engine: geometry.face.split, a face split along a datum plane (#508)

geometry.face.split {name, face, datum} records a face split; it is
applied after the merges whenever the design is opened or cut again,
follows its datum (which it holds), undoes, and is saved with the
model. geometry.face.split.delete removes it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

ATTRIBUTION.md

@@ -67,6 +67,7 @@ is committed and listed here with its exports.
 | assets/icons/geometry.partition.svg, assets/icons/geometry.partition.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/geometry.partition.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |
 | assets/icons/geometry.faces.merge.svg, assets/icons/geometry.faces.merge.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/geometry.faces.merge.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |
 | assets/icons/geometry.face.absorb.svg, assets/icons/geometry.face.absorb.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/geometry.face.absorb.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |
+| assets/icons/geometry.face.split.svg, assets/icons/geometry.face.split.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/geometry.face.split.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |
 | assets/icons/geometry.stitch.svg, assets/icons/geometry.stitch.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/geometry.stitch.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |
 | assets/icons/geometry.tessellate.svg, assets/icons/geometry.tessellate.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/geometry.tessellate.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |
 | assets/icons/interaction.bolt.svg, assets/icons/interaction.bolt.vectorcraft | FieldCraft contributors | Original toolbar icon drawn with VectorCraft from `assets/icons/src/interaction.bolt.json` (`cargo xtask icons`) | MIT OR Apache-2.0 |

assets/icons/geometry.face.split.svg

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="24" height="24" viewBox="0 0 24 24">
+  <title>geometry.face.split</title>
+  <g id="Layer_1" data-name="Layer 1">
+    <path d="M3 8 L12 4 L21 8 L12 12 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M3 8 L3 16 L12 20 L12 12 M12 20 L21 16 L21 8" fill="none" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M7.5 10 L16.5 6" fill="none" stroke="#f0b44a" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
+    <path d="M5 4.5 L9 2.5 M15 9.5 L19 7.5" fill="none" stroke="#4f9ce8" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round" stroke-dasharray="1.5 1.5"/>
+  </g>
+</svg>

assets/icons/src/geometry.face.split.json

@@ -0,0 +1 @@
+{"shapes": [{"d": "M3 8 L12 4 L21 8 L12 12 Z", "fill": "soft", "stroke": "line"}, {"d": "M3 8 L3 16 L12 20 L12 12 M12 20 L21 16 L21 8", "stroke": "line"}, {"line": [7.5, 10, 16.5, 6], "stroke": "warm", "width": 2}, {"d": "M5 4.5 L9 2.5 M15 9.5 L19 7.5", "stroke": "accent", "width": 1.5, "dash": [1.5, 1.5]}]}

crates/engine/src/catalogue.rs

@@ -309,6 +309,14 @@ pub static CATALOGUE: &[Capability] = &[
         "Prepare",
         "Absorb tiny and sliver faces into a neighbour for meshing, one picked or every one Check Geometry found",
     ),
+    covering(
+        "geometry.face.split",
+        "Split Face",
+        Geometry,
+        "Prepare",
+        "Split a face along a datum plane for meshing only: two faces on its surface, the cut an edge between them",
+        &["geometry.face.split.delete"],
+    ),
     covering(
         "geometry.stitch",
         "Stitch",

crates/engine/src/cmd/datum.rs

@@ -470,15 +470,24 @@ fn cuts(model: &Model) -> Vec<Cut> {
         .collect()
 }
 
-/// `design` cut by `model`'s partitions, then with its face merges (the design as opened when
-/// the model has neither).
+/// `design` cut by `model`'s partitions, then with its face merges and its face splits (those
+/// along a suppressed datum left out; the design as opened when the model has none).
 pub(crate) fn partitioned(design: &Design, model: &Model) -> Result<Design> {
     let merges: Vec<Merge> = model
         .face_merges()
         .iter()
         .map(|m| Merge { name: m.name.clone(), body: m.body.clone(), faces: m.faces.clone(), into: m.into.clone() })
         .collect();
-    Ok(design.partitioned(&cuts(model))?.merged(&merges)?)
+    let splits: Vec<fieldcraft_geometry::face_split::FaceSplit> = model
+        .face_splits()
+        .iter()
+        .filter(|s| !model.datum_suppressed(&s.datum))
+        .filter_map(|s| {
+            let (point, normal) = model.datum(&s.datum)?.plane()?;
+            Some(fieldcraft_geometry::face_split::FaceSplit { name: s.name.clone(), body: s.body.clone(), face: s.face.clone(), point, normal })
+        })
+        .collect();
+    Ok(design.partitioned(&cuts(model))?.merged(&merges)?.split(&splits)?)
 }
 
 /// Cut the session's design by its model's partitions again (and merge its faces again). The bodies that are gone (cells

crates/engine/src/cmd/forms.rs

@@ -1740,6 +1740,12 @@ pub const STITCH: &[Field] = &[
         .req(),
 ];
 pub const STITCH_DELETE: &[Field] = existing!("Stitch", C::Stitch);
+pub const FACE_SPLIT: &[Field] = &[
+    NAME,
+    group("face", "Face", FACE_ROW).help("The face to split.").req(),
+    reference("datum", "Datum plane", C::DatumPlane).help("The plane the face is split along.").req(),
+];
+pub const FACE_SPLIT_DELETE: &[Field] = existing!("Face split", C::FaceSplit);
 pub const FACES_MERGE_DELETE: &[Field] = existing!("Face merge", C::FaceMerge);
 pub const MESH_PLAN: &[Field] = &[
     reference("body", "Body", C::Body).help("The body of the design to plan (as opened, not a partition cell).").req(),

crates/engine/src/cmd/mod.rs

@@ -76,6 +76,8 @@ pub static ALL: &[CommandSpec] = &[
     virtual_faces::FACES_MERGE,
     virtual_faces::FACES_MERGE_DELETE,
     virtual_faces::FACE_ABSORB,
+    virtual_faces::FACE_SPLIT,
+    virtual_faces::FACE_SPLIT_DELETE,
     stitch::STITCH,
     stitch::STITCH_DELETE,
     mesh::NODE_ADD,

crates/engine/src/cmd/virtual_faces.rs

@@ -45,6 +45,26 @@ pub const FACE_ABSORB: CommandSpec = CommandSpec {
     run: absorb,
 };
 
+pub const FACE_SPLIT: CommandSpec = CommandSpec {
+    id: "geometry.face.split",
+    label: "Split Face",
+    params: r#"{"name": "Band", "face": {"body": "Base", "face": "F1:+z"}, "datum": "Mid" (a datum plane)} — splits a face along the curve where a datum plane crosses it, for meshing only (virtual topology; partitions cut whole bodies): the meshers, the checks and the viewport take it as two faces on its surface, `split:<name>:a` on the plane's negative side and `split:<name>:b` on the other, with the cut as an edge between them (a row of nodes along it; seed it like any edge), and the face's edges cut where the plane crosses them. The B-rep is not changed. Applied after the face merges, by persistent names, whenever the design is opened (a merge cannot name a split face); moving the datum moves the cut, and the datum cannot be deleted while a split uses it. The body loses its mesh (`replaced`); sets naming the face no longer resolve (`stale_sets`). A plane that does not cross the face is an error. Delete with geometry.face.split.delete. One undo step"#,
+    form: Some(super::forms::FACE_SPLIT),
+    kind: CommandKind::Model,
+    requires: &[Requirement::Design, Requirement::DatumPlane],
+    run: split,
+};
+
+pub const FACE_SPLIT_DELETE: CommandSpec = CommandSpec {
+    id: "geometry.face.split.delete",
+    label: "Delete Face Split",
+    params: r#"{"name": "Band"} — removes a face split: the face is one face again; the body loses its mesh (`replaced`)"#,
+    form: Some(super::forms::FACE_SPLIT_DELETE),
+    kind: CommandKind::Model,
+    requires: &[Requirement::FaceSplit],
+    run: split_delete,
+};
+
 /// Most faces one merge lists.
 const MAX_FACES: usize = fieldcraft_model::virtual_faces::MAX_FACE_MERGE_FACES;
 
@@ -75,12 +95,12 @@ fn stale_sets(s: &Session, body: &str) -> Vec<String> {
     s.model.sets().iter().filter(|(_, set)| set.geometry.iter().any(gone)).map(|(n, _)| n.clone()).collect()
 }
 
-/// Apply the model's partitions and merges to the design again, failing with the error of a merge
-/// named in `merges` when it has one; the meshes of `bodies` go (their faces changed).
-fn reapply(s: &mut Session, merges: &[String], bodies: &[String], reply: Value) -> Result<Value> {
+/// Apply the model's partitions, merges and splits to the design again, failing with the error of
+/// one of `features` (`merge <name>`, `split <name>`) when it has one; the meshes of `bodies` go
+/// (their faces changed).
+fn reapply(s: &mut Session, features: &[String], bodies: &[String], reply: Value) -> Result<Value> {
     let mut cleared = super::datum::recut(s)?;
     let design = s.design.as_deref().ok_or_else(|| EngineError::Params("open a design first".into()))?;
-    let features: Vec<String> = merges.iter().map(|m| format!("merge {m}")).collect();
     let errors: Vec<String> =
         design.errors.iter().filter(|e| features.contains(&e.feature)).map(|e| format!("{}: {}", e.feature, e.message)).collect();
     if !errors.is_empty() {
@@ -110,7 +130,7 @@ fn merge(s: &mut Session, p: &Value) -> Result<Value> {
     s.model.add_face_merge(FaceMerge { name: name.clone(), body: body.clone(), faces: faces.clone(), into: None })?;
     let face = merged_face_name(&name);
     let reply = json!({"name": name, "body": body, "face": face});
-    let mut reply = reapply(s, std::slice::from_ref(&name), std::slice::from_ref(&body), reply)?;
+    let mut reply = reapply(s, &[format!("merge {name}")], std::slice::from_ref(&body), reply)?;
     let shape = s.design.as_deref().and_then(|d| d.body(&body));
     let brep = shape.and_then(|b| b.face_index(&face)).and_then(|f| shape?.merged_faces().into_iter().find(|(g, _)| *g == f)).map_or(0, |(_, n)| n);
     if let Some(o) = reply.as_object_mut() {
@@ -200,7 +220,29 @@ fn absorb(s: &mut Session, p: &Value) -> Result<Value> {
         return Err(EngineError::Params(format!("no face could be absorbed: {}", json!(skipped))));
     }
     let bodies: Vec<String> = shapes.into_iter().map(|b| b.name).collect();
-    reapply(s, &merges, &bodies, json!({"absorbed": absorbed, "skipped": skipped}))
+    let features: Vec<String> = merges.iter().map(|m| format!("merge {m}")).collect();
+    reapply(s, &features, &bodies, json!({"absorbed": absorbed, "skipped": skipped}))
+}
+
+fn split(s: &mut Session, p: &Value) -> Result<Value> {
+    let name = super::name(p)?.to_string();
+    let r: GeometryRef = super::req(p, "face")?;
+    let Entity::Face(face) = r.entity else { return Err(EngineError::invalid("face", "give {\"body\", \"face\"}")) };
+    let datum: String = super::req(p, "datum")?;
+    super::datum::datum_plane_of(&s.model, &datum)?;
+    s.model.add_face_split(fieldcraft_model::FaceSplit { name: name.clone(), body: r.body.clone(), face: face.clone(), datum })?;
+    let feature = format!("split:{name}");
+    let faces = [format!("{feature}:a"), format!("{feature}:b")];
+    let reply = json!({"name": name, "body": r.body, "faces": faces});
+    reapply(s, &[format!("split {name}")], std::slice::from_ref(&r.body), reply)
+}
+
+fn split_delete(s: &mut Session, p: &Value) -> Result<Value> {
+    let name = super::name(p)?;
+    let removed = s.model.remove_face_split(name)?;
+    let mut cleared = super::datum::recut(s)?;
+    cleared.extend(super::replace::clear(&mut s.model, [removed.body.as_str()]));
+    Ok(cleared.report_if_any(s, json!({"deleted": name, "body": removed.body})))
 }
 
 fn delete(s: &mut Session, p: &Value) -> Result<Value> {
@@ -507,4 +549,75 @@ mod tests {
         assert_eq!(r["absorbed"][0]["name"], "Riser", "{r}");
         assert!(s.model.face_merges()[0].into.is_some());
     }
+
+    /// The plate's top face (z = 10), by name.
+    fn top(s: &Session) -> String {
+        let b = &s.design.as_ref().unwrap().bodies[0];
+        b.faces()
+            .iter()
+            .zip(b.face_surfaces().unwrap())
+            .find(|(_, x)| matches!(x, Some(FaceSurface::Plane { normal, point }) if normal[2] > 0.5 && point[2] > 9.0))
+            .unwrap()
+            .0
+            .name
+            .clone()
+    }
+
+    /// Mesh nodes on the top along x = `x`.
+    fn nodes_along(s: &Session, x: f64) -> usize {
+        s.model.nodes().values().filter(|n| (n.xyz[0] - x).abs() < 1e-6 && (n.xyz[2] - 10.0).abs() < 1e-6).count()
+    }
+
+    /// Splitting the top along a datum plane gives two faces the tet mesher tags, with a row of
+    /// nodes along the cut; the split follows the datum, holds it, is saved and replayed, and
+    /// undoes.
+    #[test]
+    fn a_split_face_meshes_with_a_node_row_along_the_cut() {
+        let (mut s, dir) = super::super::datum::tests::plate();
+        let body = s.design.as_ref().unwrap().bodies[0].name.clone();
+        let face = json!({"body": body, "face": top(&s)});
+        s.execute("geometry.datum", &json!({"name": "Cut", "by": {"type": "offset", "plane": "yz", "offset": 30.3}})).unwrap();
+        let mesh = json!({"size": 3});
+        s.execute("mesh.tet.generate", &mesh).unwrap();
+        let before = nodes_along(&s, 30.3);
+        let r = s.execute("geometry.face.split", &json!({"name": "Band", "face": face, "datum": "Cut"})).unwrap();
+        assert_eq!(r["faces"], json!(["split:Band:a", "split:Band:b"]), "{r}");
+        assert!(r["replaced"].is_array(), "{r}");
+        assert_eq!(faces(&s).len(), 9);
+        s.execute("mesh.tet.generate", &mesh).unwrap();
+        let after = nodes_along(&s, 30.3);
+        assert!(after >= 10 && after > before, "a node row along the 30 mm cut: {before} -> {after}");
+        let c = s.model.classification();
+        let used: BTreeSet<String> =
+            c.elements().values().flat_map(|t| t.faces.values()).filter_map(|&t| c.entity(t)?.entity.name().map(str::to_string)).collect();
+        assert!(used.contains("split:Band:a") && used.contains("split:Band:b"), "{used:?}");
+        // The datum holds; moving it moves the cut.
+        assert!(s.execute("geometry.datum.delete", &json!({"name": "Cut"})).is_err());
+        s.execute("geometry.datum.edit", &json!({"name": "Cut", "by": {"type": "offset", "plane": "yz", "offset": 35.5}})).unwrap();
+        s.execute("mesh.tet.generate", &mesh).unwrap();
+        assert!(nodes_along(&s, 35.5) >= 10);
+        // Saved and replayed.
+        let file = dir.join("split.fieldcraft");
+        s.execute("file.save", &json!({"path": file.to_str().unwrap()})).unwrap();
+        let mut t = Session::default();
+        t.execute("file.open", &json!({"path": file.to_str().unwrap()})).unwrap();
+        assert_eq!(faces(&t), faces(&s));
+        // Bad splits change nothing; delete brings the face back.
+        let journal = s.journal.len();
+        for p in [
+            json!({"name": "X", "face": face, "datum": "nope"}),
+            json!({"name": "X", "face": {"body": body, "face": "nope"}, "datum": "Cut"}),
+            json!({"name": "X", "face": {"body": body}, "datum": "Cut"}),
+            json!({"name": "Band", "face": face, "datum": "Cut"}),
+            json!({"name": "X", "face": {"body": body, "face": "split:Band:b"}, "datum": "Cut"}),
+        ] {
+            assert!(s.execute("geometry.face.split", &p).is_err(), "{p}");
+        }
+        assert_eq!(s.journal.len(), journal);
+        s.execute("geometry.face.split.delete", &json!({"name": "Band"})).unwrap();
+        assert_eq!(faces(&s).len(), 8);
+        s.execute("edit.undo", &json!({})).unwrap();
+        assert_eq!(faces(&s).len(), 9);
+        std::fs::remove_dir_all(dir).ok();
+    }
 }

crates/engine/src/requirement.rs

@@ -36,6 +36,8 @@ pub enum Requirement {
     FaceMerge,
     /// A stitch.
     Stitch,
+    /// A face split.
+    FaceSplit,
     /// An edge seed.
     Seed,
     /// At least one mesh node.
@@ -85,6 +87,7 @@ impl Requirement {
             Requirement::Partition => !m.partitions().is_empty(),
             Requirement::FaceMerge => !m.face_merges().is_empty(),
             Requirement::Stitch => !m.stitches().is_empty(),
+            Requirement::FaceSplit => !m.face_splits().is_empty(),
             Requirement::Seed => !m.seeds().is_empty(),
             Requirement::Nodes => !m.nodes().is_empty(),
             Requirement::Mesh => has_elements,
@@ -127,6 +130,7 @@ impl Requirement {
             Requirement::Partition => "a partition",
             Requirement::FaceMerge => "a face merge",
             Requirement::Stitch => "a stitch",
+            Requirement::FaceSplit => "a face split",
             Requirement::Seed => "an edge seed",
             Requirement::Nodes => "mesh nodes",
             Requirement::Mesh | Requirement::Analysable => "a mesh",
@@ -159,6 +163,7 @@ impl Requirement {
             Requirement::Partition => "geometry.partition",
             Requirement::FaceMerge => "geometry.faces.merge",
             Requirement::Stitch => "geometry.stitch",
+            Requirement::FaceSplit => "geometry.face.split",
             Requirement::Seed => "mesh.seed.edge",
             Requirement::Nodes => "mesh.node.add",
             Requirement::Mesh | Requirement::Analysable => "mesh.tet.generate",
@@ -218,6 +223,7 @@ impl crate::schema::Collection {
             C::Partition => Requirement::Partition,
             C::FaceMerge => Requirement::FaceMerge,
             C::Stitch => Requirement::Stitch,
+            C::FaceSplit => Requirement::FaceSplit,
         }
     }
 }
@@ -360,6 +366,7 @@ mod tests {
             C::Partition,
             C::FaceMerge,
             C::Stitch,
+            C::FaceSplit,
         ];
         let empty = Session::default();
         for c in all {

crates/engine/src/schema.rs

@@ -221,6 +221,8 @@ pub enum Collection {
     FaceMerge,
     /// A stitch (`geometry.stitch`).
     Stitch,
+    /// A face split (`geometry.face.split`).
+    FaceSplit,
 }
 
 impl Collection {
@@ -248,6 +250,7 @@ impl Collection {
             Collection::Partition => "partition",
             Collection::FaceMerge => "face merge",
             Collection::Stitch => "stitch",
+            Collection::FaceSplit => "face split",
         }
     }
 }
@@ -279,6 +282,7 @@ impl Session {
             Collection::Partition => Box::new(m.partitions().iter().map(|p| p.name.as_str())),
             Collection::FaceMerge => Box::new(m.face_merges().iter().map(|f| f.name.as_str())),
             Collection::Stitch => Box::new(m.stitches().iter().map(|f| f.name.as_str())),
+            Collection::FaceSplit => Box::new(m.face_splits().iter().map(|f| f.name.as_str())),
         };
         names.take(MAX_NAMES).map(str::to_string).collect()
     }

crates/geometry/src/design.rs

@@ -129,7 +129,7 @@ pub(crate) fn rebuild(doc: &Document) -> Result<Design> {
                 .map(|(i, name)| Face { name: name.clone(), exact: naming::face_name_exact(b, i), members: Vec::new() })
                 .collect())
         })?;
-        bodies.push(Shape { name: b.name.clone(), body: b.body.clone(), faces, face_map: Vec::new() });
+        bodies.push(Shape { name: b.name.clone(), body: b.body.clone(), faces, face_map: Vec::new(), cuts: Vec::new() });
     }
     // A feature that failed leaves a different solid than SolidCraft's: each one names the bodies
     // it would have changed, so opening never hands back a silently different solid.

crates/geometry/src/face_split.rs

@@ -0,0 +1,402 @@
+//! Virtual topology: a face split in two along its curve through a plane, for meshing only.
+//!
+//! A partition cuts whole bodies into cells with the kernel; a [`FaceSplit`] splits one face of
+//! a body along the curve where a plane crosses it, in what the meshers take from the body (the
+//! B-rep is unchanged): the face's triangles are cut along the plane, the pieces on the plane's
+//! negative side are `split:<name>:a` and the others `split:<name>:b` (the partition's
+//! convention), both on the face's surface, and the cut is a new edge between them, so the
+//! meshers put a row of nodes along it. Edges of the face that the plane crosses are cut there
+//! too. Splits apply after the merges ([`crate::virtual_faces`]), by persistent names, whenever
+//! the design is opened; a merge cannot name a split face.
+
+use std::collections::BTreeMap;
+
+use crate::edges::EDGE_TOLERANCE;
+use crate::{Design, Error, Face, FeatureError, Result, Shape, SurfaceMesh};
+
+/// Most splits a design takes.
+pub const MAX_SPLITS: usize = 1024;
+
+/// A face of a body split by a plane.
+#[derive(Debug, Clone, PartialEq)]
+pub struct FaceSplit {
+    /// Names the two faces (`split:<name>:a|b`).
+    pub name: String,
+    pub body: String,
+    pub face: String,
+    /// A point on the plane and its normal (any length but zero).
+    pub point: [f64; 3],
+    pub normal: [f64; 3],
+}
+
+/// One split as a body applies it: the face it cuts (kept as the `a` side), the `b` face's
+/// index, and the plane (unit normal).
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct Cut {
+    a: u32,
+    b: u32,
+    point: [f64; 3],
+    normal: [f64; 3],
+}
+
+impl Cut {
+    pub(crate) fn faces(&self) -> (u32, u32) {
+        (self.a, self.b)
+    }
+}
+
+fn signed(c: &Cut, p: [f64; 3]) -> f64 {
+    (0..3).map(|k| (p[k] - c.point[k]) * c.normal[k]).sum()
+}
+
+impl Design {
+    /// The design with `splits` applied in order (after its partitions and merges). A split that
+    /// fails (a body or face that is not there, a plane that does not cross the face) is listed
+    /// in [`Design::errors`] (feature `split <name>`) and changes nothing.
+    pub fn split(mut self, splits: &[FaceSplit]) -> Result<Design> {
+        if splits.len() > MAX_SPLITS {
+            return Err(Error::Invalid(format!("a design takes at most {MAX_SPLITS} face splits")));
+        }
+        for sp in splits {
+            let feature = format!("split {}", sp.name);
+            let Some(body) = self.bodies.iter_mut().find(|b| b.name == sp.body) else {
+                self.errors.push(FeatureError { feature, message: format!("no body `{}`", sp.body), bodies: vec![sp.body.clone()] });
+                continue;
+            };
+            match body.split_face(&sp.name, &sp.face, sp.point, sp.normal) {
+                Ok(split) => *body = split,
+                Err(e) => self.errors.push(FeatureError { feature, message: e.to_string(), bodies: vec![sp.body.clone()] }),
+            }
+        }
+        if self.bodies.iter().any(|b| !b.cuts.is_empty()) {
+            let opened = self.source.iter().map(|s| self.bodies.iter().find(|b| b.name == s.name).unwrap_or(s).clone()).collect();
+            self.opened = std::sync::Arc::new(opened);
+        }
+        Ok(self)
+    }
+}
+
+impl Shape {
+    /// This shape with face `face` split along the plane through `point` normal to `normal`:
+    /// `split:<name>:a` on its negative side, `split:<name>:b` on the other.
+    pub fn split_face(&self, name: &str, face: &str, point: [f64; 3], normal: [f64; 3]) -> Result<Shape> {
+        let len = normal.iter().map(|v| v * v).sum::<f64>().sqrt();
+        if !(point.iter().chain(&normal).all(|v| v.is_finite() && v.abs() <= 1e9) && len > 1e-12) {
+            return Err(Error::Invalid("a split needs a plane with a finite point and a non-zero normal".into()));
+        }
+        let named = self.faces_named(face);
+        let [a] = named.as_slice() else {
+            return Err(Error::Invalid(format!("body `{}` has no face `{face}` (or several pieces of it: name one)", self.name)));
+        };
+        let (an, bn) = (format!("split:{name}:a"), format!("split:{name}:b"));
+        if self.faces.iter().any(|f| f.name == an || f.name == bn) {
+            return Err(Error::Invalid(format!("body `{}` already has the faces of a split `{name}`", self.name)));
+        }
+        let b = u32::try_from(self.faces.len()).map_err(|_| Error::Invalid("too many faces".into()))?;
+        let mut shape = self.clone();
+        let original = shape.faces.get(*a as usize).cloned().ok_or_else(|| Error::Invalid("no such face".into()))?;
+        let members = if original.members.is_empty() { vec![original.name.clone()] } else { original.members.clone() };
+        if let Some(f) = shape.faces.get_mut(*a as usize) {
+            *f = Face { name: an, exact: original.exact, members: members.clone() };
+        }
+        shape.faces.push(Face { name: bn, exact: original.exact, members });
+        shape.cuts.push(Cut { a: *a, b, point, normal: normal.map(|v| v / len) });
+        // The plane must cross the face: both pieces have triangles.
+        let mesh = shape.tessellate((self.size()? * EDGE_TOLERANCE).max(1e-4))?;
+        for side in [*a, b] {
+            if !mesh.tri_face.contains(&side) {
+                return Err(Error::Invalid(format!("the plane does not cross face `{face}` of body `{}`", self.name)));
+            }
+        }
+        Ok(shape)
+    }
+}
+
+/// `mesh` with each of `cuts` applied in order (see the module docs).
+pub(crate) fn apply(mesh: &mut SurfaceMesh, cuts: &[Cut]) {
+    for c in cuts {
+        cut(mesh, c);
+    }
+}
+
+fn lerp(p: [f64; 3], q: [f64; 3], t: f64) -> [f64; 3] {
+    [p[0] + t * (q[0] - p[0]), p[1] + t * (q[1] - p[1]), p[2] + t * (q[2] - p[2])]
+}
+
+fn cut(mesh: &mut SurfaceMesh, c: &Cut) {
+    let scale = mesh.positions.iter().flatten().fold(0.0f64, |m, v| m.max(v.abs())).max(1.0);
+    let eps = 1e-12 * scale;
+    let side = |d: f64| {
+        if d > eps {
+            1
+        } else if d < -eps {
+            -1
+        } else {
+            0
+        }
+    };
+    // A new point on the side (u, v) where the plane crosses it, once per side.
+    let mut made: BTreeMap<(u32, u32), u32> = BTreeMap::new();
+    let mut segments: Vec<(u32, u32)> = Vec::new();
+    let n = mesh.triangles.len();
+    for t in 0..n {
+        let (Some(&tri), Some(&tag)) = (mesh.triangles.get(t), mesh.tri_face.get(t)) else { continue };
+        if tag != c.a {
+            continue;
+        }
+        let pos = |i: u32, m: &SurfaceMesh| m.positions.get(i as usize).copied().unwrap_or([0.0; 3]);
+        let s = tri.map(|i| side(signed(c, pos(i, mesh))));
+        let tag_of = |sg: i32| if sg > 0 { c.b } else { c.a };
+        if s.iter().all(|&x| x >= 0) || s.iter().all(|&x| x <= 0) {
+            let sg = s.iter().copied().find(|&x| x != 0).unwrap_or(-1);
+            if let Some(f) = mesh.tri_face.get_mut(t) {
+                *f = tag_of(sg);
+            }
+            // A side on the plane between pieces of both sides is the cut too (picked up when
+            // chaining from the other triangles' points; left to the edges otherwise).
+            continue;
+        }
+        let mut point_on = |u: u32, v: u32, m: &mut SurfaceMesh| -> u32 {
+            let key = (u.min(v), u.max(v));
+            if let Some(&i) = made.get(&key) {
+                return i;
+            }
+            let (p, q) = (pos(key.0, m), pos(key.1, m));
+            let (dp, dq) = (signed(c, p), signed(c, q));
+            let x = lerp(p, q, dp / (dp - dq));
+            let i = u32::try_from(m.positions.len()).unwrap_or(u32::MAX);
+            m.positions.push(x);
+            made.insert(key, i);
+            i
+        };
+        let rot = |k: usize| [tri[k], tri[(k + 1) % 3], tri[(k + 2) % 3]];
+        let rs = |k: usize| [s[k], s[(k + 1) % 3], s[(k + 2) % 3]];
+        let mut out: Vec<([u32; 3], i32)> = Vec::with_capacity(3);
+        if let Some(k) = (0..3).find(|&k| s[k] == 0) {
+            // A corner on the plane, the other two on either side.
+            let ([z, u, v], [_, su, sv]) = (rot(k), rs(k));
+            let m = point_on(u, v, mesh);
+            out.push(([z, u, m], su));
+            out.push(([z, m, v], sv));
+            segments.push((z, m));
+        } else {
+            // One corner alone on its side.
+            let k = (0..3).find(|&k| s[k] != s[(k + 1) % 3] && s[k] != s[(k + 2) % 3]).unwrap_or(0);
+            let ([l, u, v], [sl, su, _]) = (rot(k), rs(k));
+            let (pu, pv) = (point_on(l, u, mesh), point_on(l, v, mesh));
+            out.push(([l, pu, pv], sl));
+            out.push(([pu, u, v], su));
+            out.push(([pu, v, pv], su));
+            segments.push((pu, pv));
+        }
+        let mut first = true;
+        for (tri, sg) in out {
+            if first {
+                if let (Some(slot), Some(f)) = (mesh.triangles.get_mut(t), mesh.tri_face.get_mut(t)) {
+                    *slot = tri;
+                    *f = tag_of(sg);
+                }
+                first = false;
+            } else {
+                mesh.triangles.push(tri);
+                mesh.tri_face.push(tag_of(sg));
+            }
+        }
+    }
+    // The face's edges: cut where the plane crosses them, each piece on its side's face.
+    let mut edges = Vec::with_capacity(mesh.edges.len() + 1);
+    let mut edge_faces = Vec::with_capacity(mesh.edges.len() + 1);
+    for (points, faces) in std::mem::take(&mut mesh.edges).into_iter().zip(std::mem::take(&mut mesh.edge_faces)) {
+        if !faces.contains(&c.a) {
+            edges.push(points);
+            edge_faces.push(faces);
+            continue;
+        }
+        let mut piece: Vec<[f64; 3]> = Vec::new();
+        let mut piece_side = 0;
+        let mut flush = |piece: &mut Vec<[f64; 3]>, sg: i32| {
+            if piece.len() >= 2 {
+                let on = if sg > 0 { c.b } else { c.a };
+                edges.push(std::mem::take(piece));
+                edge_faces.push(faces.iter().map(|&f| if f == c.a { on } else { f }).collect());
+            }
+            piece.clear();
+        };
+        for (k, &p) in points.iter().enumerate() {
+            let sp = side(signed(c, p));
+            if k > 0
+                && let Some(&q) = points.get(k - 1)
+            {
+                let sq = side(signed(c, q));
+                if sp != 0 && sq != 0 && sp != sq {
+                    let (dq, dp) = (signed(c, q), signed(c, p));
+                    let x = lerp(q, p, dq / (dq - dp));
+                    piece.push(x);
+                    flush(&mut piece, piece_side);
+                    piece.push(x);
+                }
+            }
+            if sp != 0 {
+                piece_side = sp;
+            }
+            piece.push(p);
+        }
+        flush(&mut piece, piece_side);
+    }
+    // The cut: its segments chained into polylines, between the two faces. Copies of one point
+    // (a tessellation may repeat points) are one point for the chaining.
+    let close = (2.0 * mesh.weld_distance).max(eps);
+    let mut canon: BTreeMap<u32, u32> = BTreeMap::new();
+    let mut reps: Vec<(u32, [f64; 3])> = Vec::new();
+    for &i in segments.iter().flat_map(|(u, v)| [u, v]) {
+        if canon.contains_key(&i) {
+            continue;
+        }
+        let p = mesh.positions.get(i as usize).copied().unwrap_or([0.0; 3]);
+        let r = match reps.iter().find(|(_, q)| (0..3).map(|k| (p[k] - q[k]).powi(2)).sum::<f64>().sqrt() <= close) {
+            Some((r, _)) => *r,
+            None => {
+                reps.push((i, p));
+                i
+            }
+        };
+        canon.insert(i, r);
+    }
+    let segments: Vec<(u32, u32)> =
+        segments.iter().map(|(u, v)| (canon.get(u).copied().unwrap_or(*u), canon.get(v).copied().unwrap_or(*v))).filter(|(u, v)| u != v).collect();
+    for line in chain(&segments) {
+        let points: Vec<[f64; 3]> = line.iter().filter_map(|&i| mesh.positions.get(i as usize).copied()).collect();
+        if points.len() >= 2 {
+            edges.push(points);
+            edge_faces.push(vec![c.a, c.b]);
+        }
+    }
+    mesh.edges = edges;
+    mesh.edge_faces = edge_faces;
+}
+
+/// Segments (point index pairs) chained end to end into polylines, in a fixed order.
+fn chain(segments: &[(u32, u32)]) -> Vec<Vec<u32>> {
+    let mut at: BTreeMap<u32, Vec<usize>> = BTreeMap::new();
+    for (i, &(u, v)) in segments.iter().enumerate() {
+        at.entry(u).or_default().push(i);
+        at.entry(v).or_default().push(i);
+    }
+    let mut used = vec![false; segments.len()];
+    let mut lines = Vec::new();
+    // Start at ends (points on one segment) first, then what is left (closed loops).
+    let ends: Vec<usize> = at.values().filter(|s| s.len() == 1).flatten().copied().collect();
+    let starts: Vec<usize> = ends.into_iter().chain(0..segments.len()).collect();
+    for s in starts {
+        if used.get(s).copied().unwrap_or(true) {
+            continue;
+        }
+        let Some(&(u, v)) = segments.get(s) else { continue };
+        // From an end segment, start at its free point.
+        let (mut line, mut last) = if at.get(&v).is_some_and(|x| x.len() == 1) { (vec![v, u], u) } else { (vec![u, v], v) };
+        if let Some(x) = used.get_mut(s) {
+            *x = true;
+        }
+        while let Some(&next) = at.get(&last).and_then(|x| x.iter().find(|&&i| !used.get(i).copied().unwrap_or(true))) {
+            if let Some(x) = used.get_mut(next) {
+                *x = true;
+            }
+            let Some(&(a, b)) = segments.get(next) else { break };
+            last = if a == last { b } else { a };
+            line.push(last);
+        }
+        lines.push(line);
+    }
+    lines
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::open_design;
+    use crate::sample::{self, Plate};
+
+    use super::*;
+
+    /// The plate's top (z = 10, 40 × 30 with the Ø10 hole at (15, 15)) split at x = 30: two
+    /// faces on the top's plane, a straight cut edge between them along x = 30, the top's rim
+    /// edges at y = 0 and y = 30 cut there; triangles all on one side of the plane each.
+    #[test]
+    fn the_plates_top_splits_along_a_plane() {
+        let d = open_design(&sample::plate_design(&Plate::default()).unwrap()).unwrap();
+        let body = d.bodies[0].name.clone();
+        let b = &d.bodies[0];
+        let surfaces = b.face_surfaces().unwrap();
+        let top = b
+            .faces()
+            .iter()
+            .zip(&surfaces)
+            .find(|(_, s)| matches!(s, Some(crate::FaceSurface::Plane { normal, point }) if normal[2] > 0.5 && point[2] > 9.0))
+            .unwrap()
+            .0
+            .name
+            .clone();
+        let split = |x: f64| FaceSplit { name: "S".into(), body: body.clone(), face: top.clone(), point: [x, 0.0, 0.0], normal: [1.0, 0.0, 0.0] };
+        let d = d.split(&[split(30.0)]).unwrap();
+        assert!(d.errors.is_empty(), "{:?}", d.errors);
+        let b = &d.bodies[0];
+        assert_eq!(b.faces().len(), 9);
+        let (fa, fb) = (b.face_index("split:S:a").unwrap(), b.face_index("split:S:b").unwrap());
+        let s = b.face_surfaces().unwrap();
+        assert_eq!(s[fa as usize], s[fb as usize]);
+        let mesh = b.tessellate(0.01).unwrap();
+        for (t, &f) in mesh.triangles.iter().zip(&mesh.tri_face) {
+            let x = t.map(|i| mesh.positions[i as usize][0]);
+            if f == fa {
+                assert!(x.iter().all(|&v| v <= 30.0 + 1e-9), "{x:?}");
+            } else if f == fb {
+                assert!(x.iter().all(|&v| v >= 30.0 - 1e-9), "{x:?}");
+            }
+        }
+        // The area is kept: 40 × 30 less the hole, split 30 × 30 − π25 and 10 × 30.
+        let area = |f: u32| -> f64 {
+            mesh.triangles
+                .iter()
+                .zip(&mesh.tri_face)
+                .filter(|(_, g)| **g == f)
+                .map(|(t, _)| {
+                    let [a, b, c] = t.map(|i| mesh.positions[i as usize]);
+                    let (u, v) = ([b[0] - a[0], b[1] - a[1], b[2] - a[2]], [c[0] - a[0], c[1] - a[1], c[2] - a[2]]);
+                    0.5 * ((u[1] * v[2] - u[2] * v[1]).powi(2) + (u[2] * v[0] - u[0] * v[2]).powi(2) + (u[0] * v[1] - u[1] * v[0]).powi(2)).sqrt()
+                })
+                .sum()
+        };
+        assert!((area(fb) - 300.0).abs() < 1e-6, "{}", area(fb));
+        assert!((area(fa) - (900.0 - std::f64::consts::PI * 25.0)).abs() < 0.5, "{}", area(fa));
+        let edges = b.edges().unwrap();
+        let cut: Vec<_> = edges.iter().filter(|e| e.faces.contains(&fa) && e.faces.contains(&fb)).collect();
+        assert_eq!(cut.len(), 1, "{:?}", edges.iter().map(|e| &e.name).collect::<Vec<_>>());
+        assert!((cut[0].length() - 30.0).abs() < 1e-9 && cut[0].points.iter().all(|p| (p[0] - 30.0).abs() < 1e-9));
+        // The top's rim edges along x are cut at x = 30: four pieces where there were two.
+        let rim = edges.iter().filter(|e| e.faces.iter().any(|&f| f == fa || f == fb) && !(e.faces.contains(&fa) && e.faces.contains(&fb))).count();
+        assert_eq!(rim, 2 + 2 + 2 + 2, "four sides, the two along x in two pieces, and the hole's two rim arcs");
+    }
+
+    #[test]
+    fn bad_splits_are_listed() {
+        let d = open_design(&sample::plate_design(&Plate::default()).unwrap()).unwrap();
+        let body = d.bodies[0].name.clone();
+        let face = d.bodies[0].faces()[0].name.clone();
+        let sp = |n: &str, b: &str, f: &str, x: f64, normal: [f64; 3]| FaceSplit {
+            name: n.into(),
+            body: b.into(),
+            face: f.into(),
+            point: [x, 0.0, 0.0],
+            normal,
+        };
+        let d = d
+            .split(&[
+                sp("Far", &body, &face, 1000.0, [1.0, 0.0, 0.0]),
+                sp("NoFace", &body, "nope", 20.0, [1.0, 0.0, 0.0]),
+                sp("NoBody", "x", &face, 20.0, [1.0, 0.0, 0.0]),
+                sp("Zero", &body, &face, 20.0, [0.0; 3]),
+            ])
+            .unwrap();
+        assert_eq!(d.errors.len(), 4, "{:?}", d.errors);
+        assert_eq!(d.bodies[0].faces().len(), 8);
+    }
+}

crates/geometry/src/lib.rs

@@ -20,6 +20,7 @@
 pub mod check;
 mod design;
 mod edges;
+pub mod face_split;
 pub mod features;
 pub mod partition;
 pub mod sample;
@@ -70,6 +71,8 @@ pub struct Shape {
     faces: Vec<Face>,
     /// The virtual face (index into `faces`) of each B-rep face; empty: they are the same.
     face_map: Vec<u32>,
+    /// Faces split along a plane ([`face_split`]), applied after the merges, in order.
+    cuts: Vec<face_split::Cut>,
 }
 
 /// A B-rep face's persistent identity.
@@ -156,7 +159,7 @@ fn xyz(v: solidcraft_geom::Vec3) -> [f64; 3] {
 pub fn block(a: [f64; 3], b: [f64; 3]) -> Result<Shape> {
     let v = |p: [f64; 3]| solidcraft_geom::Vec3 { x: p[0], y: p[1], z: p[2] };
     let body = guard("block", || solidcraft_kernel::box_solid(v(a), v(b)))?;
-    Ok(Shape { name: "Block".into(), body, faces: Vec::new(), face_map: Vec::new() })
+    Ok(Shape { name: "Block".into(), body, faces: Vec::new(), face_map: Vec::new(), cuts: Vec::new() })
 }
 
 impl Shape {
@@ -246,7 +249,7 @@ impl Shape {
             centroid: xyz(m.centroid),
             bbox_min: xyz(m.bbox.min),
             bbox_max: xyz(m.bbox.max),
-            faces: if self.face_map.is_empty() { m.faces } else { self.faces.len() },
+            faces: if self.face_map.is_empty() && self.cuts.is_empty() { m.faces } else { self.faces.len() },
         })
     }
 

crates/geometry/src/partition.rs

@@ -277,7 +277,7 @@ fn split(body: &Shape, plane: &Plane, n: [f64; 3], cut: &Cut) -> Result<Split> {
     let parent = Faces::of(body, tol)?;
     let mut cells: Vec<(bool, [f64; 3], Shape)> = Vec::with_capacity(parts.len());
     for part in parts {
-        let mut shape = Shape { name: String::new(), body: part, faces: Vec::new(), face_map: Vec::new() };
+        let mut shape = Shape { name: String::new(), body: part, faces: Vec::new(), face_map: Vec::new(), cuts: Vec::new() };
         let m = shape.measure()?;
         let negative = dot(sub(m.centroid, cut.point), n) < 0.0;
         shape.faces = (0..m.faces).map(|_| Face { name: String::new(), exact: false, members: Vec::new() }).collect();

crates/geometry/src/virtual_faces.rs

@@ -50,7 +50,7 @@ pub fn merged_face_name(name: &str) -> String {
 /// Whether a feature error comes from the model's partitions or merges (applied again from the
 /// design as opened) rather than from the design's own timeline.
 pub(crate) fn derived(feature: &str) -> bool {
-    feature.starts_with("partition ") || feature.starts_with("merge ")
+    feature.starts_with("partition ") || feature.starts_with("merge ") || feature.starts_with("split ")
 }
 
 impl Design {
@@ -119,6 +119,9 @@ impl Shape {
         if faces.len() > MAX_MERGE_FACES {
             return Err(Error::Invalid(format!("a merge takes at most {MAX_MERGE_FACES} faces")));
         }
+        if !self.cuts.is_empty() {
+            return Err(Error::Invalid("merges come before splits: a merge cannot take a split face".into()));
+        }
         let mut members = BTreeSet::new();
         for f in faces {
             let found = self.faces_named(f);
@@ -162,7 +165,7 @@ impl Shape {
         }
         let brep = if self.face_map.is_empty() { (0..u32::try_from(self.faces.len()).unwrap_or(0)).collect() } else { self.face_map.clone() };
         let face_map = brep.iter().map(|&v| renumber.get(v as usize).copied().unwrap_or(v)).collect();
-        Ok(Shape { name: self.name.clone(), body: self.body.clone(), faces: kept, face_map })
+        Ok(Shape { name: self.name.clone(), body: self.body.clone(), faces: kept, face_map, cuts: Vec::new() })
     }
 
     /// The neighbour face `face` shares the longest edges with (by name): where a tiny or sliver
@@ -214,6 +217,8 @@ impl Shape {
     /// for a free-form one.
     pub fn member_surfaces(&self, face: u32) -> Result<Vec<Option<FaceSurface>>> {
         let brep = self.brep_face_surfaces()?;
+        // The b side of a split is made of the face it was split from.
+        let face = self.cuts.iter().rev().fold(face, |f, c| if c.faces().1 == f { c.faces().0 } else { f });
         if self.face_map.is_empty() {
             return Ok(brep.get(face as usize).copied().into_iter().collect());
         }
@@ -223,6 +228,21 @@ impl Shape {
     /// `brep` (surfaces by B-rep face) by face: a merged face has its B-rep faces' surface when
     /// they share one, else none.
     pub(crate) fn virtual_surfaces(&self, brep: Vec<Option<FaceSurface>>) -> Result<Vec<Option<FaceSurface>>> {
+        let mut out = self.merged_surfaces(brep)?;
+        for c in &self.cuts {
+            let (a, b) = c.faces();
+            let s = out.get(a as usize).copied().flatten();
+            if out.len() <= b as usize {
+                out.resize(b as usize + 1, None);
+            }
+            if let Some(slot) = out.get_mut(b as usize) {
+                *slot = s;
+            }
+        }
+        Ok(out)
+    }
+
+    fn merged_surfaces(&self, brep: Vec<Option<FaceSurface>>) -> Result<Vec<Option<FaceSurface>>> {
         if self.face_map.is_empty() {
             return Ok(brep);
         }
@@ -243,7 +263,13 @@ impl Shape {
 
     /// `mesh` (a tessellation of the B-rep) with the merged faces: triangles tagged by face,
     /// the edges inside a merged face dropped and the pieces of an edge it leaves joined.
-    pub(crate) fn virtual_mesh(&self, mut mesh: SurfaceMesh) -> SurfaceMesh {
+    pub(crate) fn virtual_mesh(&self, mesh: SurfaceMesh) -> SurfaceMesh {
+        let mut mesh = self.merged_mesh(mesh);
+        crate::face_split::apply(&mut mesh, &self.cuts);
+        mesh
+    }
+
+    fn merged_mesh(&self, mut mesh: SurfaceMesh) -> SurfaceMesh {
         if self.face_map.is_empty() {
             return mesh;
         }

crates/model/src/datums.rs

@@ -174,7 +174,8 @@ impl Model {
     /// The definitions that refer to a datum (the partitions that cut by it), as `kind \`name\``
     /// strings.
     pub fn datum_users(&self, name: &str) -> Vec<String> {
-        self.partitions.iter().filter(|p| p.datum == name).map(|p| format!("partition `{}`", p.name)).collect()
+        let partitions = self.partitions.iter().filter(|p| p.datum == name).map(|p| format!("partition `{}`", p.name));
+        partitions.chain(self.face_splits().iter().filter(|s| s.datum == name).map(|s| format!("face split `{}`", s.name))).collect()
     }
 
     /// The partitions, in the order they cut.

crates/model/src/lib.rs

@@ -85,7 +85,7 @@ pub use steps::{
 };
 pub use tie::Tie;
 pub use units::UnitSystem;
-pub use virtual_faces::{FaceMerge, Stitch};
+pub use virtual_faces::{FaceMerge, FaceSplit, Stitch};
 
 /// The largest node or element label. Nastran's GRID and element ids stop at 99 999 999 (eight
 /// digits in a small field), the tightest of the three formats.
@@ -180,6 +180,9 @@ pub struct Model {
     /// Stitches of imported surfaces, in the order they apply.
     #[serde(skip_serializing_if = "Vec::is_empty")]
     stitches: Vec<Stitch>,
+    /// Face splits along datum planes, in the order they apply (after the merges).
+    #[serde(skip_serializing_if = "Vec::is_empty")]
+    face_splits: Vec<FaceSplit>,
 }
 
 /// A [`Model`] as stored, before it is checked.
@@ -236,6 +239,8 @@ struct ModelData {
     face_merges: Vec<FaceMerge>,
     #[serde(default)]
     stitches: Vec<Stitch>,
+    #[serde(default)]
+    face_splits: Vec<FaceSplit>,
 }
 
 impl TryFrom<ModelData> for Model {
@@ -331,6 +336,9 @@ impl TryFrom<ModelData> for Model {
         for v in d.stitches {
             m.add_stitch(v)?;
         }
+        for v in d.face_splits {
+            m.add_face_split(v)?;
+        }
         Ok(m)
     }
 }
@@ -465,6 +473,7 @@ impl Model {
             mesh_plans: self.mesh_plans.clone(),
             face_merges: self.face_merges.clone(),
             stitches: self.stitches.clone(),
+            face_splits: self.face_splits.clone(),
         }
     }
 
@@ -515,7 +524,7 @@ impl Model {
     }
 
     /// Counts of each kind of definition, for summaries.
-    pub fn definition_counts(&self) -> [(&'static str, usize); 18] {
+    pub fn definition_counts(&self) -> [(&'static str, usize); 19] {
         [
             ("sets", self.sets.len()),
             ("materials", self.materials.len()),
@@ -535,6 +544,7 @@ impl Model {
             ("seeds", self.seeds.len()),
             ("face_merges", self.face_merges.len()),
             ("stitches", self.stitches.len()),
+            ("face_splits", self.face_splits.len()),
         ]
     }
 }

crates/model/src/virtual_faces.rs

@@ -33,6 +33,59 @@ pub struct FaceMerge {
     pub into: Option<String>,
 }
 
+/// Most face splits a model holds.
+pub const MAX_FACE_SPLITS: usize = 1024;
+
+/// A face of a body split along a datum plane for meshing (`split:<name>:a|b`).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct FaceSplit {
+    pub name: String,
+    pub body: String,
+    pub face: String,
+    /// The datum plane it splits along.
+    pub datum: String,
+}
+
+impl Model {
+    /// The face splits, in the order they apply (after the merges).
+    pub fn face_splits(&self) -> &[FaceSplit] {
+        &self.face_splits
+    }
+
+    /// Add a face split after the others.
+    pub fn add_face_split(&mut self, split: FaceSplit) -> Result<()> {
+        let name = split.name.clone();
+        check_name("face split", &name)?;
+        if self.face_splits.iter().any(|s| s.name == name) {
+            return Err(Error::Invalid(format!("face split `{name}` already exists")));
+        }
+        if self.face_splits.len() >= MAX_FACE_SPLITS {
+            return Err(Error::Limit(format!("a model holds at most {MAX_FACE_SPLITS} face splits")));
+        }
+        let invalid = |m: String| context("face split", &name, Error::Invalid(m));
+        for n in [&split.body, &split.face] {
+            if n.trim().is_empty() || n.len() > MAX_GEOMETRY_NAME_LEN {
+                return Err(invalid(format!("body and face names are 1 to {MAX_GEOMETRY_NAME_LEN} bytes")));
+            }
+        }
+        match self.datum(&split.datum) {
+            None => return Err(context("face split", &name, Error::NotFound(format!("datum `{}`", split.datum)))),
+            Some(d) if d.plane().is_none() => {
+                return Err(invalid(format!("datum `{}` is a datum {}; a split is along a plane", split.datum, d.kind())));
+            }
+            Some(_) => {}
+        }
+        self.face_splits.push(split);
+        Ok(())
+    }
+
+    pub fn remove_face_split(&mut self, name: &str) -> Result<FaceSplit> {
+        let i = self.face_splits.iter().position(|s| s.name == name).ok_or_else(|| Error::NotFound(format!("face split `{name}`")))?;
+        Ok(self.face_splits.remove(i))
+    }
+}
+
 /// Most stitches a model holds.
 pub const MAX_STITCHES: usize = 1024;
 
@@ -132,6 +185,23 @@ mod tests {
         FaceMerge { name: name.into(), body: "Base".into(), faces: faces.iter().map(|f| f.to_string()).collect(), into: None }
     }
 
+    #[test]
+    fn splits_need_a_datum_plane_and_hold_it() {
+        let mut m = Model::default();
+        m.add_datum("Mid", crate::Datum::Plane { point: [0.0; 3], normal: [1.0, 0.0, 0.0] }).unwrap();
+        m.add_datum("P", crate::Datum::Point { point: [0.0; 3] }).unwrap();
+        let sp = |n: &str, d: &str| FaceSplit { name: n.into(), body: "Base".into(), face: "F1:+z".into(), datum: d.into() };
+        m.add_face_split(sp("S", "Mid")).unwrap();
+        for bad in [sp("S", "Mid"), sp("T", "P"), sp("U", "nope"), FaceSplit { face: String::new(), ..sp("V", "Mid") }] {
+            assert!(m.add_face_split(bad).is_err());
+        }
+        assert!(m.remove_datum("Mid").is_err(), "in use by the split");
+        let back: Model = serde_json::from_str(&serde_json::to_string(&m).unwrap()).unwrap();
+        assert_eq!(back.face_splits(), m.face_splits());
+        m.remove_face_split("S").unwrap();
+        m.remove_datum("Mid").unwrap();
+    }
+
     #[test]
     fn stitches_are_kept_in_order_and_checked() {
         let mut m = Model::default();

docs/features.md

@@ -6,7 +6,7 @@ Every entry is a toolbar button. **Ready** entries run their command; **coming s
 shown greyed with the roadmap milestone that brings them (ROADMAP.md). Scripts, the command line,
 the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 
-81 ready, 37 coming soon.
+82 ready, 37 coming soon.
 
 ## Geometry
 
@@ -24,6 +24,7 @@ the control channel and AI agents over MCP (docs/mcp.md) run the same commands.
 | Prepare | Partition — Cut bodies into cells by a datum plane; the two faces of each cut are paired by name | ready | `geometry.partition`, (`geometry.partition.delete`), (`geometry.partition.suppress`) |
 | Prepare | Merge Faces — Mesh adjacent faces as one (virtual topology): a fillet in pieces, a face split by an imprint; the B-rep is unchanged | ready | `geometry.faces.merge`, (`geometry.faces.merge.delete`) |
 | Prepare | Absorb Faces — Absorb tiny and sliver faces into a neighbour for meshing, one picked or every one Check Geometry found | ready | `geometry.face.absorb` |
+| Prepare | Split Face — Split a face along a datum plane for meshing only: two faces on its surface, the cut an edge between them | ready | `geometry.face.split`, (`geometry.face.split.delete`) |
 | Prepare | Stitch — Close an imported surface's gaps under a tolerance: open points welded, T-junctions split | ready | `geometry.stitch`, (`geometry.stitch.delete`) |
 | Prepare | Midsurface — Extract midsurfaces of thin parts for shell meshes | coming soon (M8) |  |