Tetrahedron (Equilateral Triangular Pyramid)

Add the tetrahedron module

src/lib.rs additions:

#![allow(unused)]
fn main() {
use truck_meshalgo::prelude::*;

// ...keep `write_polygon_mesh`, `triangle()`, and `square()` above...
pub mod tetrahedron; // add this
pub use tetrahedron::tetrahedron; // add this
}

Construct Main Function

src/tetrahedron.rs:

#![allow(unused)]
fn main() {
use std::iter::FromIterator;
use truck_meshalgo::prelude::*;

/// Equilateral tetrahedron (triangular pyramid).
pub fn tetrahedron() -> PolygonMesh {

    //PLACE STEP 1-4 HERE

}
}

Step 1: Define vertex positions

#![allow(unused)]
fn main() {
    let positions = vec![
        Point3::new(0.0, 0.0, 0.0), // base 1 [0]
        Point3::new(1.0, 0.0, 0.0), // base 2 [1]
        Point3::new(0.5, f64::sqrt(3.0) / 2.0, 0.0), // base 3 [2]
        Point3::new(0.5, f64::sqrt(3.0) / 6.0, f64::sqrt(6.0) / 3.0), // apex [3]
    ];
}

Step 2: Build attribute set

#![allow(unused)]
fn main() {
    let attrs = StandardAttributes {
        positions,
        ..Default::default()
    };
}

Step 3: Define mesh faces

#![allow(unused)]
fn main() {
    let faces = Faces::from_iter([
        [2, 1, 0], // base
        [0, 1, 3], // side 1
        [1, 2, 3], // side 2
        [2, 0, 3], // side 3
    ]);
}

Trigonal pyramid illustration

Step 4: Construct the mesh

#![allow(unused)]
fn main() {
    PolygonMesh::new(attrs, faces)
}

Export the tetrahedron

Add examples/tetrahedron.rs:

fn main() {
    let mesh = truck_meshes::tetrahedron();
    truck_meshes::write_polygon_mesh(&mesh, "output/tetrahedron.obj");
}
What does the `&` mean in `&mesh`?

write_polygon_mesh expects a reference to a PolygonMesh, so &mesh borrows the mesh instead of moving it. That lets the function read the mesh without taking ownership, so you could keep using mesh afterward if needed.

Run it:

cargo run --example tetrahedron

View it

Open output/tetrahedron.obj in Preview/3D Viewer/ParaView/Blender. You should see a single tetrahedron.

gif below from Preview (mac).

Tetrahedron

File tree after this step 
truck_meshes/
├─ Cargo.toml
├─ src/
│  ├─ lib.rs
│  ├─ triangle.rs
│  ├─ square.rs
│  └─ tetrahedron.rs
├─ examples/
│  ├─ triangle.rs
│  ├─ square.rs
│  └─ tetrahedron.rs
└─ output/          # exported OBJ files (e.g., output/tetrahedron.obj)
Full code:

src/lib.rs:

#![allow(unused)]
fn main() {
use truck_meshalgo::prelude::*;

/// Write any mesh to an OBJ file.
pub fn write_polygon_mesh(mesh: &PolygonMesh, path: &str) {
    let mut obj = std::fs::File::create(path).unwrap();
    obj::write(mesh, &mut obj).unwrap();
}

pub mod triangle;
pub use triangle::triangle;

pub mod square;
pub use square::square;

pub mod tetrahedron;
pub use tetrahedron::tetrahedron;
}

src/tetrahedron.rs:

#![allow(unused)]
fn main() {
use std::iter::FromIterator;
use truck_meshalgo::prelude::*;

pub fn tetrahedron() -> PolygonMesh {
    let positions = vec![
        Point3::new(0.0, 0.0, 0.0),
        Point3::new(1.0, 0.0, 0.0),
        Point3::new(0.5, f64::sqrt(3.0) / 2.0, 0.0),
        Point3::new(0.5, f64::sqrt(3.0) / 6.0, f64::sqrt(6.0) / 3.0),
    ];

    let attrs = StandardAttributes {
        positions,
        ..Default::default()
    };

    let faces = Faces::from_iter([
        [2, 1, 0],
        [0, 1, 3],
        [1, 2, 3],
        [2, 0, 3],
    ]);

    PolygonMesh::new(attrs, faces)
}
}

examples/tetrahedron.rs:

fn main() {
    let mesh = truck_meshes::tetrahedron();
    truck_meshes::write_polygon_mesh(&mesh, "output/tetrahedron.obj");
}