Faces split on purpose stay split: the body remembers its Split Face lines, export and healing keep the pieces apart

Split Face records the new edges as polylines on the body (Body::split_lines); the document carries them through later features. STEP export no longer joins pieces of one surface across such a line, and healing (fillets, chamfers, booleans, shell, draft) no longer merges coplanar pieces there, while incidental pieces (boolean leftovers, seam halves) still merge as before. Tests: plane and cylinder splits keep their face count through a STEP round trip (and join without the record), a second split keeps both lines, and an engine test exports a split box after a fillet with 8 faces.

Fixes #11

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/doc/src/eval.rs

@@ -283,6 +283,12 @@ impl ModelState {
                     (key, &b.name).hash(&mut h);
                     b.key = h.finish();
                     b.names = crate::naming::NameCell::new(Some(origin.clone()));
+                    // Faces split on purpose stay split through later features (for export).
+                    if b.body.split_lines().is_none()
+                        && let Some(lines) = before.bodies.iter().find(|x| x.name == b.name).and_then(|x| x.body.split_lines())
+                    {
+                        b.body = b.body.clone().with_split_lines(Some(lines.clone()));
+                    }
                     // An imported look outlives the operations on the body.
                     if b.body.paint().is_none()
                         && b.body.color().is_none()

crates/engine/src/split_face_tests.rs

@@ -62,3 +62,21 @@ fn a_tool_that_misses_is_refused() {
     assert!(s.execute("solid.split_face", &json!({"faces": [[10, 15, 20]], "plane": {"origin": [100, 0, 0], "normal": [1, 0, 0]}})).is_err());
     assert_eq!(s.doc.features.len(), n);
 }
+
+#[test]
+fn export_keeps_split_faces_through_later_features() {
+    let mut s = boxed();
+    run(&mut s, "solid.split_face", json!({"faces": [[10, 15, 20]], "plane": {"origin": [20, 0, 0], "normal": [1, 0, 0]}}));
+    // A later feature rebuilds the body; the split stays a split in the file.
+    run(&mut s, "solid.fillet", json!({"edges": [[20, 0, 0]], "radius": 2}));
+    let (f, _) = faces_volume(&mut s, "B");
+    assert_eq!(f, 8);
+    let dir = std::env::temp_dir().join(format!("solvecraft-split-{}", std::process::id()));
+    std::fs::create_dir_all(&dir).unwrap();
+    let step = dir.join("split.step");
+    run(&mut s, "file.export", json!({"path": step.to_string_lossy()}));
+    let text = std::fs::read_to_string(&step).unwrap();
+    let _ = std::fs::remove_dir_all(&dir);
+    // Six box faces, the top's second piece and the fillet.
+    assert_eq!(text.matches("ADVANCED_FACE(").count(), 8, "{text}");
+}

crates/kernel/src/blend.rs

@@ -285,7 +285,7 @@ fn blend(body: &Body, edges: &[Vec3], r: f64, shape: Shape, what: &str) -> Resul
         return Err(KernelError::Invalid("too many edges".into()));
     }
     // Coplanar neighbours as one face (as after a join), so corners look as they should.
-    let mut cur = Body::new(crate::heal::heal(body.deep_copy(), body.size()))?;
+    let mut cur = Body::new(crate::heal::heal_keep(body.deep_copy(), body.size(), body.split_keep()))?;
     // Every edge of a convex planar body: rounded as a whole (sphere corners).
     if shape == Shape::Round && edges.len() >= 6 {
         let solid = cur.deep_copy();
@@ -700,7 +700,7 @@ pub fn chamfer(body: &Body, edges: &[Vec3], distance: f64) -> Result<Body> {
         }
     }
     // Coplanar neighbours as one face (as after a join), so corners look as they should.
-    let mut cur = Body::new(crate::heal::heal(body.deep_copy(), body.size()))?;
+    let mut cur = Body::new(crate::heal::heal_keep(body.deep_copy(), body.size(), body.split_keep()))?;
     if let Some(groups) = loop_groups(&cur, edges)? {
         for g in groups {
             let size = cur.size();

crates/kernel/src/blend_corner.rs

@@ -33,7 +33,7 @@ pub fn fillet_radii(body: &Body, edges: &[(Vec3, f64)]) -> Result<Body> {
     if edges.iter().any(|(p, r)| !(p.is_finite() && r.is_finite() && *r > 1e-6 && *r < 1e6)) {
         return Err(KernelError::Invalid("fillet size must be positive".into()));
     }
-    let mut cur = Body::new(crate::heal::heal(body.deep_copy(), body.size()))?;
+    let mut cur = Body::new(crate::heal::heal_keep(body.deep_copy(), body.size(), body.split_keep()))?;
     let mut sel = edges.to_vec();
     let mut first: Option<KernelError> = None;
     // Earlier rounds at the edges' ends come off and are blended again with the new edges.

crates/kernel/src/body.rs

@@ -19,6 +19,9 @@ pub struct Body {
     pub(crate) color: Option<[f32; 3]>,
     /// Opacity and face colours for export (set by the document; operations drop it).
     pub(crate) paint: Option<std::sync::Arc<Paint>>,
+    /// Lines the user split faces along (Split Face), as polylines: export keeps the faces on
+    /// either side apart where it would otherwise join pieces of one surface.
+    pub(crate) splits: Option<std::sync::Arc<Vec<Vec<Vec3>>>>,
 }
 
 /// How a body looks beyond its colour, for export: its opacity and its faces' own colours.
@@ -278,6 +281,7 @@ impl Body {
             }
             && self.color == other.color
             && self.paint == other.paint
+            && self.splits == other.splits
     }
 
     pub(crate) fn new(solid: Solid) -> Result<Body> {
@@ -302,7 +306,7 @@ impl Body {
         if v < 0.0 {
             solid.not();
         }
-        Ok(Body { solid: std::sync::Arc::new(solid), mesh: None, color: None, paint: None })
+        Ok(Body { solid: std::sync::Arc::new(solid), mesh: None, color: None, paint: None, splits: None })
     }
 
     /// A deep copy of the solid that can be mutated without affecting other bodies.
@@ -335,6 +339,21 @@ impl Body {
         self
     }
 
+    /// Lines faces were split along on purpose (Split Face), as polylines.
+    pub fn split_lines(&self) -> Option<&std::sync::Arc<Vec<Vec<Vec3>>>> {
+        self.splits.as_ref()
+    }
+
+    pub(crate) fn split_keep(&self) -> &[Vec<Vec3>] {
+        self.splits.as_ref().map(|l| l.as_slice()).unwrap_or(&[])
+    }
+
+    /// The same body with the lines its faces were split along on purpose.
+    pub fn with_split_lines(mut self, s: Option<std::sync::Arc<Vec<Vec<Vec3>>>>) -> Body {
+        self.splits = s.filter(|l| !l.is_empty());
+        self
+    }
+
     /// Opacity and face colours for export, if any.
     pub fn paint(&self) -> Option<&Paint> {
         self.paint.as_deref()

crates/kernel/src/heal.rs

@@ -79,7 +79,26 @@ fn inside(p: Vec3, edges: &[mt::Edge], n: Vec3) -> bool {
 
 /// Merge each group of edge-connected coplanar faces into one face. Returns None when nothing
 /// changed or the result would not be a valid solid.
-fn merge_coplanar(shell: &mt::Shell, tol: f64) -> Option<mt::Shell> {
+/// Do the points lie along one of the lines faces were split along on purpose (polylines;
+/// chords of a curved line sag by under a tenth of their length)?
+pub(crate) fn on_split_line(pts: &[Vec3], keep: &[Vec<Vec3>]) -> bool {
+    !pts.is_empty()
+        && keep.iter().any(|line| {
+            let seg = line.windows(2).filter_map(|w| Some(w.first()?.dist(*w.get(1)?))).fold(0.0, f64::max);
+            let tol = 1e-6 * (1.0 + seg) + if line.len() > 2 { seg * 0.1 } else { 0.0 };
+            pts.iter().all(|q| line.windows(2).any(|w| w.first().zip(w.get(1)).is_some_and(|(a, b)| q.dist_to_segment(*a, *b) <= tol)))
+        })
+}
+
+/// Points along an edge (ends and three inside).
+fn edge_points(e: &mt::Edge) -> Vec<Vec3> {
+    use mt::{BoundedCurve, ParametricCurve};
+    let c = e.curve();
+    let (t0, t1) = c.range_tuple();
+    (0..=4).map(|k| from_p3(c.subs(t0 + (t1 - t0) * k as f64 / 4.0))).collect()
+}
+
+fn merge_coplanar(shell: &mt::Shell, tol: f64, keep: &[Vec<Vec3>]) -> Option<mt::Shell> {
     let faces: Vec<mt::Face> = shell.face_iter().cloned().collect();
     let planes: Vec<Option<(Vec3, f64)>> = faces.iter().map(plane_of).collect();
     let mut parent: Vec<usize> = (0..faces.len()).collect();
@@ -90,9 +109,11 @@ fn merge_coplanar(shell: &mt::Shell, tol: f64) -> Option<mt::Shell> {
         }
     }
     let mut merged_any = false;
-    for fs in by_edge.values() {
+    let edges: HashMap<mt::EdgeID, mt::Edge> = shell.edge_iter().map(|e| (e.id(), e)).collect();
+    for (id, fs) in &by_edge {
         if let [a, b] = fs[..]
             && a != b
+            && (keep.is_empty() || !edges.get(id).is_some_and(|e| on_split_line(&edge_points(e), keep)))
             && let (Some(Some((na, da))), Some(Some((nb, db)))) = (planes.get(a), planes.get(b))
             && na.dot(*nb) > 1.0 - 1e-9
             && (da - db).abs() < tol
@@ -378,6 +399,11 @@ fn planar_surfaces(shell: &mt::Shell, tol: f64) -> Option<mt::Shell> {
 
 /// Heal a solid; returns the input unchanged when nothing applies or healing fails.
 pub fn heal(solid: Solid, size: f64) -> Solid {
+    heal_keep(solid, size, &[])
+}
+
+/// [`heal`], keeping faces apart along lines they were split along on purpose.
+pub(crate) fn heal_keep(solid: Solid, size: f64, keep: &[Vec<Vec3>]) -> Solid {
     let tol = (size * 1e-7).max(1e-9);
     let mut shells = Vec::new();
     let mut changed = false;
@@ -391,7 +417,7 @@ pub fn heal(solid: Solid, size: f64) -> Solid {
             cur = m;
             changed = true;
         }
-        if let Some(m) = merge_coplanar(&cur, tol) {
+        if let Some(m) = merge_coplanar(&cur, tol, keep) {
             cur = m;
             changed = true;
         }

crates/kernel/src/meshbody.rs

@@ -101,7 +101,7 @@ pub fn mesh_body(positions: &[Vec3], triangles: &[[u32; 3]]) -> Result<Body> {
             t.swap(1, 2);
         }
     }
-    Ok(Body { solid: Arc::new(crate::body::Solid::new_unchecked(Vec::new())), mesh: Some(Arc::new(out)), color: None, paint: None })
+    Ok(Body { solid: Arc::new(crate::body::Solid::new_unchecked(Vec::new())), mesh: Some(Arc::new(out)), color: None, paint: None, splits: None })
 }
 
 /// Wind every connected piece one way: a triangle that runs a side in the same direction as
@@ -185,6 +185,6 @@ impl Body {
                 t.swap(1, 2);
             }
         }
-        Some(Body { solid: self.solid.clone(), mesh: Some(Arc::new(out)), color: self.color, paint: self.paint.clone() })
+        Some(Body { solid: self.solid.clone(), mesh: Some(Arc::new(out)), color: self.color, paint: self.paint.clone(), splits: self.splits.clone() })
     }
 }

crates/kernel/src/ops.rs

@@ -241,6 +241,7 @@ pub fn boolean(a: &Body, b: &Body, op: BoolOp) -> Result<Option<Body>> {
         }
     }
     let mut empty_votes = 0;
+    let keep: Vec<Vec<Vec3>> = a.split_keep().iter().chain(b.split_keep()).cloned().collect();
     for (attempt, j) in JITTER.iter().enumerate() {
         let shift = mt::Vector3::new(j[0], j[1], j[2]) * (size * 0.01);
         for k in [2e-3, 5e-4] {
@@ -267,7 +268,7 @@ pub fn boolean(a: &Body, b: &Body, op: BoolOp) -> Result<Option<Body>> {
                     empty_votes += 1;
                     last = "empty result".into();
                 }
-                Ok(s) => match Body::new(crate::heal::heal(s, size)) {
+                Ok(s) => match Body::new(crate::heal::heal_keep(s, size, &keep)) {
                     Ok(body) => {
                         let (ok, v) = check(&body);
                         if ok {

crates/kernel/src/polyhedron.rs

@@ -192,7 +192,7 @@ pub fn draft(b: &Body, faces: &[Vec3], neutral: &Plane, pull: Vec3, angle: f64)
 
 /// Draft of a non-convex body: walls (planes, cylinders) next to caps square to `pull`.
 fn draft_walls(b: &Body, at: &[Vec3], neutral: &Plane, pull: Vec3, angle: f64) -> Result<Body> {
-    let healed = Body::new(crate::heal::heal(b.deep_copy(), b.size()))?;
+    let healed = Body::new(crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep()))?;
     let size = healed.size();
     let mesh = healed.tessellate((size * 1e-3).max(1e-3))?;
     let mut chosen: Vec<usize> = Vec::new();
@@ -338,7 +338,7 @@ pub fn round_all_edges(b: &Body, r: f64) -> Result<Body> {
 /// by the thickness (open faces outward, past the body) keeping the body's topology, which
 /// gives the cavity; the shell is the body minus the cavity.
 fn shell_planar(b: &Body, open: &[Vec3], thickness: f64) -> Result<Body> {
-    let healed = Body::new(crate::heal::heal(b.deep_copy(), b.size()))?;
+    let healed = Body::new(crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep()))?;
     let size = healed.size();
     let mesh = healed.tessellate((size * 1e-3).max(1e-3))?;
     // Faces to open: the face of the triangle nearest each point.
@@ -422,7 +422,7 @@ pub fn offset_faces(b: &Body, at: &[Vec3], distance: f64) -> Result<Body> {
     if !distance.is_finite() || distance.abs() > 1e6 {
         return Err(KernelError::Invalid("offset distance".into()));
     }
-    let healed = Body::new(crate::heal::heal(b.deep_copy(), b.size()))?;
+    let healed = Body::new(crate::heal::heal_keep(b.deep_copy(), b.size(), b.split_keep()))?;
     let size = healed.size();
     let mesh = healed.tessellate((size * 1e-3).max(1e-3))?;
     let mut chosen: Vec<usize> = Vec::new();

crates/kernel/src/splitface.rs

@@ -845,6 +845,16 @@ pub fn split_faces(body: &Body, faces: &[usize], tool: &SplitTool) -> Result<Bod
 /// Like [`split_faces`], with the body face each face of the result came from (in face order).
 pub fn split_faces_with_map(body: &Body, faces: &[usize], tool: &SplitTool) -> Result<(Body, Vec<usize>)> {
     let (b, imp) = split_any(body, faces, tool)?;
+    // The new edges, remembered so that export keeps the pieces apart.
+    let mut lines: Vec<Vec<Vec3>> = body.splits.as_ref().map(|l| l.to_vec()).unwrap_or_default();
+    lines.extend(imp.new_edges.iter().filter_map(|e| {
+        let c = e.curve();
+        let (t0, t1) = c.range_tuple();
+        let n = if matches!(c, mt::Curve::Line(_)) { 1 } else { 64 };
+        let pts: Vec<Vec3> = (0..=n).map(|k| from_p3(c.subs(t0 + (t1 - t0) * k as f64 / n as f64))).collect();
+        pts.iter().all(|p| p.is_finite()).then_some(pts)
+    }));
+    let b = b.with_split_lines(Some(std::sync::Arc::new(lines)));
     if let Some(missed) = faces.iter().find(|f| imp.made.get(f).copied().unwrap_or(0) == 0) {
         return Err(KernelError::Invalid(format!("the tool does not cross face {missed}")));
     }
@@ -867,7 +877,7 @@ pub(crate) fn split_any(body: &Body, faces: &[usize], tool: &SplitTool) -> Resul
         }
         let shells = std::mem::take(&mut imp.shells);
         let solid = mt::Solid::new_unchecked(shells.into_iter().map(mt::Shell::from).collect());
-        Ok((Body { solid: std::sync::Arc::new(solid), mesh: None, color: body.color, paint: None }, imp))
+        Ok((Body { solid: std::sync::Arc::new(solid), mesh: None, color: body.color, paint: None, splits: body.splits.clone() }, imp))
     })
 }
 
@@ -1156,6 +1166,42 @@ mod tests {
         assert!(rel(v, 6000.0 - std::f64::consts::PI * 4.0 * 30.0) < 1e-3, "{v}");
     }
 
+    /// Faces after a STEP round trip.
+    fn round_trip_faces(b: &Body) -> usize {
+        let ex = crate::ExportBody { name: "B".into(), body: b, color: None };
+        let text = crate::step_export_bodies(&[ex], &crate::StepHeader::default()).unwrap();
+        let imp = crate::step_import(&text).unwrap();
+        imp.bodies.iter().map(|x| x.body.face_count()).sum()
+    }
+
+    #[test]
+    fn export_keeps_faces_split_on_purpose() {
+        // A box top split by a plane, and a cylinder's side split by a plane across its axis.
+        let b = box_solid(Vec3::ZERO, Vec3::new(10.0, 20.0, 30.0)).unwrap();
+        let pl = Plane::new(Vec3::new(4.0, 0.0, 0.0), Vec3::Y, Vec3::Z).unwrap();
+        let s = split_faces(&b, &[top(&b, 30.0)], &SplitTool::Plane(pl)).unwrap();
+        assert_eq!(s.face_count(), 7);
+        assert_eq!(round_trip_faces(&s), 7);
+        let c = cylinder(Vec3::ZERO, Vec3::Z, 10.0, 20.0).unwrap();
+        let side = c.faces(0.01).unwrap().iter().position(|f| f.plane_normal.is_none()).unwrap();
+        let mid = Plane::new(Vec3::new(0.0, 0.0, 5.0), Vec3::X, Vec3::Y).unwrap();
+        let cs = split_faces(&c, &[side], &SplitTool::Plane(mid)).unwrap();
+        let n = cs.face_count();
+        assert!(n > c.face_count());
+        // The pieces stay apart; the cylinder's incidental seam pieces still join as before.
+        assert_eq!(round_trip_faces(&cs), round_trip_faces(&c) + 1);
+        // Without the record the export joins the top's pieces again (as it does for pieces a
+        // boolean left).
+        assert_eq!(round_trip_faces(&s.clone().with_split_lines(None)), 6);
+        // A split after a split keeps both lines.
+        let pl2 = Plane::new(Vec3::new(0.0, 12.0, 0.0), Vec3::X, Vec3::Z).unwrap();
+        let tops: Vec<usize> =
+            s.faces(0.01).unwrap().iter().enumerate().filter(|(_, f)| (f.centroid.z - 30.0).abs() < 1e-6).map(|(i, _)| i).collect();
+        let s2 = split_faces(&s, &tops, &SplitTool::Plane(pl2)).unwrap();
+        assert_eq!(s2.face_count(), 9);
+        assert_eq!(round_trip_faces(&s2), 9);
+    }
+
     #[test]
     fn sketch_curve_splits_a_cylinder_side_and_a_circle_splits_a_top() {
         let c = cylinder(Vec3::ZERO, Vec3::Z, 10.0, 20.0).unwrap();

crates/kernel/src/step_out.rs

@@ -11,6 +11,8 @@
 use std::collections::HashMap;
 use std::fmt::Write as _;
 
+use solvecraft_geom::Vec3;
+
 use crate::body::Body;
 use crate::step_in::p21::{self, Param};
 use crate::{KernelError, Result};
@@ -832,7 +834,13 @@ fn face_edges(ex: &p21::Exchange, face: &p21::Entity) -> Option<Vec<Vec<(u64, bo
 /// Faces that truck splits (a revolution in two or three turns) joined back into one face per
 /// analytic surface, by dropping the edges between them. Faces whose union would lose all its
 /// edges, or whose leftover edges do not close into loops, stay apart.
-fn merge_split_faces(ex: &mut p21::Exchange, paint: &mut HashMap<u64, FaceLook>) -> HashMap<u64, V3> {
+fn merge_split_faces(ex: &mut p21::Exchange, paint: &mut HashMap<u64, FaceLook>, keep: &[Vec<Vec3>]) -> HashMap<u64, V3> {
+    // An edge on a line the user split along: every sample within a hair of the polyline.
+    let on_split = |ex: &p21::Exchange, e: u64| -> bool {
+        let Some(pts) = edge_samples(ex, e) else { return false };
+        let pts: Vec<Vec3> = pts.iter().map(|q| Vec3::new(q[0], q[1], q[2])).collect();
+        crate::heal::on_split_line(&pts, keep)
+    };
     let mut merged_surfaces = std::collections::HashSet::new();
     let mut next = ex.entities.keys().max().copied().unwrap_or(0) + 1;
     let shells: Vec<u64> = ex.entities.iter().filter(|(_, e)| matches!(e.name(), "CLOSED_SHELL" | "OPEN_SHELL")).map(|(i, _)| *i).collect();
@@ -923,7 +931,7 @@ fn merge_split_faces(ex: &mut p21::Exchange, paint: &mut HashMap<u64, FaceLook>)
                     let all: Vec<(u64, bool)> = ea.into_iter().chain(eb).flatten().collect();
                     let shared: std::collections::HashSet<u64> =
                         all.iter().filter(|(e, d)| all.iter().any(|(e2, d2)| e2 == e && d2 != d)).map(|(e, _)| *e).collect();
-                    if shared.is_empty() {
+                    if shared.is_empty() || shared.iter().any(|e| on_split(ex, *e)) {
                         continue;
                     }
                     // Chain oriented edges into loops by vertices (None: they do not close).
@@ -1200,7 +1208,8 @@ fn brep_exchange(b: &Body, merge: bool) -> Result<(p21::Exchange, HashMap<u64, V
         }
     }
     let before = shell_face_count(&ex);
-    let merged = if merge { merge_split_faces(&mut ex, &mut paint) } else { Default::default() };
+    let keep = b.split_lines().map(|l| l.as_slice()).unwrap_or(&[]);
+    let merged = if merge { merge_split_faces(&mut ex, &mut paint, keep) } else { Default::default() };
     MERGES.with(|m| m.set(m.get() + before.saturating_sub(shell_face_count(&ex))));
     Ok((ex, merged, paint))
 }

crates/kernel/src/surfaces.rs

@@ -22,7 +22,7 @@ impl Body {
 /// A body of faces as they are (no orientation fix: a surface has no inside).
 fn surface_body(faces: Vec<mt::Face>) -> Body {
     let shell = mt::Shell::from(faces);
-    Body { solid: std::sync::Arc::new(mt::Solid::new_unchecked(vec![shell])), mesh: None, color: None, paint: None }
+    Body { solid: std::sync::Arc::new(mt::Solid::new_unchecked(vec![shell])), mesh: None, color: None, paint: None, splits: None }
 }
 
 /// A body from faces: a solid when they close, else a surface.
@@ -31,7 +31,7 @@ fn body_of(faces: Vec<mt::Face>) -> Result<Body> {
     if shell.shell_condition() == mt::ShellCondition::Closed {
         Body::new(mt::Solid::new_unchecked(vec![shell]))
     } else {
-        Ok(Body { solid: std::sync::Arc::new(mt::Solid::new_unchecked(vec![shell])), mesh: None, color: None, paint: None })
+        Ok(Body { solid: std::sync::Arc::new(mt::Solid::new_unchecked(vec![shell])), mesh: None, color: None, paint: None, splits: None })
     }
 }