graphics/

meshes.rs

//! This module generates meshes

use std::{
    collections::HashMap,
    f32::consts::TAU,
    fs::File,
    io::{BufReader, Read},
};

use lin_alg::f32::Vec3;

use crate::{
    graphics::UP_VEC,
    types::{Mesh, Vertex},
};

/// Rotate a 2d vector counter-clockwise a given angle.
fn rotate_vec_2d(vec: [f32; 2], θ: f32) -> [f32; 2] {
    // Self-contained 2d rotation matrix (col-maj)
    let (sin_θ, cos_θ) = θ.sin_cos();
    let mat = [cos_θ, sin_θ, -sin_θ, cos_θ];

    [
        vec[0] * mat[0] + vec[1] * mat[2],
        vec[0] * mat[1] + vec[1] * mat[3],
    ]
}

impl Mesh {
    // /// Create a triangular face, with no volume. Only visible from one side.
    // /// Useful for building a grid surface like terrain, or a surface plot.
    // pub fn new_tri_face(a: [f32; 3], b: [f32; 3], c: [f32; 3]) -> Self {
    //     let norm = Vec3::new(0., 0., -1.);

    //     let vertices = vec![
    //         Vertex::new(a, norm),
    //         Vertex::new(b, norm),
    //         Vertex::new(c, norm),
    //     ];

    //     let indices = vec![0, 1, 2];

    //     Self {
    //         vertices,
    //         indices,
    //         material: 0,
    //     }
    // }

    /// Create a grid surface of triangles.
    /// Useful for building a grid surface like terrain, or a surface plot.
    /// `grid`'s outer vec is rows; inner vec is col-associated values within that
    /// row. The grid is evenly-spaced.
    /// todo:  You should draw both sides.

    /// Create a sided surface. Useful as terrain, or as a 2-sided plot.
    /// Note that the grid is viewed here as x, z, with values in y direction, to be compatible
    /// with the convention of Z-up used elsewhere.
    ///
    /// Points are (x, y, z), with Z being the vertical component. Their indices correspond to how they're
    /// tied together in a mesh.
    // pub fn new_surface(grid: &Vec<Vec<f32>>, start: f32, step: f32, two_sided: bool) -> Self {
    pub fn new_surface(points: &Vec<Vec<Vec3>>, two_sided: bool) -> Self {
        let mut vertices = Vec::new();
        let mut indices = Vec::new();

        // let mut x = start;
        let mut this_vert_i = 0;

        // Note: Y is
        for (i, rows) in points.into_iter().enumerate() {
            for (j, point) in rows.into_iter().enumerate() {
                let x = point.x;
                let y = point.z; // Swap y and z coords.
                let z = point.y;

                // for (i, row) in posits.iter().enumerate() {
                //     let mut z = start;
                //     for (j, y_posit) in row.into_iter().enumerate() {
                vertices.push(Vertex::new([x, y, z], Vec3::new_zero()));

                // To understand how we set up the triangles (index buffer),
                // it's best to draw it out.

                // Upper triangle: This exists for every vertex except
                // the bottom and right edges.
                // (grid.length is num_rows)
                if i != points.len() - 1 && j != rows.len() - 1 {
                    indices.append(&mut vec![
                        this_vert_i,
                        this_vert_i + points.len(),
                        this_vert_i + 1,
                    ]);
                }

                // Lower triangle: This exists for every vertex except
                // the top and left edges.
                if i != 0 && j != 0 {
                    indices.append(&mut vec![
                        this_vert_i,
                        this_vert_i - points.len(),
                        this_vert_i - 1,
                    ]);
                }

                // z += step;
                this_vert_i += 1;
                // }
            }
            // x += step;
        }

        // Now that we've populated our vertices, update their normals.
        for i in 0..indices.len() / 3 {
            let tri_start_i = i * 3;
            // Find the vertices that make up each triangle.
            let vert0 = vertices[indices[tri_start_i]];
            let vert1 = vertices[indices[tri_start_i + 1]];
            let vert2 = vertices[indices[tri_start_i + 2]];

            // Convert from arrays to Vec3.
            let v0 = Vec3::new(vert0.position[0], vert0.position[1], vert0.position[2]);
            let v1 = Vec3::new(vert1.position[0], vert1.position[1], vert1.position[2]);
            let v2 = Vec3::new(vert2.position[0], vert2.position[1], vert2.position[2]);

            let norm = (v2 - v0).to_normalized().cross((v1 - v0).to_normalized());

            // todo: DRY on this indexing.
            vertices[indices[tri_start_i]].normal = norm;
            vertices[indices[tri_start_i + 1]].normal = norm;
            vertices[indices[tri_start_i + 1]].normal = norm;
        }

        // If dual-sided, We need to replicate vertices, since the normal will be opposite.
        // Then, update the index buffer with these new vertices, using the opposite triangle order.
        if two_sided {
            let orig_vert_len = vertices.len();
            let mut vertices_other_side = Vec::new();
            for vertex in &vertices {
                let mut new_vertex = vertex.clone();
                new_vertex.normal *= -1.;
                vertices_other_side.push(new_vertex);
            }
            vertices.append(&mut vertices_other_side);

            let mut new_indices = Vec::new();
            for i in 0..indices.len() / 3 {
                let tri_start_i = i * 3;
                // Opposite direction of first-side indices.
                new_indices.push(indices[tri_start_i] + orig_vert_len);
                new_indices.push(indices[tri_start_i + 2] + orig_vert_len);
                new_indices.push(indices[tri_start_i + 1] + orig_vert_len);
            }
            indices.append(&mut new_indices);
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Creates an isosphere, from subdividing an icosahedron. 2-3 subdivisions works well for most uses.
    pub fn new_sphere(radius: f32, mut subdivisions: u32) -> Self {
        if subdivisions > 4 {
            println!(
                "Warning: Sphere subdivisions > 4 is not allowed due to extreme performance cost. Setting to 4."
            );
            subdivisions = 4;
        }
        let mut vertices = Vec::new();
        let mut indices = Vec::new();

        // Create the initial icosahedron.
        // 't' is the golden ratio.
        let t = (1.0 + 5.0_f32.sqrt()) / 2.0;

        let mut initial_points = vec![
            Vec3::new(-1.0, t, 0.0),
            Vec3::new(1.0, t, 0.0),
            Vec3::new(-1.0, -t, 0.0),
            Vec3::new(1.0, -t, 0.0),
            Vec3::new(0.0, -1.0, t),
            Vec3::new(0.0, 1.0, t),
            Vec3::new(0.0, -1.0, -t),
            Vec3::new(0.0, 1.0, -t),
            Vec3::new(t, 0.0, -1.0),
            Vec3::new(t, 0.0, 1.0),
            Vec3::new(-t, 0.0, -1.0),
            Vec3::new(-t, 0.0, 1.0),
        ];

        // Normalize each vertex to lie on the sphere and scale by the radius.
        for point in &mut initial_points {
            *point = point.to_normalized() * radius;
            vertices.push(Vertex::new(point.to_arr(), point.to_normalized()));
        }

        // The 20 faces of the icosahedron defined as triangles (each index refers to a vertex).
        let mut faces = vec![
            [0, 11, 5],
            [0, 5, 1],
            [0, 1, 7],
            [0, 7, 10],
            [0, 10, 11],
            [1, 5, 9],
            [5, 11, 4],
            [11, 10, 2],
            [10, 7, 6],
            [7, 1, 8],
            [3, 9, 4],
            [3, 4, 2],
            [3, 2, 6],
            [3, 6, 8],
            [3, 8, 9],
            [4, 9, 5],
            [2, 4, 11],
            [6, 2, 10],
            [8, 6, 7],
            [9, 8, 1],
        ];

        // Cache to avoid duplicating vertices at the midpoints.
        let mut middle_point_cache = HashMap::<(usize, usize), usize>::new();

        // Helper closure to get the midpoint between two vertices.
        // The midpoint is computed, normalized to the sphere and added to the vertex list.
        let mut get_middle_point = |a: usize, b: usize| -> usize {
            let key = if a < b { (a, b) } else { (b, a) };
            if let Some(&index) = middle_point_cache.get(&key) {
                return index;
            }
            let point_a = Vec3::from_slice(&vertices[a].position).unwrap();
            let point_b = Vec3::from_slice(&vertices[b].position).unwrap();
            let middle = (point_a + point_b) * 0.5;
            let normalized = middle.to_normalized() * radius;
            let index = vertices.len();
            vertices.push(Vertex::new(normalized.to_arr(), normalized.to_normalized()));
            middle_point_cache.insert(key, index);
            index
        };

        // Subdivide each triangular face.
        // For each subdivision iteration, every triangle is split into 4 smaller ones.
        for _ in 0..subdivisions {
            let mut new_faces = Vec::new();
            for tri in &faces {
                let a = tri[0];
                let b = tri[1];
                let c = tri[2];

                let ab = get_middle_point(a, b);
                let bc = get_middle_point(b, c);
                let ca = get_middle_point(c, a);

                new_faces.push([a, ca, ab]);
                new_faces.push([b, ab, bc]);
                new_faces.push([c, bc, ca]);
                new_faces.push([ab, ca, bc]);
            }
            faces = new_faces;
        }

        // Flatten the faces into the indices vector.
        for face in &mut faces {
            if subdivisions % 2 == 0 {
                let first = face[0];
                face[0] = face[1];
                face[1] = first;
            }
            indices.extend_from_slice(face);
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Create a UV sphere mesh. A higher number of latitudes and longitudes results in a
    /// a smoother sphere.
    pub fn new_sphere_uv(radius: f32, num_lats: usize, num_lons: usize) -> Self {
        let mut vertices = Vec::new();
        // We use faces to construct indices (triangles)
        let mut faces = Vec::new();
        let mut indices = Vec::new();

        // In radians
        let lat_size = TAU / (2. * num_lats as f32);
        let lon_size = TAU / num_lons as f32;

        let mut current_i = 0;

        // Bottom vertex and faces
        vertices.push(Vertex::new([0., -radius, 0.], Vec3::new(0., -1., 0.)));
        current_i += 1;

        // Faces connected to the bottom vertex.
        for k in 0..num_lons {
            if k == num_lons - 1 {
                indices.append(&mut vec![0, k + 2 - num_lons, k + 1]);
            } else {
                indices.append(&mut vec![0, k + 2, k + 1]);
            }
        }

        // Don't include the top or bottom (0, TAU/2) angles in lats.
        for i in 1..num_lats {
            let θ = i as f32 * lat_size;

            for j in 0..num_lons {
                let φ = j as f32 * lon_size;

                // https://en.wikipedia.org/wiki/Spherical_coordinate_system
                let x = radius * φ.cos() * θ.sin();
                let y = radius * φ.sin() * θ.sin();
                let z = radius * θ.cos();

                vertices.push(Vertex::new([x, y, z], Vec3::new(x, y, z).to_normalized()));

                if i < num_lats - 1 {
                    // In CCW order
                    if j == num_lons - 1 {
                        faces.push([
                            current_i,
                            current_i + 1 - num_lons,
                            current_i + 1,
                            current_i + num_lons,
                        ]);
                    } else {
                        faces.push([
                            current_i,
                            current_i + 1,
                            current_i + num_lons + 1,
                            current_i + num_lons,
                        ]);
                    }
                }
                current_i += 1;
            }
        }

        // Top vertex and faces
        vertices.push(Vertex::new([0., radius, 0.], Vec3::new(0., 1., 0.)));

        // Faces connected to the bottom vertex.
        let top_ring_start_i = current_i - num_lons;

        // todo: There's a rougue internal triangle on both the top and bottom caps, but it
        // todo does'nt appear to be visible from the outside. Possibly related: The caps look wrong.

        for k in 0..num_lons {
            if k == num_lons - 1 {
                indices.append(&mut vec![current_i, top_ring_start_i + k, top_ring_start_i]);
            } else {
                indices.append(&mut vec![
                    current_i,
                    top_ring_start_i + k,
                    top_ring_start_i + k + 1,
                ]);
            }
        }

        for f in faces {
            indices.append(&mut vec![f[0], f[1], f[2], f[0], f[2], f[3]]);
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    // todo: Fill this in.
    // todo: DRY with cylinder
    pub fn new_ring(len: f32, radius_inner: f32, radius_outer: f32, num_sides: usize) -> Self {
        let angle_between_vertices = TAU / num_sides as f32;

        let mut circle_vertices_inner = Vec::new();
        for i in 0..num_sides {
            circle_vertices_inner.push(rotate_vec_2d(
                [radius_inner, 0.],
                i as f32 * angle_between_vertices,
            ));
        }

        // todo DRY
        let mut circle_vertices_outer = Vec::new();
        for i in 0..num_sides {
            circle_vertices_outer.push(rotate_vec_2d(
                [radius_outer, 0.],
                i as f32 * angle_between_vertices,
            ));
        }

        let half_len = len * 0.5;
        let mod_ = 2 * num_sides;

        let mut vertices = Vec::new();
        let mut indices = Vec::new();

        let mut i_vertex = 0;

        // Set up the sides
        for vert in &circle_vertices_inner {
            // The number of faces is the number of angles - 1.
            // Triangle 1: This top, this bottom, next top.
            indices.append(&mut vec![i_vertex, i_vertex + 1, (i_vertex + 2) % mod_]);
            // Triangle 2: This bottom, next bottom, next top.
            indices.append(&mut vec![
                // Note: Order swapped from outer.
                i_vertex + 1,
                (i_vertex + 2) % mod_,
                (i_vertex + 3) % mod_,
            ]);

            // On edge face, top
            vertices.push(Vertex::new(
                [vert[0], half_len, vert[1]],
                Vec3::new(vert[0], 0., vert[1]).to_normalized(),
            ));
            i_vertex += 1;

            // On edge face, bottom
            vertices.push(Vertex::new(
                [vert[0], -half_len, vert[1]],
                Vec3::new(vert[0], 0., vert[1]).to_normalized(),
            ));
            i_vertex += 1;
        }

        // todo DRY!
        for vert in &circle_vertices_outer {
            // The number of faces is the number of angles - 1.
            // Triangle 1: This top, this bottom, next top.
            indices.append(&mut vec![i_vertex, i_vertex + 1, (i_vertex + 2) % mod_]);
            // Triangle 2: This bottom, next bottom, next top.
            indices.append(&mut vec![
                i_vertex + 1,
                (i_vertex + 3) % mod_,
                (i_vertex + 2) % mod_,
            ]);

            // On edge face, top
            vertices.push(Vertex::new(
                [vert[0], half_len, vert[1]],
                Vec3::new(vert[0], 0., vert[1]).to_normalized(),
            ));
            i_vertex += 1;

            // On edge face, bottom
            vertices.push(Vertex::new(
                [vert[0], -half_len, vert[1]],
                Vec3::new(vert[0], 0., vert[1]).to_normalized(),
            ));
            i_vertex += 1;
        }

        let top_anchor = i_vertex;
        let bottom_anchor = i_vertex + 1;

        // for (j, vert) in circle_vertices_inner.iter().enumerate() {
        //     // We need num_sides - 2 triangles using this anchor-vertex algorithm.
        //     if j != 0 && j != num_sides - 1 {
        //         indices.append(&mut vec![top_anchor, i_vertex, i_vertex + 2]);
        //         // We need CCW triangles for both, so reverse order on the bottom face.
        //         indices.append(&mut vec![bottom_anchor, i_vertex + 3, i_vertex + 1]);
        //     }
        //
        //     // Top face
        //     vertices.push(Vertex::new([vert[0], half_len, vert[1]], UP_VEC));
        //     i_vertex += 1;
        //
        //     // Bottom face
        //     vertices.push(Vertex::new([vert[0], -half_len, vert[1]], -UP_VEC));
        //     i_vertex += 1;
        // }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Create a box (rectangular prism) mesh.
    pub fn new_box(len_x: f32, len_y: f32, len_z: f32) -> Self {
        let x = len_x / 2.;
        let y = len_y / 2.;
        let z = len_z / 2.;

        // Aft face
        let abl = [-x, -y, -z];
        let abr = [x, -y, -z];
        let atr = [x, y, -z];
        let atl = [-x, y, -z];

        // Forward face
        let fbl = [-x, -y, z];
        let fbr = [x, -y, z];
        let ftr = [x, y, z];
        let ftl = [-x, y, z];

        // Normal vectors
        let aft = Vec3::new(0., 0., -1.);
        let fwd = Vec3::new(0., 0., 1.);
        let l = Vec3::new(-1., 0., 0.);
        let r = Vec3::new(1., 0., 0.);
        let t = Vec3::new(0., 1., 0.);
        let b = Vec3::new(0., -1., 0.);

        let vertices = vec![
            // Aft
            Vertex::new(abl, aft),
            Vertex::new(abr, aft),
            Vertex::new(atr, aft),
            Vertex::new(atl, aft),
            // Fwd
            Vertex::new(fbl, fwd),
            Vertex::new(ftl, fwd),
            Vertex::new(ftr, fwd),
            Vertex::new(fbr, fwd),
            // Left
            Vertex::new(fbl, l),
            Vertex::new(abl, l),
            Vertex::new(atl, l),
            Vertex::new(ftl, l),
            // Right
            Vertex::new(abr, r),
            Vertex::new(fbr, r),
            Vertex::new(ftr, r),
            Vertex::new(atr, r),
            // Top
            Vertex::new(atl, t),
            Vertex::new(atr, t),
            Vertex::new(ftr, t),
            Vertex::new(ftl, t),
            // Bottom
            Vertex::new(abl, b),
            Vertex::new(fbl, b),
            Vertex::new(fbr, b),
            Vertex::new(abr, b),
        ];

        let faces = [
            [0, 1, 2, 3],
            [4, 5, 6, 7],
            [8, 9, 10, 11],
            [12, 13, 14, 15],
            [16, 17, 18, 19],
            [20, 21, 22, 23],
        ];

        let mut indices = Vec::new();
        for face in &faces {
            indices.append(&mut vec![
                face[0], face[1], face[2], face[0], face[2], face[3],
            ]);
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Create a tetrahedron mesh
    pub fn new_tetrahedron(side_len: f32) -> Self {
        let c = side_len / 2.;

        let v_0 = [c, c, c];
        let v_1 = [c, -c, -c];
        let v_2 = [-c, c, -c];
        let v_3 = [-c, -c, c];

        // Note: For tetrahedrons, the normals are the corners of the cube we
        // didn't use for vertices.
        let n_0 = Vec3::new(1., 1., -1.).to_normalized();
        let n_1 = Vec3::new(1., -1., 1.).to_normalized();
        let n_2 = Vec3::new(-1., 1., 1.).to_normalized();
        let n_3 = Vec3::new(-1., -1., -1.).to_normalized();

        let vertices = vec![
            // Face 0
            Vertex::new(v_0, n_0),
            Vertex::new(v_2, n_0),
            Vertex::new(v_1, n_0),
            // Face 1
            Vertex::new(v_0, n_1),
            Vertex::new(v_1, n_1),
            Vertex::new(v_3, n_1),
            // Face 2
            Vertex::new(v_0, n_2),
            Vertex::new(v_3, n_2),
            Vertex::new(v_2, n_2),
            // Face 3
            Vertex::new(v_1, n_3),
            Vertex::new(v_2, n_3),
            Vertex::new(v_3, n_3),
        ];

        // These indices define faces by triangles. (each 3 represent a triangle, starting at index 0.
        // Indices are arranged CCW, from front of face
        // Note that because we're using "hard" lighting on faces, we can't repeat any vertices, since
        // they each have a different normal.
        #[rustfmt::skip]
            // let indices: &[u32] = &[
            let indices = vec![
            0, 1, 2,
            3, 4, 5,
            6, 7, 8,
            9, 10, 11,
        ];

        Self {
            vertices,
            indices,
            // vertex_buffer: Vec<usize>,
            // index_buffer: Vec<usize>,
            // num_elements: u32,
            material: 0,
        }
    }

    /// Create a cylinder
    pub fn new_cylinder(len: f32, radius: f32, num_sides: usize) -> Self {
        let angle_between_vertices = TAU / num_sides as f32;

        let mut circle_vertices = Vec::new();
        for i in 0..num_sides {
            circle_vertices.push(rotate_vec_2d(
                [radius, 0.],
                i as f32 * angle_between_vertices,
            ));
        }

        let half_len = len * 0.5;
        let mod_ = 2 * num_sides;

        let mut vertices = Vec::new();
        let mut indices = Vec::new();

        let mut i_vertex = 0;

        // Set up the sides
        for vert in &circle_vertices {
            // The number of faces is the number of angles - 1.
            // Triangle 1: This top, this bottom, next top.
            indices.append(&mut vec![i_vertex, i_vertex + 1, (i_vertex + 2) % mod_]);
            // Triangle 2: This bottom, next bottom, next top.
            indices.append(&mut vec![
                i_vertex + 1,
                (i_vertex + 3) % mod_,
                (i_vertex + 2) % mod_,
            ]);

            // On edge face, top
            vertices.push(Vertex::new(
                [vert[0], half_len, vert[1]],
                Vec3::new(vert[0], 0., vert[1]).to_normalized(),
            ));
            i_vertex += 1;

            // On edge face, bottom
            vertices.push(Vertex::new(
                [vert[0], -half_len, vert[1]],
                Vec3::new(vert[0], 0., vert[1]).to_normalized(),
            ));
            i_vertex += 1;
        }

        let top_anchor = i_vertex;
        let bottom_anchor = i_vertex + 1;

        for (j, vert) in circle_vertices.iter().enumerate() {
            // We need num_sides - 2 triangles using this anchor-vertex algorithm.
            if j != 0 && j != num_sides - 1 {
                indices.append(&mut vec![top_anchor, i_vertex, i_vertex + 2]);
                // We need CCW triangles for both, so reverse order on the bottom face.
                indices.append(&mut vec![bottom_anchor, i_vertex + 3, i_vertex + 1]);
            }

            // Top face
            vertices.push(Vertex::new([vert[0], half_len, vert[1]], UP_VEC));
            i_vertex += 1;

            // Bottom face
            vertices.push(Vertex::new([vert[0], -half_len, vert[1]], -UP_VEC));
            i_vertex += 1;
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    pub fn new_pyramid(len: f32, radius: f32, num_sides: usize) -> Self {
        // Note: DRY with cylinder; this is similar, but one of the faces consists of a single, central point.
        let angle_between_vertices = TAU / num_sides as f32;

        let mut circle_vertices = Vec::new();
        for i in 0..num_sides {
            circle_vertices.push(rotate_vec_2d(
                [radius, 0.],
                i as f32 * angle_between_vertices,
            ));
        }

        let half_len = len * 0.5;

        let mut vertices = Vec::new();
        let mut indices = Vec::new();

        let tip = 0;

        let mut i_vertex = 0;

        // A central point; the tip of the arrow.
        vertices.push(Vertex::new([0., half_len, 0.], UP_VEC));
        i_vertex += 1;

        indices.push(tip);

        // Set up the sides.
        for (j, vert) in circle_vertices.iter().enumerate() {
            // The number of faces is the number of angles - 1.

            let next = if j == num_sides - 1 {
                i_vertex + 1 - num_sides // Wrap to the first.
            } else {
                i_vertex + 1
            };
            // Triangle: This edge, next edge, central;
            indices.append(&mut vec![i_vertex, next, tip]);

            vertices.push(Vertex::new(
                [vert[0], -half_len, vert[1]],
                Vec3::new(vert[0], len, vert[1]).to_normalized(),
            ));
            i_vertex += 1;
        }

        let bottom_anchor = i_vertex; // An arbitrary point shared by all bottom triangles.

        // Set up the bottom face.
        for (j, vert) in circle_vertices.iter().enumerate() {
            // We need num_sides - 2 triangles using this anchor-vertex algorithm.
            if j != 0 && j != num_sides - 1 {
                indices.append(&mut vec![bottom_anchor, i_vertex + 1, i_vertex]);
            }

            vertices.push(Vertex::new([vert[0], -half_len, vert[1]], -UP_VEC));
            i_vertex += 1;
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Create an arrow, oriented up.
    pub fn new_arrow(len: f32, radius: f32, num_sides: usize) -> Self {
        let tip_offset = len / 2.;
        let cylinder = Self::new_cylinder(len, radius, num_sides);

        let tip = Self::new_pyramid(len * 0.5, radius * 3., num_sides);

        let mut vertices = cylinder.vertices.clone();
        let mut indices = cylinder.indices.clone();

        for vertex in tip.vertices {
            vertices.push(Vertex {
                position: [
                    vertex.position[0],
                    vertex.position[1] + tip_offset,
                    vertex.position[2],
                ],
                ..vertex
            });
        }

        let ci2 = cylinder.indices.clone();
        let tip_start_index = ci2.iter().max().unwrap() + 1;

        for index in tip.indices {
            indices.push(index + tip_start_index);
        }

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Load a mesh from obj data.
    /// [File type description](https://en.wikipedia.org/wiki/Wavefront_.obj_file)
    /// [Example](https://github.com/gfx-rs/wgpu/blob/master/wgpu/examples/skybox/main.rs)
    pub fn from_obj(obj_data: &[u8]) -> Self {
        let data = obj::ObjData::load_buf(&obj_data[..]).unwrap();
        let mut vertices = Vec::new();

        for object in data.objects {
            for group in object.groups {
                vertices.clear();

                for poly in group.polys {
                    for end_index in 2..poly.0.len() {
                        for &index in &[0, end_index - 1, end_index] {
                            let obj::IndexTuple(position_id, _texture_id, normal_id) =
                                poly.0[index];

                            let n = data.normal[normal_id.unwrap()];

                            vertices.push(Vertex::new(
                                data.position[position_id],
                                Vec3::new(n[0], n[1], n[2]),
                            ));
                        }
                    }
                }
            }
        }

        // todo: Is this right?
        let indices = (0..vertices.len()).collect();

        Self {
            vertices,
            indices,
            material: 0,
        }
    }

    /// Load a mesh from a obj file.
    /// [File type description](https://en.wikipedia.org/wiki/Wavefront_.obj_file)
    /// [Example](https://github.com/gfx-rs/wgpu/blob/master/wgpu/examples/skybox/main.rs)
    pub fn from_obj_file(filename: &str) -> Self {
        // todo: Add a way to load from an obj bytestream etc instead of just file API.
        let f = File::open(filename).unwrap();
        let mut reader = BufReader::new(f);
        let mut file_buf = Vec::new();

        reader.read_to_end(&mut file_buf).unwrap();

        Self::from_obj(&file_buf)
    }
}