2. / 3., 0.]);
122        assert_eq!(c_uv, [1., 1.]);
123
124        triangle.vertices[2] = vec3(2., 1., 0.);
125        let [a_uv, b_uv, c_uv] = uv_coords(&triangle);
126        assert_eq!(a_uv, [0., 0.]);
127        assert_eq!(b_uv, [1., 0.]);
128        assert_eq!(c_uv, [1., 1.]);
129    }
130}
bevy_mesh/primitives/dim3/triangle3d.rs

bevy_mesh/primitives/dim3/
triangle3d.rs
