mesh: projection and relaxation keep to a merged face's B-rep pieces

A merged face on several surfaces (a sliver absorbed into its neighbour)
had no exact surface, so the projection target fell back to its
triangles: flat curvature probes gave fewer divisions round the boss and
the enclosure base's symmetric mesh dropped from 0.70 to 0.51 once its
absorbs reached the cells. The target now projects a point of such a
face onto the exact surface of the B-rep piece it is on, trimmed to that
piece (Tessellation::with_pieces, Shape::tessellate_pieces); node
relaxation slides on it the same way.

Test from commands: the enclosure base's four slivers absorbed, meshed
whole and by symmetry, the absorbs kept in every cell.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/cmd/hex.rs

@@ -42,6 +42,9 @@ pub(super) struct Input<'a> {
     /// The design's edge seeds ([`super::seeds::edge_seeds`]); the mesher applies those on its
     /// edges.
     pub(super) seeds: Vec<fieldcraft_mesh::seeds::EdgeSeed>,
+    /// Per triangle of `mesh`, the B-rep face it lies on: its piece of a merged face (empty for
+    /// a surface that is not a body's).
+    pub(super) pieces: Vec<u32>,
 }
 
 fn analytic(s: FaceSurface) -> Surface {
@@ -73,7 +76,8 @@ pub(super) fn shape_input(shape: &Shape, tolerance: Option<f64>) -> Result<Input
     }
     let surfaces = shape.face_surfaces()?.into_iter().map(|f| f.map(analytic)).collect();
     let face_names = shape.faces().iter().map(|f| f.name.clone()).collect();
-    Ok(Input { name: shape.name.clone(), mesh: Cow::Owned(shape.tessellate(tol)?), surfaces, face_names, diagonal: diag, seeds: Vec::new() })
+    let (mesh, pieces) = shape.tessellate_pieces(tol)?;
+    Ok(Input { name: shape.name.clone(), mesh: Cow::Owned(mesh), surfaces, face_names, diagonal: diag, seeds: Vec::new(), pieces })
 }
 
 /// The imported surface `surface` names, with the body name its mesh is tagged with.
@@ -109,7 +113,15 @@ pub(super) fn inputs<'a>(s: &'a Session, p: &Value) -> Result<Vec<Input<'a>>> {
             }
             // Faces named by index, as mesh.tet.generate names them.
             let face_names = (0..t.faces).map(|f| f.to_string()).collect();
-            Ok(vec![Input { name, mesh: Cow::Borrowed(&t.mesh), surfaces: Vec::new(), face_names, diagonal: diagonal(lo, hi), seeds: Vec::new() }])
+            Ok(vec![Input {
+                name,
+                mesh: Cow::Borrowed(&t.mesh),
+                surfaces: Vec::new(),
+                face_names,
+                diagonal: diagonal(lo, hi),
+                seeds: Vec::new(),
+                pieces: Vec::new(),
+            }])
         }
         (None, None) => {
             let design = s.design.as_deref().ok_or_else(|| EngineError::Params("open a design first, or give `surface`".into()))?;

crates/engine/src/cmd/mesh.rs

@@ -216,21 +216,30 @@ pub(super) fn quality_options(p: &Value, max_tets: u64) -> Result<(QualityOption
 /// The exact surfaces of a body's faces, for the quality stage's projections (none when the
 /// kernel cannot give them: the stage then keeps to the tessellation).
 pub fn analytic_surfaces(shape: &fieldcraft_geometry::Shape) -> Vec<Option<AnalyticSurface>> {
+    shape.face_surfaces().unwrap_or_default().into_iter().map(|f| f.map(analytic_surface)).collect()
+}
+
+fn analytic_surface(f: fieldcraft_geometry::FaceSurface) -> AnalyticSurface {
     use fieldcraft_geometry::FaceSurface as F;
-    shape
-        .face_surfaces()
-        .unwrap_or_default()
-        .into_iter()
-        .map(|f| {
-            f.map(|f| match f {
-                F::Plane { normal, point } => AnalyticSurface::Plane { normal, point },
-                F::Cylinder { origin, axis, radius } => AnalyticSurface::Cylinder { origin, axis, radius },
-                F::Cone { apex, axis, half_angle } => AnalyticSurface::Cone { apex, axis, half_angle },
-                F::Sphere { center, radius } => AnalyticSurface::Sphere { center, radius },
-                F::Torus { center, axis, major, minor } => AnalyticSurface::Torus { center, axis, major, minor },
-            })
-        })
-        .collect()
+    match f {
+        F::Plane { normal, point } => AnalyticSurface::Plane { normal, point },
+        F::Cylinder { origin, axis, radius } => AnalyticSurface::Cylinder { origin, axis, radius },
+        F::Cone { apex, axis, half_angle } => AnalyticSurface::Cone { apex, axis, half_angle },
+        F::Sphere { center, radius } => AnalyticSurface::Sphere { center, radius },
+        F::Torus { center, axis, major, minor } => AnalyticSurface::Torus { center, axis, major, minor },
+    }
+}
+
+/// The B-rep pieces of `shape`'s merged faces (virtual topology) for a projection target built
+/// on its tessellation whose triangles lie on the B-rep faces `tri_piece`
+/// ([`fieldcraft_geometry::Shape::tessellate_pieces`]): a merged face on no one surface then
+/// projects onto each piece's own; none when no face is merged.
+pub(crate) fn pieces(shape: &fieldcraft_geometry::Shape, tri_piece: &[u32]) -> fieldcraft_mesh::tet::geometry::Pieces {
+    if !shape.has_merged_faces() {
+        return fieldcraft_mesh::tet::geometry::Pieces::default();
+    }
+    let analytic = shape.brep_face_surfaces().unwrap_or_default().into_iter().map(|f| f.map(analytic_surface)).collect();
+    fieldcraft_mesh::tet::geometry::Pieces { tri_piece: tri_piece.to_vec(), analytic }
 }
 
 pub(super) fn quality_json(q: &TetQuality) -> Value {

crates/engine/src/cmd/plan.rs

@@ -2405,6 +2405,42 @@ mod acceptance {
         accept(&mut s, &r, 0.5, 10.0);
     }
 
+    /// The enclosure base's bosses cut a 0.42 mm strip out of the middle of the inner fillet
+    /// (#511, #513): geometry.check finds the slivers and geometry.face.absorb absorbs them.
+    /// The absorbed faces stay absorbed in the cells the plan's symmetry cuts the body into, and
+    /// the projection keeps to the exact surfaces of a merged face's pieces, so the mesh is no
+    /// worse for them, whole or by symmetry.
+    #[test]
+    fn absorbed_slivers_stay_absorbed_in_the_plan_s_cells() {
+        let mut commands = vec![
+            json!({"command": "solid.box", "params": {"length": 56, "width": 42, "height": 16, "body_name": "Base"}}),
+            json!({"command": "solid.fillet", "params": {"edges": [[0, 0, 8], [56, 0, 8], [56, 42, 8], [0, 42, 8]], "radius": 5}}),
+            json!({"command": "solid.shell", "params": {"faces": [[28, 21, 16]], "thickness": 3, "body": "Base"}}),
+        ];
+        for (x, y) in [(-1.0, -1.0), (57.0, -1.0), (-1.0, 43.0), (57.0, 43.0)] {
+            commands.push(json!({"command": "solid.cylinder", "params": {"diameter": 13, "height": 16, "base": [x, y, 0], "operation": "join", "targets": ["Base"]}}));
+        }
+        let (_d, mut s) = open(json!(commands));
+        let check = s.execute("geometry.check", &json!({"body": "Base", "size": 1.5, "mark": true})).unwrap();
+        assert_eq!(check["counts"]["sliver_face"], 4, "{check}");
+        let absorbed = s.execute("geometry.face.absorb", &json!({"from_check": true})).unwrap();
+        assert_eq!(absorbed["absorbed"].as_array().map(Vec::len), Some(4), "{absorbed}");
+        let plan = s.execute("mesh.plan", &json!({"body": "Base", "size": 1.5, "layers": 2})).unwrap();
+        assert_eq!(plan["mesher"], "project", "{plan}");
+        let r = s.execute("mesh.plan.apply", &json!({"plan": plan["name"], "symmetry": false})).unwrap();
+        accept(&mut s, &r, 0.6, 10.0);
+        let r = s.execute("mesh.plan.apply", &json!({"plan": plan["name"]})).unwrap();
+        assert_eq!(r["symmetry"].as_array().map(Vec::len), Some(2), "{r}");
+        let design = s.design.as_deref().unwrap();
+        assert!(design.errors.is_empty(), "{:?}", design.errors);
+        // Each quarter holds one corner, its sliver absorbed.
+        let cells: Vec<_> = design.bodies.iter().filter(|b| b.name.starts_with("Base/")).collect();
+        assert_eq!(cells.len(), 4);
+        assert!(cells.iter().all(|c| c.merged_faces().len() == 1), "{:?}", cells.iter().map(|c| c.merged_faces()).collect::<Vec<_>>());
+        assert_eq!(design.source_body("Base").map(|b| b.merged_faces().len()), Some(4));
+        accept(&mut s, &r, 0.65, 10.0);
+    }
+
     /// The crank arm sample, whole (#455, #190): its sides are at an angle to the axes and its
     /// root arcs meet the bosses at a kink, so it takes the ring-and-pillow layout: each boss an
     /// annulus from its bore or pedal hole to its circle through every level, the arm a carved

crates/engine/src/cmd/project.rs

@@ -286,7 +286,7 @@ pub(crate) fn open_body(s: &Session, name: &str, tolerance: Option<f64>, raw: &V
     let surface =
         BoundarySurface::new(&input.mesh.positions, &input.mesh.triangles, &input.mesh.tri_face, &super::mesh::surface_options(raw, &input.mesh)?)
             .map_err(|e| EngineError::Params(format!("body `{name}`: {e}")))?;
-    let target = Tessellation::new(&surface, super::mesh::analytic_surfaces(shape));
+    let target = Tessellation::new(&surface, super::mesh::analytic_surfaces(shape)).with_pieces(&surface, super::mesh::pieces(shape, &input.pieces));
     let m = shape.measure()?;
     Ok(Body { input, surface, target, lo: m.bbox_min, hi: m.bbox_max })
 }

crates/engine/src/cmd/relax.rs

@@ -194,7 +194,7 @@ fn design_geometry(s: &Session, nodes: &BTreeSet<u32>) -> Result<DesignGeometry>
             None => None,
             Some(shape) => {
                 let tol = (shape.size()? * 1e-3).max(1e-6);
-                let mesh = shape.tessellate(tol)?;
+                let (mesh, pieces) = shape.tessellate_pieces(tol)?;
                 let surface = BoundarySurface::new(
                     &mesh.positions,
                     &mesh.triangles,
@@ -202,7 +202,7 @@ fn design_geometry(s: &Session, nodes: &BTreeSet<u32>) -> Result<DesignGeometry>
                     &SurfaceOptions { weld_distance: mesh.weld_distance, ..SurfaceOptions::default() },
                 )?;
                 let analytic = super::mesh::analytic_surfaces(shape);
-                g.bodies.push(Tessellation::new(&surface, analytic.clone()));
+                g.bodies.push(Tessellation::new(&surface, analytic.clone()).with_pieces(&surface, super::mesh::pieces(shape, &pieces)));
                 Some((g.bodies.len() - 1, analytic))
             }
         };

crates/geometry/src/lib.rs

@@ -206,8 +206,8 @@ impl Shape {
         self.virtual_surfaces(brep)
     }
 
-    /// [`Shape::face_surfaces`] by B-rep face index.
-    pub(crate) fn brep_face_surfaces(&self) -> Result<Vec<Option<FaceSurface>>> {
+    /// [`Shape::face_surfaces`] by B-rep face index (the pieces of [`Shape::tessellate_pieces`]).
+    pub fn brep_face_surfaces(&self) -> Result<Vec<Option<FaceSurface>>> {
         guard("face surfaces", || {
             Ok((0..self.body.face_count())
                 .map(|f| {
@@ -257,18 +257,37 @@ impl Shape {
     /// by face index ([`Shape::faces`]): merged faces are one face, without the edges between
     /// their B-rep faces.
     pub fn tessellate(&self, tolerance: f64) -> Result<SurfaceMesh> {
+        Ok(self.virtual_mesh(self.tessellate_brep(tolerance)?))
+    }
+
+    /// [`Shape::tessellate`], and per triangle the B-rep face it lies on: its piece of a merged
+    /// face, whose surface is in [`Shape::brep_face_surfaces`] (the same as its tag when no face
+    /// is merged).
+    pub fn tessellate_pieces(&self, tolerance: f64) -> Result<(SurfaceMesh, Vec<u32>)> {
+        let brep = self.tessellate_brep(tolerance)?;
+        let pieces = brep.tri_face.clone();
+        Ok((self.virtual_mesh(brep), pieces))
+    }
+
+    /// The B-rep's own tessellation, tagged by B-rep face.
+    fn tessellate_brep(&self, tolerance: f64) -> Result<SurfaceMesh> {
         if !(tolerance > 0.0 && tolerance.is_finite()) {
             return Err(Error::Invalid(format!("tessellation tolerance must be a positive number, got {tolerance}")));
         }
         let m = guard("tessellate", || self.body.tessellate(tolerance))?;
-        Ok(self.virtual_mesh(SurfaceMesh {
+        Ok(SurfaceMesh {
             positions: m.positions.into_iter().map(xyz).collect(),
             triangles: m.triangles,
             tri_face: m.tri_face,
             weld_distance: 2.0 * LINEAR_TOLERANCE,
             edges: m.edges.into_iter().map(|e| e.into_iter().map(xyz).collect()).collect(),
             edge_faces: m.edge_faces,
-        }))
+        })
+    }
+
+    /// Whether some face is made of several B-rep faces ([`virtual_faces`]).
+    pub fn has_merged_faces(&self) -> bool {
+        !self.face_map.is_empty()
     }
 }
 

crates/mesh/src/tet/geometry.rs

@@ -429,6 +429,20 @@ pub struct Tessellation {
     /// A tree over each face pair's segments.
     pair_trees: std::collections::BTreeMap<[u32; 2], Tree>,
     analytic: Vec<Option<AnalyticSurface>>,
+    /// Faces made of several B-rep faces (virtual topology): each triangle's B-rep face (its
+    /// piece) and each piece's exact surface, for a face with no one surface of its own. Empty
+    /// when no face is merged.
+    tri_piece: Vec<u32>,
+    piece_analytic: Vec<Option<AnalyticSurface>>,
+}
+
+/// The B-rep faces of merged faces: per triangle of the surface a [`Tessellation`] is built on
+/// (as given to [`BoundarySurface::new`]) the B-rep face it lies on, and each B-rep face's exact
+/// surface when known.
+#[derive(Debug, Clone, Default)]
+pub struct Pieces {
+    pub tri_piece: Vec<u32>,
+    pub analytic: Vec<Option<AnalyticSurface>>,
 }
 
 impl Tessellation {
@@ -480,7 +494,31 @@ impl Tessellation {
             by_pair.entry(*pair).or_default().push(i as u32);
         }
         let pair_trees = by_pair.into_iter().map(|(pair, items)| (pair, Tree::build_over(&seg_boxes, items))).collect();
-        Tessellation { points, triangles, tri_face, tri_tree, face_trees, segments, seg_tree, pair_trees, analytic }
+        Tessellation {
+            points,
+            triangles,
+            tri_face,
+            tri_tree,
+            face_trees,
+            segments,
+            seg_tree,
+            pair_trees,
+            analytic,
+            tri_piece: Vec::new(),
+            piece_analytic: Vec::new(),
+        }
+    }
+
+    /// The same target with the B-rep pieces of its merged faces (`surface` the boundary
+    /// surface it was built on): a point on a merged face that lies on no one surface goes onto
+    /// the exact surface of the piece it is on, trimmed to that piece, instead of its triangles.
+    pub fn with_pieces(mut self, surface: &BoundarySurface, pieces: Pieces) -> Tessellation {
+        if pieces.tri_piece.is_empty() {
+            return self;
+        }
+        self.tri_piece = surface.source_triangle().iter().map(|&t| pieces.tri_piece.get(t as usize).copied().unwrap_or(u32::MAX)).collect();
+        self.piece_analytic = pieces.analytic;
+        self
     }
 
     fn pt(&self, v: u32) -> P {
@@ -492,19 +530,42 @@ impl Tessellation {
         self.analytic.get(face as usize).and_then(|a| a.as_ref())
     }
 
-    /// The closest point on the face's triangles, and the longest edge of the triangle it is
-    /// on (the local scale of the tessellation).
-    fn on_tessellation(&self, face: u32, p: P) -> Option<(P, f64)> {
+    /// The exact surface of a point of face `face` on its triangle `t`: the face's own, else
+    /// that of the B-rep piece of a merged face the triangle lies on.
+    fn exact(&self, face: u32, t: u32) -> Option<&AnalyticSurface> {
+        self.analytic(face).or_else(|| {
+            let piece = *self.tri_piece.get(t as usize)?;
+            self.piece_analytic.get(piece as usize).and_then(|a| a.as_ref())
+        })
+    }
+
+    /// Whether triangle `u` lies on the same piece of face `face` as triangle `t` (always, for
+    /// a face with a surface of its own or without pieces).
+    fn same_piece(&self, face: u32, t: u32, u: u32) -> bool {
+        self.analytic(face).is_some() || self.tri_piece.get(t as usize) == self.tri_piece.get(u as usize)
+    }
+
+    /// The nearest triangle of face `face` to `p` and the closest point on it.
+    fn nearest_triangle(&self, face: u32, p: P) -> Option<(P, u32)> {
         let accept = |t: u32| self.tri_face.get(t as usize) == Some(&face);
         let dist = |t: u32| {
             let tri = self.triangles.get(t as usize).copied().unwrap_or([0; 3]);
-            let (a, b, c) = (self.pt(tri[0]), self.pt(tri[1]), self.pt(tri[2]));
-            let q = closest_on_triangle(p, a, b, c);
+            let q = closest_on_triangle(p, self.pt(tri[0]), self.pt(tri[1]), self.pt(tri[2]));
             let d = sub(q, p);
-            let longest = measure::dist(a, b).max(measure::dist(b, c)).max(measure::dist(c, a));
-            (dot(d, d), [q[0], q[1], q[2], longest])
+            // The triangle index travels through the f64 exactly (counts are capped far below
+            // 2^53).
+            (dot(d, d), [q[0], q[1], q[2], f64::from(t)])
         };
-        self.face_tree(face).nearest4(p, &accept, &dist).map(|r| ([r[0], r[1], r[2]], r[3]))
+        self.face_tree(face).nearest4(p, &accept, &dist).map(|r| ([r[0], r[1], r[2]], r[3] as u32))
+    }
+
+    /// The closest point on the face's triangles, the longest edge of the triangle it is on
+    /// (the local scale of the tessellation) and that triangle.
+    fn on_tessellation(&self, face: u32, p: P) -> Option<(P, f64, u32)> {
+        let (q, t) = self.nearest_triangle(face, p)?;
+        let tri = self.triangles.get(t as usize).copied().unwrap_or([0; 3]);
+        let (a, b, c) = (self.pt(tri[0]), self.pt(tri[1]), self.pt(tri[2]));
+        Some((q, measure::dist(a, b).max(measure::dist(b, c)).max(measure::dist(c, a)), t))
     }
 
     /// The B-rep face whose triangles lie nearest to `p`.
@@ -552,24 +613,17 @@ impl Tessellation {
     /// closest point of the face's triangles. Unlike [`BoundaryGeometry::project_face`] it never
     /// returns a point of the surface's extension beyond the face's boundary, whatever the
     /// distance from `p` — nor of the same surface's other faces: the pieces of one fillet split
-    /// by a cut share their cylinder, and a point on one piece is off the others.
+    /// by a cut share their cylinder, and a point on one piece is off the others. On a merged
+    /// face lying on no one surface, the surface is that of the piece its nearest triangle is
+    /// on, trimmed to that piece.
     pub fn project_trimmed(&self, face: u32, p: P) -> Option<P> {
-        let accept = |t: u32| self.tri_face.get(t as usize) == Some(&face);
         let corners = |t: u32| {
             let tri = self.triangles.get(t as usize).copied().unwrap_or([0; 3]);
             (self.pt(tri[0]), self.pt(tri[1]), self.pt(tri[2]))
         };
-        let near = {
-            let dist = |t: u32| {
-                let (a, b, c) = corners(t);
-                let q = closest_on_triangle(p, a, b, c);
-                let d = sub(q, p);
-                (dot(d, d), [q[0], q[1], q[2], 0.0])
-            };
-            let r = self.face_tree(face).nearest4(p, &accept, &dist)?;
-            [r[0], r[1], r[2]]
-        };
-        let Some(surface) = self.analytic(face) else { return Some(near) };
+        let (near, nearest) = self.nearest_triangle(face, p)?;
+        let accept = |t: u32| self.tri_face.get(t as usize) == Some(&face) && self.same_piece(face, nearest, t);
+        let Some(surface) = self.exact(face, nearest) else { return Some(near) };
         let Some(q) = surface.project(p) else { return Some(near) };
         // The nearest triangle to the surface's point, with its slack: the largest gap between
         // the triangle and the surface (at its edges' midpoints and centre: a chord's deepest
@@ -589,19 +643,14 @@ impl Tessellation {
     }
 
     /// The outward unit normal of B-rep face `face` near `p`: the exact surface's (oriented like
-    /// the nearest triangle) when known, else the nearest triangle's.
+    /// the nearest triangle; on a merged face, its piece's) when known, else the nearest
+    /// triangle's.
     pub fn face_normal(&self, face: u32, p: P) -> Option<P> {
-        let accept = |t: u32| self.tri_face.get(t as usize) == Some(&face);
-        let dist = |t: u32| {
-            let tri = self.triangles.get(t as usize).copied().unwrap_or([0; 3]);
-            let (a, b, c) = (self.pt(tri[0]), self.pt(tri[1]), self.pt(tri[2]));
-            let d = sub(closest_on_triangle(p, a, b, c), p);
-            let n = unit(cross(sub(b, a), sub(c, a))).unwrap_or([0.0; 3]);
-            (dot(d, d), [n[0], n[1], n[2], 0.0])
-        };
-        let r = self.face_tree(face).nearest4(p, &accept, &dist)?;
-        let flat = unit([r[0], r[1], r[2]])?;
-        match self.analytic(face).and_then(|s| s.normal_at(p)) {
+        let (_, t) = self.nearest_triangle(face, p)?;
+        let tri = self.triangles.get(t as usize).copied().unwrap_or([0; 3]);
+        let (a, b, c) = (self.pt(tri[0]), self.pt(tri[1]), self.pt(tri[2]));
+        let flat = unit(cross(sub(b, a), sub(c, a)))?;
+        match self.exact(face, t).and_then(|s| s.normal_at(p)) {
             Some(n) if dot(n, flat) < 0.0 => Some(scale(n, -1.0)),
             Some(n) => Some(n),
             None => Some(flat),
@@ -611,8 +660,8 @@ impl Tessellation {
 
 impl BoundaryGeometry for Tessellation {
     fn project_face(&self, face: u32, p: P) -> Option<P> {
-        let (near, scale_len) = self.on_tessellation(face, p)?;
-        match self.analytic(face) {
+        let (near, scale_len, t) = self.on_tessellation(face, p)?;
+        match self.exact(face, t) {
             // The exact surface's closest point, kept when it is near the tessellation's: the
             // tessellation deviates from its surface by far less than half its triangles' size,
             // so a point further away is a projection onto another part of the surface (the far
@@ -664,8 +713,8 @@ impl Tessellation {
     /// tessellation's chordal tolerance, as [`BoundaryGeometry::project_face`]).
     pub fn slide_face(&self, face: u32, p: P, d: P) -> Option<P> {
         let target = add(p, d);
-        let (near, scale_len) = self.on_tessellation(face, target)?;
-        if let Some(q) = self.analytic(face).and_then(|s| s.slide(p, d)) {
+        let (near, scale_len, t) = self.on_tessellation(face, target)?;
+        if let Some(q) = self.exact(face, t).and_then(|s| s.slide(p, d)) {
             let tol = (0.5 * scale_len).max(2.0 * measure::dist(target, near));
             if measure::dist(q, near) <= tol {
                 return Some(q);