feat(scene): tessellate loaded ACIS solids via the truck B-rep kernel

Rebuild every ACIS face as a truck topological Face and mesh it with truck,
routing loaded 3DSOLID/BODY/REGION/SURFACE through the same kernel the Model
tab uses. plane→try_attach_plane, cone/cylinder/sphere/torus→revolution faces
(truck's Surface has no cone/sphere/torus), spline→BSplineSurface grid sample.
Per-vertex normals are computed analytically outward per surface type and
triangle winding is oriented to match, since truck's own orientation isn't
consistent across independently built faces. `tessellate_solid3d|body|region|
surface` try this first and fall back to the bespoke sampler.

Light the mesh shader with three spread directional lights plus ambient
(abs(dot) per light, two-sided) so faces — including the inner walls of an
open lofted surface — never read as a flat dark mass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/acis_to_truck.rs

@@ -0,0 +1,431 @@
+// ACIS B-rep → truck B-rep → mesh.
+//
+// Rebuilds every ACIS face as a truck topological `Face` and lets truck's
+// meshing kernel triangulate it, routing loaded 3DSOLID / BODY / REGION /
+// SURFACE geometry through the same NURBS/B-rep kernel the Model tab builds on
+// instead of the bespoke per-surface sampler in `solid3d_tess`.
+//
+//   plane-surface   → planar face from the sampled boundary loop
+//   cone-surface    → surface of revolution (cylinder / cone) via rsweep
+//   sphere-surface  → meridian revolved about the pole
+//   torus-surface   → tube circle revolved about the spine
+//   spline-surface  → truck BSplineSurface, grid-sampled (see spline_tess)
+//
+// Each face is meshed independently and its triangles are oriented outward
+// using an analytic per-surface normal — truck's own face orientation is not
+// consistent across independently built faces, so normals/winding are derived
+// from geometry instead. Faces whose surface type isn't handled are skipped;
+// the caller falls back to `solid3d_tess` when this returns `None`.
+
+use truck_meshalgo::tessellation::{MeshableShape, MeshedShape};
+use truck_modeling::{builder, Face, InnerSpace, Point3, Rad, Shell, Vector3, Wire};
+use truck_polymesh::PolygonMesh;
+
+use acadrust::entities::acis::types::Sense;
+use acadrust::entities::acis::{
+    SatConeSurface, SatDocument, SatFace, SatPlaneSurface, SatSphereSurface, SatTorusSurface,
+};
+
+use crate::scene::mesh_model::{MeshLodSet, MeshModel};
+use crate::scene::solid3d_tess::{
+    apply_body_transform, body_transform, collect_face_polygon, cone_axis_span, mesh_aabb,
+};
+
+/// Slightly over 2π so revolution builders close the loop.
+const FULL: f64 = std::f64::consts::TAU + 0.2;
+/// Boundary sampling density for planar faces with curved (circular) edges.
+const BOUNDARY_SEGS: usize = 64;
+/// Triangle mesh chord tolerance (world units).
+const MESH_TOL: f64 = 0.01;
+
+/// Analytic outward-normal rule for a face, used to orient triangles and
+/// supply smooth per-vertex normals (truck's face orientation is unreliable
+/// for independently built faces).
+enum Outward {
+    /// Constant normal (planar face).
+    Const([f64; 3]),
+    /// Cone/cylinder: radial away from the axis, tilted by the half-angle.
+    Cone {
+        center: [f64; 3],
+        axis: [f64; 3],
+        sin: f64,
+        cos: f64,
+    },
+    /// Sphere: radial from the centre.
+    Sphere { center: [f64; 3] },
+    /// Torus: from the nearest point on the spine circle.
+    Torus {
+        center: [f64; 3],
+        axis: [f64; 3],
+        major: f64,
+    },
+}
+
+impl Outward {
+    fn at(&self, p: [f64; 3]) -> [f64; 3] {
+        match self {
+            Outward::Const(n) => *n,
+            Outward::Cone {
+                center,
+                axis,
+                sin,
+                cos,
+            } => {
+                let d = vsub(p, *center);
+                let h = vdot(d, *axis);
+                let radial = vnorm(vsub(d, vscale(*axis, h)));
+                vnorm(vsub(vscale(radial, *cos), vscale(*axis, *sin)))
+            }
+            Outward::Sphere { center } => vnorm(vsub(p, *center)),
+            Outward::Torus {
+                center,
+                axis,
+                major,
+            } => {
+                let d = vsub(p, *center);
+                let h = vdot(d, *axis);
+                let radial = vsub(d, vscale(*axis, h));
+                let rl = vlen(radial);
+                let spine = if rl > 1e-9 {
+                    vadd(*center, vscale(radial, major / rl))
+                } else {
+                    *center
+                };
+                vnorm(vsub(p, spine))
+            }
+        }
+    }
+}
+
+/// Tessellate an ACIS SAT document by rebuilding it as truck faces.
+///
+/// Returns `None` when no face could be rebuilt (caller should fall back to
+/// the bespoke sampler).
+pub fn tessellate_sat_truck(
+    sat: &SatDocument,
+    name: String,
+    color: [f32; 4],
+    _facet_res: f64,
+) -> Option<MeshLodSet> {
+    let mut mesh = MeshModel {
+        name: name.clone(),
+        verts: Vec::new(),
+        normals: Vec::new(),
+        indices: Vec::new(),
+        color,
+        selected: false,
+    };
+
+    for face in sat.faces() {
+        let Some(surf_rec) = sat.resolve(face.surface()) else {
+            continue;
+        };
+        let Some((faces, outward)) = build_face_group(sat, &face, surf_rec) else {
+            continue;
+        };
+        if faces.is_empty() {
+            continue;
+        }
+        let shell: Shell = faces.into();
+        let poly = shell.triangulation(MESH_TOL).to_polygon();
+        append_group(&mut mesh, &poly, &outward);
+    }
+
+    // Spline (NURBS) faces are meshed by direct grid sampling of the truck
+    // BSplineSurface — see spline_tess — and merged into the same buffers.
+    append_spline_faces(sat, &mut mesh);
+
+    if mesh.indices.is_empty() {
+        return None;
+    }
+
+    if let Some((m, tr, scale)) = body_transform(sat) {
+        apply_body_transform(&mut mesh, &m, &tr, scale);
+    }
+    let world_aabb = mesh_aabb(&mesh);
+    Some(MeshLodSet {
+        lods: vec![mesh],
+        world_aabb,
+    })
+}
+
+/// Build the truck face(s) + outward rule for one analytic ACIS face.
+fn build_face_group(
+    sat: &SatDocument,
+    face: &SatFace,
+    surf_rec: &acadrust::entities::acis::SatRecord,
+) -> Option<(Vec<Face>, Outward)> {
+    match surf_rec.entity_type.as_str() {
+        "plane-surface" => {
+            let plane = SatPlaneSurface::from_record(surf_rec)?;
+            let f = plane_face(sat, face)?;
+            let (nx, ny, nz) = plane.normal();
+            let n = if matches!(face.sense(), Sense::Reversed) {
+                [-nx, -ny, -nz]
+            } else {
+                [nx, ny, nz]
+            };
+            Some((vec![f], Outward::Const(vnorm(n))))
+        }
+        "cone-surface" => {
+            let cone = SatConeSurface::from_record(surf_rec)?;
+            let (faces, out) = cone_faces(sat, &cone)?;
+            Some((faces, out))
+        }
+        "sphere-surface" => {
+            let sphere = SatSphereSurface::from_record(surf_rec)?;
+            let (cx, cy, cz) = sphere.center();
+            Some((sphere_faces(&sphere), Outward::Sphere { center: [cx, cy, cz] }))
+        }
+        "torus-surface" => {
+            let torus = SatTorusSurface::from_record(surf_rec)?;
+            let (cx, cy, cz) = torus.center();
+            let (nx, ny, nz) = torus.normal();
+            let out = Outward::Torus {
+                center: [cx, cy, cz],
+                axis: vnorm([nx, ny, nz]),
+                major: torus.major_radius(),
+            };
+            Some((torus_faces(&torus), out))
+        }
+        _ => None,
+    }
+}
+
+// ── Planar face ────────────────────────────────────────────────────────────
+
+/// Build a planar truck face from a face's sampled boundary loop. Curved
+/// boundary edges (circles) are sampled into line segments, which keeps the
+/// wire planar so `try_attach_plane` can fit the plane.
+fn plane_face(sat: &SatDocument, face: &SatFace) -> Option<Face> {
+    let poly = collect_face_polygon(sat, face, BOUNDARY_SEGS);
+    if poly.len() < 3 {
+        return None;
+    }
+    let verts: Vec<_> = poly
+        .iter()
+        .map(|p| builder::vertex(Point3::new(p[0], p[1], p[2])))
+        .collect();
+    let n = verts.len();
+    let edges: Vec<_> = (0..n)
+        .map(|i| builder::line(&verts[i], &verts[(i + 1) % n]))
+        .collect();
+    let wire: Wire = edges.into();
+    builder::try_attach_plane(&[wire]).ok()
+}
+
+// ── Cone / cylinder face ─────────────────────────────────────────────────────
+
+/// Build the lateral surface of a cone/cylinder as revolution faces. The
+/// height span comes from the solid's coaxial rims (plus a true cone's apex).
+fn cone_faces(sat: &SatDocument, cone: &SatConeSurface) -> Option<(Vec<Face>, Outward)> {
+    let (cx, cy, cz) = cone.center();
+    let (ax, ay, az) = cone.axis();
+    let (ux, uy, uz) = cone.major_axis();
+    let radius = cone.radius();
+    let sin = cone.sin_half_angle();
+    let cos = cone.cos_half_angle();
+
+    let axis = norm(Vector3::new(ax, ay, az));
+    let udir = norm(Vector3::new(ux, uy, uz));
+    let center = Point3::new(cx, cy, cz);
+
+    let (hmin, hmax) = cone_axis_span(sat, cone, [axis.x, axis.y, axis.z], [cx, cy, cz])?;
+    let r_at = |h: f64| {
+        if cos.abs() > 1e-9 {
+            radius + h * sin / cos
+        } else {
+            radius
+        }
+    };
+
+    let p0 = center + udir * r_at(hmin) + axis * hmin;
+    let p1 = center + udir * r_at(hmax) + axis * hmax;
+    let profile = builder::line(&builder::vertex(p0), &builder::vertex(p1));
+    let shell: Shell = builder::rsweep(&profile, center, axis, Rad(FULL));
+
+    let out = Outward::Cone {
+        center: [cx, cy, cz],
+        axis: [axis.x, axis.y, axis.z],
+        sin,
+        cos,
+    };
+    Some((shell.face_iter().cloned().collect(), out))
+}
+
+// ── Sphere face ──────────────────────────────────────────────────────────────
+
+fn sphere_faces(sphere: &SatSphereSurface) -> Vec<Face> {
+    let (cx, cy, cz) = sphere.center();
+    let r = sphere.radius();
+    let (px, py, pz) = sphere.pole();
+    let (ux, uy, uz) = sphere.u_direction();
+
+    let center = Point3::new(cx, cy, cz);
+    let pole = norm(Vector3::new(px, py, pz));
+    let mut perp = Vector3::new(ux, uy, uz);
+    if perp.magnitude2() < 1e-12 || perp.dot(pole).abs() > 0.999 {
+        perp = perpendicular(pole);
+    } else {
+        perp = norm(perp - pole * perp.dot(pole));
+    }
+
+    let top = builder::vertex(center + pole * r);
+    let meridian: Wire = builder::rsweep(&top, center, perp, Rad(std::f64::consts::PI));
+    let shell: Shell = builder::cone(&meridian, pole, Rad(FULL));
+    shell.face_iter().cloned().collect()
+}
+
+// ── Torus face ───────────────────────────────────────────────────────────────
+
+fn torus_faces(torus: &SatTorusSurface) -> Vec<Face> {
+    let (cx, cy, cz) = torus.center();
+    let (nx, ny, nz) = torus.normal();
+    let (ux, uy, uz) = torus.u_direction();
+    let major = torus.major_radius();
+    let minor = torus.minor_radius();
+
+    let center = Point3::new(cx, cy, cz);
+    let axis = norm(Vector3::new(nx, ny, nz));
+    let udir = norm(Vector3::new(ux, uy, uz));
+    let binormal = norm(axis.cross(udir));
+
+    let ring_center = center + udir * major;
+    let tube_start = builder::vertex(ring_center + axis * minor);
+    let tube: Wire = builder::rsweep(&tube_start, ring_center, binormal, Rad(FULL));
+    let shell: Shell = builder::rsweep(&tube, center, axis, Rad(FULL));
+    shell.face_iter().cloned().collect()
+}
+
+// ── Spline faces (NURBS) ─────────────────────────────────────────────────────
+
+/// Append meshes for every `spline-surface` face, reusing the truck
+/// BSplineSurface grid sampler in `spline_tess`.
+fn append_spline_faces(sat: &SatDocument, mesh: &mut MeshModel) {
+    use crate::scene::solid3d_tess::LodConfig;
+    for face in sat.faces() {
+        let Some(surf_rec) = sat.resolve(face.surface()) else {
+            continue;
+        };
+        if surf_rec.entity_type != "spline-surface" {
+            continue;
+        }
+        let mut verts: Vec<[f32; 3]> = Vec::new();
+        let mut normals: Vec<[f32; 3]> = Vec::new();
+        let mut indices: Vec<u32> = Vec::new();
+        crate::scene::spline_tess::tess_spline_face(
+            sat,
+            &face,
+            LodConfig::HIGH,
+            &mut verts,
+            &mut normals,
+            &mut indices,
+        );
+        let base = mesh.verts.len() as u32;
+        mesh.verts.extend_from_slice(&verts);
+        mesh.normals.extend_from_slice(&normals);
+        mesh.indices.extend(indices.iter().map(|i| i + base));
+    }
+}
+
+// ── Mesh append with analytic outward normals ────────────────────────────────
+
+/// Append one face's triangulation to `mesh`, computing smooth per-vertex
+/// normals from the outward rule and flipping any triangle whose winding
+/// disagrees with that outward direction.
+fn append_group(mesh: &mut MeshModel, poly: &PolygonMesh, outward: &Outward) {
+    let positions = poly.positions();
+    let base = mesh.verts.len() as u32;
+    for p in positions {
+        let pos = [p.x, p.y, p.z];
+        mesh.verts.push([p.x as f32, p.y as f32, p.z as f32]);
+        let n = outward.at(pos);
+        mesh.normals.push([n[0] as f32, n[1] as f32, n[2] as f32]);
+    }
+    for tri in poly.tri_faces() {
+        let (i0, i1, i2) = (tri[0].pos, tri[1].pos, tri[2].pos);
+        let a = pt(positions[i0]);
+        let b = pt(positions[i1]);
+        let c = pt(positions[i2]);
+        let gn = vcross(vsub(b, a), vsub(c, a));
+        let cen = [
+            (a[0] + b[0] + c[0]) / 3.0,
+            (a[1] + b[1] + c[1]) / 3.0,
+            (a[2] + b[2] + c[2]) / 3.0,
+        ];
+        let out = outward.at(cen);
+        let (j0, j1, j2) = (base + i0 as u32, base + i1 as u32, base + i2 as u32);
+        if vdot(gn, out) < 0.0 {
+            mesh.indices.extend_from_slice(&[j0, j2, j1]);
+        } else {
+            mesh.indices.extend_from_slice(&[j0, j1, j2]);
+        }
+    }
+}
+
+#[inline]
+fn pt(p: Point3) -> [f64; 3] {
+    [p.x, p.y, p.z]
+}
+
+// ── Vector helpers (cgmath Vector3) ──────────────────────────────────────────
+
+#[inline]
+fn norm(v: Vector3) -> Vector3 {
+    let len = v.magnitude();
+    if len < 1e-12 {
+        Vector3::unit_z()
+    } else {
+        v / len
+    }
+}
+
+/// Any unit vector perpendicular to `v`.
+fn perpendicular(v: Vector3) -> Vector3 {
+    let a = if v.x.abs() < 0.9 {
+        Vector3::unit_x()
+    } else {
+        Vector3::unit_y()
+    };
+    norm(v.cross(a))
+}
+
+// ── Vector helpers ([f64; 3]) ────────────────────────────────────────────────
+
+#[inline]
+fn vsub(a: [f64; 3], b: [f64; 3]) -> [f64; 3] {
+    [a[0] - b[0], a[1] - b[1], a[2] - b[2]]
+}
+#[inline]
+fn vadd(a: [f64; 3], b: [f64; 3]) -> [f64; 3] {
+    [a[0] + b[0], a[1] + b[1], a[2] + b[2]]
+}
+#[inline]
+fn vscale(a: [f64; 3], s: f64) -> [f64; 3] {
+    [a[0] * s, a[1] * s, a[2] * s]
+}
+#[inline]
+fn vdot(a: [f64; 3], b: [f64; 3]) -> f64 {
+    a[0] * b[0] + a[1] * b[1] + a[2] * b[2]
+}
+#[inline]
+fn vcross(a: [f64; 3], b: [f64; 3]) -> [f64; 3] {
+    [
+        a[1] * b[2] - a[2] * b[1],
+        a[2] * b[0] - a[0] * b[2],
+        a[0] * b[1] - a[1] * b[0],
+    ]
+}
+#[inline]
+fn vlen(a: [f64; 3]) -> f64 {
+    vdot(a, a).sqrt()
+}
+#[inline]
+fn vnorm(a: [f64; 3]) -> [f64; 3] {
+    let l = vlen(a);
+    if l < 1e-12 {
+        [0.0, 0.0, 1.0]
+    } else {
+        [a[0] / l, a[1] / l, a[2] / l]
+    }
+}

src/scene/mod.rs

@@ -25,6 +25,7 @@ pub(super) struct EntityIndex {
 }
 pub(crate) mod render;
 mod selection;
+pub mod acis_to_truck;
 pub mod solid3d_tess;
 pub mod spline_tess;
 pub mod tess_util;

src/scene/solid3d_tess.rs

@@ -132,6 +132,31 @@ fn tessellate_sat(
     })
 }
 
+/// Tessellate an ACIS document, preferring the truck B-rep kernel and falling
+/// back to the bespoke per-surface sampler when truck can't rebuild the shell
+/// (e.g. an unhandled surface type).
+fn tessellate_acis(
+    sat: &SatDocument,
+    name: String,
+    color: [f32; 4],
+    facet_res: f64,
+) -> Option<MeshLodSet> {
+    if let Some(set) = crate::scene::acis_to_truck::tessellate_sat_truck(sat, name.clone(), color, facet_res) {
+        if std::env::var_os("OCS_TESS_DEBUG").is_some() {
+            let tris = set.lods.first().map(|m| m.indices.len() / 3).unwrap_or(0);
+            eprintln!("acis_tess[{name}]: truck ({tris} tris)");
+        }
+        return Some(set);
+    }
+    // truck couldn't rebuild the shell — fall back to the bespoke sampler.
+    let fallback = tessellate_sat_lods(sat, name.clone(), color, facet_res);
+    eprintln!(
+        "acis_tess[{name}]: manual fallback ({})",
+        if fallback.is_some() { "ok" } else { "empty" }
+    );
+    fallback
+}
+
 /// Tessellate a SAT document at all three LODs and bundle them into a
 /// `MeshLodSet` ready for the render pipeline to pick a level per frame.
 fn tessellate_sat_lods(
@@ -164,7 +189,7 @@ fn tessellate_sat_lods(
 /// geometry in body-local space and records the placement in a `transform`
 /// record (`<3×3> <tx ty tz> <scale> rotate reflect shear`). `None` when the
 /// document has no transform (treated as identity).
-fn body_transform(sat: &SatDocument) -> Option<([f64; 9], [f64; 3], f64)> {
+pub(crate) fn body_transform(sat: &SatDocument) -> Option<([f64; 9], [f64; 3], f64)> {
     let t = sat.records.iter().find(|r| r.entity_type == "transform")?;
     let mut m = [0.0f64; 9];
     for (i, slot) in m.iter_mut().enumerate() {
@@ -178,7 +203,7 @@ fn body_transform(sat: &SatDocument) -> Option<([f64; 9], [f64; 3], f64)> {
 /// Apply a body placement transform to a mesh. ACIS treats points as row
 /// vectors (`p' = scale·(p·M) + T`), so the 3×3 is indexed transposed relative
 /// to a column-vector multiply. Normals get the rotation only, renormalized.
-fn apply_body_transform(mesh: &mut MeshModel, m: &[f64; 9], tr: &[f64; 3], scale: f64) {
+pub(crate) fn apply_body_transform(mesh: &mut MeshModel, m: &[f64; 9], tr: &[f64; 3], scale: f64) {
     for v in &mut mesh.verts {
         let (x, y, z) = (v[0] as f64, v[1] as f64, v[2] as f64);
         let wx = scale * (x * m[0] + y * m[3] + z * m[6]) + tr[0];
@@ -212,7 +237,7 @@ fn scale_lod(base: LodConfig, facet_res: f64) -> LodConfig {
 
 /// World-XY AABB of the mesh — used by the render-pipeline LOD selector
 /// to pick a level based on projected pixel diagonal.
-fn mesh_aabb(mesh: &MeshModel) -> [f32; 4] {
+pub(crate) fn mesh_aabb(mesh: &MeshModel) -> [f32; 4] {
     let mut min_x = f32::INFINITY;
     let mut min_y = f32::INFINITY;
     let mut max_x = f32::NEG_INFINITY;
@@ -251,7 +276,7 @@ pub fn tessellate_region(region: &Region, color: [f32; 4], facet_res: f64) -> Op
         &region.acis_data.sab_data,
     )?;
     let name = region.common.handle.value().to_string();
-    tessellate_sat_lods(&sat, name, color, facet_res)
+    tessellate_acis(&sat, name, color, facet_res)
 }
 
 /// Tessellate a `Body` entity (3D ACIS body) at all three LOD levels.
@@ -262,7 +287,7 @@ pub fn tessellate_body(body: &Body, color: [f32; 4], facet_res: f64) -> Option<M
         &body.acis_data.sab_data,
     )?;
     let name = body.common.handle.value().to_string();
-    tessellate_sat_lods(&sat, name, color, facet_res)
+    tessellate_acis(&sat, name, color, facet_res)
 }
 
 /// Tessellate a `Surface` entity (ACAD_SURFACE family) at all three LOD
@@ -280,7 +305,7 @@ pub fn tessellate_surface(
         &surface.acis_data.sab_data,
     )?;
     let name = surface.common.handle.value().to_string();
-    tessellate_sat_lods(&sat, name, color, facet_res)
+    tessellate_acis(&sat, name, color, facet_res)
 }
 
 /// Tessellate a `Solid3D` entity at all three LOD levels.
@@ -295,7 +320,7 @@ pub fn tessellate_solid3d(solid: &Solid3D, color: [f32; 4], facet_res: f64) -> O
         &solid.acis_data.sab_data,
     )?;
     let name = solid.common.handle.value().to_string();
-    tessellate_sat_lods(&sat, name, color, facet_res)
+    tessellate_acis(&sat, name, color, facet_res)
 }
 
 // ── Topology helpers ──────────────────────────────────────────────────────────
@@ -310,7 +335,7 @@ pub fn tessellate_solid3d(solid: &Solid3D, color: [f32; 4], facet_res: f64) -> O
 ///
 /// Returns an empty `Vec` when the loop topology is broken or has fewer than
 /// three distinct points.
-fn collect_face_polygon(sat: &SatDocument, face: &SatFace, circ_segs: usize) -> Vec<[f64; 3]> {
+pub(crate) fn collect_face_polygon(sat: &SatDocument, face: &SatFace, circ_segs: usize) -> Vec<[f64; 3]> {
     let loop_ptr = face.first_loop();
     let Some(loop_rec) = sat.resolve(loop_ptr) else {
         return vec![];
@@ -355,7 +380,7 @@ fn collect_face_polygon(sat: &SatDocument, face: &SatFace, circ_segs: usize) ->
 /// sampled along their parametric arc (excluding the end param so the next
 /// coedge's start point provides the junction); all other curve types fall
 /// back to the single start vertex, respecting coedge sense.
-fn append_coedge_points(
+pub(crate) fn append_coedge_points(
     sat: &SatDocument,
     coedge: &SatCoedge,
     circ_segs: usize,
@@ -442,7 +467,7 @@ fn sample_ellipse_arc(
 }
 
 /// Resolve a vertex pointer all the way to its `[x, y, z]` coordinate.
-fn resolve_point(sat: &SatDocument, v_ptr: SatPointer) -> Option<[f64; 3]> {
+pub(crate) fn resolve_point(sat: &SatDocument, v_ptr: SatPointer) -> Option<[f64; 3]> {
     let v_rec = sat.resolve(v_ptr)?;
     let vertex = SatVertex::from_record(v_rec)?;
     let pt_rec = sat.resolve(vertex.point())?;
@@ -702,7 +727,7 @@ fn angular_range(
 /// projects their centres onto the axis to get rim heights. For a true cone
 /// with a single rim, the tip is added analytically (the height where the
 /// radius reaches zero). Returns `None` when no coaxial rim is found.
-fn cone_axis_span(
+pub(crate) fn cone_axis_span(
     sat: &SatDocument,
     cone: &SatConeSurface,
     axis: [f64; 3],

src/shaders/mesh.wgsl

@@ -49,8 +49,6 @@ fn vs_main(v: VertexIn) -> VertexOut {
 
 @fragment
 fn fs_main(in: VertexOut) -> @location(0) vec4<f32> {
-    // Fixed directional light (world space, tilted toward the viewer).
-    let light_dir = normalize(vec3<f32>(0.4, 0.8, 0.6));
     var n: vec3<f32>;
     if (u.flat_shade > 0.5) {
         // Per-triangle face normal: derivatives of the interpolated
@@ -60,8 +58,19 @@ fn fs_main(in: VertexOut) -> @location(0) vec4<f32> {
     } else {
         n = normalize(in.normal);
     }
-    // Half-Lambert: [0.5, 1.0] so the dark side isn't fully black.
-    let diff = clamp(dot(n, light_dir), 0.0, 1.0) * 0.5 + 0.5;
-    let rgb  = in.color.rgb * diff;
+
+    // Three-point-ish lighting (world space) plus ambient. Spread directions
+    // keep every face — and the back faces seen through an open surface — lit
+    // from at least one source, so the model never reads as a flat dark mass.
+    // `abs(dot)` makes each light two-sided (independent of normal direction).
+    let l0 = normalize(vec3<f32>( 0.5,  0.8,  0.6)); // key (upper front)
+    let l1 = normalize(vec3<f32>(-0.7,  0.3,  0.4)); // fill (left)
+    let l2 = normalize(vec3<f32>( 0.2, -0.6, -0.8)); // back/under
+    let ambient = 0.35;
+    let diff = ambient
+        + 0.45 * abs(dot(n, l0))
+        + 0.30 * abs(dot(n, l1))
+        + 0.25 * abs(dot(n, l2));
+    let rgb = in.color.rgb * clamp(diff, 0.0, 1.0);
     return vec4<f32>(rgb, in.color.a);
 }