OneCAD-Draft/src/io/stl.rs

// STL binary export — converts all tessellated MeshModels in the scene to a
// single binary STL file.
//
// Binary STL format:
//   80-byte header
//   4-byte triangle count (u32 LE)
//   Per triangle (50 bytes):
//     3 × f32 normal
//     3 × 3 × f32 vertices
//     2-byte attribute (0)

use std::io::Write;

use crate::scene::model::mesh_model::MeshModel;

/// Build a binary STL byte buffer from a slice of mesh models.
/// Returns `None` if there are no triangles to export.
pub fn build_stl(meshes: &[&MeshModel]) -> Option<Vec<u8>> {
    // Collect all triangles.
    struct Tri {
        normal: [f32; 3],
        v: [[f32; 3]; 3],
    }

    let mut tris: Vec<Tri> = Vec::new();

    for mesh in meshes {
        let verts = &mesh.verts;
        let idx = &mesh.indices;
        let n_tri = idx.len() / 3;
        for t in 0..n_tri {
            let i0 = idx[t * 3] as usize;
            let i1 = idx[t * 3 + 1] as usize;
            let i2 = idx[t * 3 + 2] as usize;
            if i0 >= verts.len() || i1 >= verts.len() || i2 >= verts.len() {
                continue;
            }
            let a = verts[i0];
            let b = verts[i1];
            let c = verts[i2];

            // The facet normal is the triangle's own. A smoothed vertex normal
            // leans off it, so only a triangle whose cross product is zero
            // falls back to the normal the mesh gives it. Small facets have a
            // tiny cross product but still a normal of their own.
            let ab = [b[0] - a[0], b[1] - a[1], b[2] - a[2]];
            let ac = [c[0] - a[0], c[1] - a[1], c[2] - a[2]];
            let nx = ab[1] * ac[2] - ab[2] * ac[1];
            let ny = ab[2] * ac[0] - ab[0] * ac[2];
            let nz = ab[0] * ac[1] - ab[1] * ac[0];
            let len = nx.hypot(ny).hypot(nz);
            let normal = if len == 0.0 && i0 < mesh.normals.len() {
                mesh.normals[i0]
            } else {
                let len = len.max(f32::MIN_POSITIVE);
                [nx / len, ny / len, nz / len]
            };

            tris.push(Tri {
                normal,
                v: [a, b, c],
            });
        }
    }

    if tris.is_empty() {
        return None;
    }

    let mut buf: Vec<u8> = Vec::with_capacity(84 + tris.len() * 50);

    // 80-byte header.
    let mut header = [0u8; 80];
    let title = b"Open CAD Studio STL export";
    header[..title.len()].copy_from_slice(title);
    buf.extend_from_slice(&header);

    // Triangle count.
    buf.extend_from_slice(&(tris.len() as u32).to_le_bytes());

    for tri in &tris {
        // Normal.
        for &f in &tri.normal {
            buf.write_all(&f.to_le_bytes()).ok()?;
        }
        // Vertices.
        for v in &tri.v {
            for &f in v {
                buf.write_all(&f.to_le_bytes()).ok()?;
            }
        }
        // Attribute byte count = 0.
        buf.extend_from_slice(&0u16.to_le_bytes());
    }

    Some(buf)
}

/// Web: no save dialog — the export continues with a virtual filename and
/// the generated bytes are delivered as a browser download.
#[cfg(target_arch = "wasm32")]
pub async fn pick_stl_path_owned(stem: String) -> Option<std::path::PathBuf> {
    Some(std::path::PathBuf::from(format!("{stem}.stl")))
}

#[cfg(test)]
mod tests {
    use super::build_stl;
    use crate::scene::model::mesh_model::MeshModel;

    fn triangle(verts: Vec<[f32; 3]>, normals: Vec<[f32; 3]>) -> MeshModel {
        MeshModel {
            name: String::new(),
            verts,
            verts_low: Vec::new(),
            normals,
            indices: vec![0, 1, 2],
            triangle_material_handles: Vec::new(),
            triangle_colors: Vec::new(),
            color: [1.0; 4],
            selected: false,
        }
    }

    /// The first facet's normal: the 12 bytes after the 80-byte header and
    /// the 4-byte triangle count.
    fn first_facet_normal(stl: &[u8]) -> [f32; 3] {
        [0, 1, 2].map(|k| {
            let at = 84 + 4 * k;
            f32::from_le_bytes(stl[at..at + 4].try_into().expect("4 bytes"))
        })
    }

    /// A curved surface shares smoothed normals across its facets, so a
    /// vertex normal leans away from the triangle. STL stores the facet's
    /// own normal.
    #[test]
    fn the_facet_normal_is_the_triangles_own() {
        let leaning = [
            std::f32::consts::FRAC_1_SQRT_2,
            0.0,
            std::f32::consts::FRAC_1_SQRT_2,
        ];
        let mesh = triangle(
            vec![[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [0.0, 1.0, 0.0]],
            vec![leaning; 3],
        );
        let stl = build_stl(&[&mesh]).expect("stl");
        assert_eq!(first_facet_normal(&stl), [0.0, 0.0, 1.0]);
    }

    /// A small facet still has a normal of its own. Its cross product is tiny
    /// (1e-8 here), far below f32::EPSILON, but it is not zero.
    #[test]
    fn a_small_facet_keeps_its_own_normal() {
        let leaning = [
            std::f32::consts::FRAC_1_SQRT_2,
            0.0,
            std::f32::consts::FRAC_1_SQRT_2,
        ];
        let mesh = triangle(
            vec![[0.0, 0.0, 0.0], [1e-4, 0.0, 0.0], [0.0, 1e-4, 0.0]],
            vec![leaning; 3],
        );
        let stl = build_stl(&[&mesh]).expect("stl");
        assert_eq!(first_facet_normal(&stl), [0.0, 0.0, 1.0]);
    }

    #[test]
    fn a_facet_whose_squared_cross_product_underflows_keeps_its_own_normal() {
        let mesh = triangle(
            vec![[0.0, 0.0, 0.0], [1.0e-12, 0.0, 0.0], [0.0, 1.0e-12, 0.0]],
            vec![[0.0, 1.0, 0.0]; 3],
        );
        let stl = build_stl(&[&mesh]).expect("stl");
        assert_eq!(first_facet_normal(&stl), [0.0, 0.0, 1.0]);
    }

    /// A facet with no area has no normal of its own, so it keeps the one
    /// the mesh gives it rather than a zero vector.
    #[test]
    fn a_degenerate_facet_keeps_its_vertex_normal() {
        let mesh = triangle(
            vec![[0.0, 0.0, 0.0], [1.0, 0.0, 0.0], [2.0, 0.0, 0.0]],
            vec![[0.0, 1.0, 0.0]; 3],
        );
        let stl = build_stl(&[&mesh]).expect("stl");
        assert_eq!(first_facet_normal(&stl), [0.0, 1.0, 0.0]);
    }
}