bevy_render/mesh/primitives/dim3/
triangle3d.rs

1use bevy_math::{primitives::Triangle3d, Vec3};
2use wgpu::PrimitiveTopology;
3
4use crate::{
5    mesh::{Indices, Mesh, MeshBuilder, Meshable},
6    render_asset::RenderAssetUsages,
7};
8
9/// A builder used for creating a [`Mesh`] with a [`Triangle3d`] shape.
10pub struct Triangle3dMeshBuilder {
11    triangle: Triangle3d,
12}
13
14impl MeshBuilder for Triangle3dMeshBuilder {
15    fn build(&self) -> Mesh {
16        let positions: Vec<_> = self.triangle.vertices.into();
17        let uvs: Vec<_> = uv_coords(&self.triangle).into();
18
19        // Every vertex has the normal of the face of the triangle (or zero if the triangle is degenerate).
20        let normal: Vec3 = normal_vec(&self.triangle);
21        let normals = vec![normal; 3];
22
23        let indices = Indices::U32(vec![0, 1, 2]);
24
25        Mesh::new(
26            PrimitiveTopology::TriangleList,
27            RenderAssetUsages::default(),
28        )
29        .with_inserted_indices(indices)
30        .with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, positions)
31        .with_inserted_attribute(Mesh::ATTRIBUTE_NORMAL, normals)
32        .with_inserted_attribute(Mesh::ATTRIBUTE_UV_0, uvs)
33    }
34}
35
36impl Meshable for Triangle3d {
37    type Output = Triangle3dMeshBuilder;
38
39    fn mesh(&self) -> Self::Output {
40        Triangle3dMeshBuilder { triangle: *self }
41    }
42}
43
44/// The normal of a [`Triangle3d`] with zeroing so that a [`Vec3`] is always obtained for meshing.
45#[inline]
46pub(crate) fn normal_vec(triangle: &Triangle3d) -> Vec3 {
47    triangle.normal().map_or(Vec3::ZERO, |n| n.into())
48}
49
50/// Unskewed uv-coordinates for a [`Triangle3d`].
51#[inline]
52pub(crate) fn uv_coords(triangle: &Triangle3d) -> [[f32; 2]; 3] {
53    let [a, b, c] = triangle.vertices;
54
55    let main_length = a.distance(b);
56    let Some(x) = (b - a).try_normalize() else {
57        return [[0., 0.], [1., 0.], [0., 1.]];
58    };
59    let y = c - a;
60
61    // `x` corresponds to one of the axes in uv-coordinates;
62    // to uv-map the triangle without skewing, we use the orthogonalization
63    // of `y` with respect to `x` as the second direction and construct a rectangle that
64    // contains `triangle`.
65    let y_proj = y.project_onto_normalized(x);
66
67    // `offset` represents the x-coordinate of the point `c`; note that x has been shrunk by a
68    // factor of `main_length`, so `offset` follows it.
69    let offset = y_proj.dot(x) / main_length;
70
71    // Obtuse triangle leaning to the left => x direction extends to the left, shifting a from 0.
72    if offset < 0. {
73        let total_length = 1. - offset;
74        let a_uv = [offset.abs() / total_length, 0.];
75        let b_uv = [1., 0.];
76        let c_uv = [0., 1.];
77
78        [a_uv, b_uv, c_uv]
79    }
80    // Obtuse triangle leaning to the right => x direction extends to the right, shifting b from 1.
81    else if offset > 1. {
82        let a_uv = [0., 0.];
83        let b_uv = [1. / offset, 0.];
84        let c_uv = [1., 1.];
85
86        [a_uv, b_uv, c_uv]
87    }
88    // Acute triangle => no extending necessary; a remains at 0 and b remains at 1.
89    else {
90        let a_uv = [0., 0.];
91        let b_uv = [1., 0.];
92        let c_uv = [offset, 1.];
93
94        [a_uv, b_uv, c_uv]
95    }
96}
97
98impl From<Triangle3d> for Mesh {
99    fn from(triangle: Triangle3d) -> Self {
100        triangle.mesh().build()
101    }
102}
103
104#[cfg(test)]
105mod tests {
106    use super::uv_coords;
107    use bevy_math::primitives::Triangle3d;
108
109    #[test]
110    fn uv_test() {
111        use bevy_math::vec3;
112        let mut triangle = Triangle3d::new(vec3(0., 0., 0.), vec3(2., 0., 0.), vec3(-1., 1., 0.));
113
114        let [a_uv, b_uv, c_uv] = uv_coords(&triangle);
115        assert_eq!(a_uv, [1. / 3., 0.]);
116        assert_eq!(b_uv, [1., 0.]);
117        assert_eq!(c_uv, [0., 1.]);
118
119        triangle.vertices[2] = vec3(3., 1., 0.);
120        let [a_uv, b_uv, c_uv] = uv_coords(&triangle);
121        assert_eq!(a_uv, [0., 0.]);
122        assert_eq!(b_uv, [2. / 3., 0.]);
123        assert_eq!(c_uv, [1., 1.]);
124
125        triangle.vertices[2] = vec3(2., 1., 0.);
126        let [a_uv, b_uv, c_uv] = uv_coords(&triangle);
127        assert_eq!(a_uv, [0., 0.]);
128        assert_eq!(b_uv, [1., 0.]);
129        assert_eq!(c_uv, [1., 1.]);
130    }
131}
bevy_render/mesh/primitives/dim3/triangle3d.rs

bevy_render/mesh/primitives/dim3/
triangle3d.rs
