Fix/shell saddle corners (#1316)

Thanks @Ian-cmd-ops.

crates/engine/tests/shell_regressions.rs

@@ -0,0 +1,88 @@
+//! Shell regressions found by the robustness fuzzer (tests/robustness/goldens.json).
+//!
+//! The box-on-box designs below have a saddle corner: where the top box's left face meets the
+//! lower box's back face, four planar faces meet with convex and concave edges in turn (the
+//! top box overhangs the lower box's back face). A shell splits that corner into two corners
+//! joined by a new edge.
+//!
+//! Lower box `[0, 25] × [0, 17.5] × [0, 35]`, top box `[7.5, 22.5] × [0, 17.5 + o] × [35, 42.5]`.
+//! With the bottom face open, a shell of thickness `t` leaves this cavity (for `o < 17.5 − 2t`):
+//!
+//! - in the lower box: `[t, 25 − t] × [t, 17.5 − t] × [0, 35 − t]`
+//! - in the top box, over the lower box: `[7.5 + t, 22.5 − t] × [t, 17.5 − t] × [35 − t, 42.5 − t]`
+//! - in the overhang: `[7.5 + t, 22.5 − t] × [17.5 − t, 17.5 + o − t] × [35 + t, 42.5 − t]`
+//!
+//! The shell's volume is the solid's volume less those three boxes.
+use serde_json::json;
+use solidcraft_engine::Session;
+
+fn run(s: &mut Session, id: &str, p: serde_json::Value) {
+    if let Err(e) = s.execute(id, &p) {
+        panic!("{id} {p}: {e}");
+    }
+}
+
+/// The box on a box, overhanging the lower box's back face by `o`.
+fn box_on_box(o: f64) -> Session {
+    let mut s = Session::default();
+    run(&mut s, "solid.box", json!({"height": 35.0, "length": 25.0, "width": 17.5}));
+    run(&mut s, "solid.box", json!({"corner": [7.5, 0.0, 35.0], "height": 7.5, "length": 15.0, "operation": "join", "width": 17.5 + o}));
+    s
+}
+
+/// The shell's volume with the bottom face open (see the module comment).
+fn open_bottom_shell_volume(o: f64, t: f64) -> f64 {
+    let solid = 25.0 * 17.5 * 35.0 + 15.0 * (17.5 + o) * 7.5;
+    let lower = (25.0 - 2.0 * t) * (17.5 - 2.0 * t) * (35.0 - t);
+    let above = (15.0 - 2.0 * t) * (17.5 - 2.0 * t) * 7.5;
+    let overhang = (15.0 - 2.0 * t) * o * (7.5 - 2.0 * t);
+    solid - lower - above - overhang
+}
+
+/// Seed 83 (minimised): the overhang is 2.5 mm, and the shell opens the top box's left face.
+/// The shell refused the saddle corner (`a corner whose faces don't meet the same way`).
+#[test]
+fn shell_box_on_box_overhanging_saddle_corner() {
+    let mut s = box_on_box(2.5);
+    run(&mut s, "solid.shell", json!({"faces": [[7.5, 17.5, 38.75]], "thickness": 1.5}));
+    sound(&s);
+}
+
+/// The same saddle corner with the bottom open: every face round the corner moves, and the
+/// volume is exact.
+#[test]
+fn shell_saddle_corner_volume_is_exact() {
+    for t in [1.0, 1.5] {
+        let mut s = box_on_box(2.5);
+        run(&mut s, "solid.shell", json!({"faces": [[12.5, 8.75, 0.0]], "thickness": t}));
+        sound(&s);
+        let (got, want) = (volume(&s), open_bottom_shell_volume(2.5, t));
+        assert!((got - want).abs() < 1e-6 * want, "thickness {t}: shell volume {got}, want {want}");
+    }
+}
+
+/// An overhang (1 mm) thinner than the wall (1.5 mm): the overhang's cavity is only 1 mm deep.
+/// A corner landing outside the solid, or a wall left thin, would change the volume.
+#[test]
+fn shell_saddle_corner_with_an_overhang_thinner_than_the_wall() {
+    let mut s = box_on_box(1.0);
+    run(&mut s, "solid.shell", json!({"faces": [[12.5, 8.75, 0.0]], "thickness": 1.5}));
+    sound(&s);
+    let (got, want) = (volume(&s), open_bottom_shell_volume(1.0, 1.5));
+    assert!((got - want).abs() < 1e-6 * want, "shell volume {got}, want {want}");
+}
+
+fn volume(s: &Session) -> f64 {
+    s.model.state().bodies.iter().map(|b| solidcraft_kernel::measure(&b.body).map(|m| m.volume).unwrap_or(f64::NAN)).sum()
+}
+
+fn sound(s: &Session) {
+    let errors: Vec<_> = s.model.results.iter().filter_map(|r| r.error.clone()).collect();
+    assert!(errors.is_empty(), "{errors:?}");
+    let st = s.model.state();
+    assert_eq!(st.bodies.len(), 1, "one body");
+    for b in &st.bodies {
+        assert_eq!(b.body.shell_count(), 1, "one shell");
+        assert!(b.body.validity().is_empty(), "{:?}", b.body.validity());
+    }
+}

crates/kernel/src/offset.rs

@@ -459,6 +459,7 @@ fn split_corner(
     // normal, so its inside is to the left; the edge is convex when the face beyond it falls
     // away from there.
     let mut sign = 0.0f64;
+    let mut mixed = false;
     let mut pairs: Vec<(usize, usize)> = Vec::new();
     for &(fi, na, ..) in &planes {
         for w in faces.get(fi)?.boundaries() {
@@ -474,7 +475,7 @@ fn split_corner(
                     continue;
                 }
                 if sign * s < 0.0 {
-                    return None;
+                    mixed = true;
                 }
                 sign = s.signum();
             }
@@ -483,33 +484,55 @@ fn split_corner(
     if sign == 0.0 {
         return None;
     }
+    // Which triples of faces may meet at a new corner. All convex or all concave: any triple
+    // whose point is inside all the other moved planes. Mixed (a saddle, such as a box on a
+    // box that overhangs its side): the faces round the corner in order, and each new corner
+    // on three faces in a row (the two new edges on the middle face meet there). A saddle's
+    // faces bound no convex region, so a right corner can be outside another face's moved
+    // plane, and the inside-all-the-others test does not apply. What keeps a corner in place
+    // is that it is where its middle face's two new edges meet; a corner past the far end of
+    // one of the corner's edges turns that edge round, and a face whose loops cross after the
+    // move is refused (both checked in `offset_body`).
+    let triples: Vec<[usize; 3]> = if mixed {
+        let ring = face_ring(&pairs, planes.len())?;
+        let n = ring.len();
+        (0..n).map(|i| [ring[(i + n - 1) % n], ring[i], ring[(i + 1) % n]]).collect()
+    } else {
+        let mut t = Vec::new();
+        for ia in 0..planes.len() {
+            for ib in ia + 1..planes.len() {
+                for ic in ib + 1..planes.len() {
+                    t.push([planes[ia].0, planes[ib].0, planes[ic].0]);
+                }
+            }
+        }
+        t
+    };
+    let plane_of = |fi: usize| planes.iter().find(|x| x.0 == fi);
     // Convex: the body's side, n·x ≤ d; concave: the outside's, n·x ≥ d.
     let side = -sign;
     let merge = size * 1e-6;
     let mut nodes: Vec<(Vec3, Vec<usize>)> = Vec::new();
-    for (ia, a) in planes.iter().enumerate() {
-        for (ib, b) in planes.iter().enumerate().skip(ia + 1) {
-            for c in planes.iter().skip(ib + 1) {
-                let den = a.1.dot(b.1.cross(c.1));
-                if den.abs() < 1e-9 {
-                    continue;
-                }
-                let x = (b.1.cross(c.1) * a.3 + c.1.cross(a.1) * b.3 + a.1.cross(b.1) * c.3) * (1.0 / den);
-                if !x.is_finite() || x.dist(p) > size || planes.iter().any(|q| side * (q.1.dot(x) - q.3) > merge) {
-                    continue;
-                }
-                let fs = [a.0, b.0, c.0];
-                match nodes.iter_mut().find(|(q, _)| q.dist(x) < merge) {
-                    Some((_, nf)) => {
-                        for f in fs {
-                            if !nf.contains(&f) {
-                                nf.push(f);
-                            }
-                        }
+    for [fa, fb, fc] in triples {
+        let (a, b, c) = (plane_of(fa)?, plane_of(fb)?, plane_of(fc)?);
+        let den = a.1.dot(b.1.cross(c.1));
+        if den.abs() < 1e-9 {
+            continue;
+        }
+        let x = (b.1.cross(c.1) * a.3 + c.1.cross(a.1) * b.3 + a.1.cross(b.1) * c.3) * (1.0 / den);
+        if !x.is_finite() || x.dist(p) > size || (!mixed && planes.iter().any(|q| side * (q.1.dot(x) - q.3) > merge)) {
+            continue;
+        }
+        let fs = [fa, fb, fc];
+        match nodes.iter_mut().find(|(q, _)| q.dist(x) < merge) {
+            Some((_, nf)) => {
+                for f in fs {
+                    if !nf.contains(&f) {
+                        nf.push(f);
                     }
-                    None => nodes.push((x, fs.to_vec())),
                 }
             }
+            None => nodes.push((x, fs.to_vec())),
         }
     }
     // Every face keeps a corner here, and every edge an end.
@@ -527,6 +550,39 @@ fn split_corner(
     Some(nodes)
 }
 
+/// The faces round a corner in order, from the pairs of faces on each of its edges: `None`
+/// unless they form one simple ring of `count` faces (each face meets the corner once, between
+/// exactly two neighbours).
+fn face_ring(pairs: &[(usize, usize)], count: usize) -> Option<Vec<usize>> {
+    let mut next: HashMap<usize, Vec<usize>> = HashMap::new();
+    for &(a, b) in pairs {
+        for (x, y) in [(a, b), (b, a)] {
+            let ns = next.entry(x).or_default();
+            if !ns.contains(&y) {
+                ns.push(y);
+            }
+        }
+    }
+    if next.len() != count || next.values().any(|ns| ns.len() != 2) {
+        return None;
+    }
+    let first = *next.keys().min()?;
+    let mut ring = vec![first];
+    let mut prev = first;
+    let mut cur = *next.get(&first)?.first()?;
+    while cur != first {
+        if ring.len() >= count {
+            return None;
+        }
+        ring.push(cur);
+        let ns = next.get(&cur)?;
+        let to = if ns[0] == prev { ns[1] } else { ns[0] };
+        prev = cur;
+        cur = to;
+    }
+    (ring.len() == count).then_some(ring)
+}
+
 /// The corners of a split corner (see [`split_corner`]) on face `face`, in order from corner
 /// `from` to corner `to`, along the new edges between them.
 fn split_path(nodes: &[(Vec3, Vec<usize>)], face: usize, from: usize, to: usize) -> Option<Vec<usize>> {

tests/robustness/goldens.json

@@ -84,7 +84,7 @@
   "80": {"area":8652.533342,"bodies":1,"edges":20,"error":null,"faces":9,"outcome":"ok","rejected":0,"shells":1,"steps":["box","fillet","fillet"],"unsupported":0,"volume":37387.449469},
   "81": {"area":4001.847983,"bodies":2,"edges":75,"error":null,"faces":29,"outcome":"ok","rejected":1,"shells":2,"steps":["revolved section","cut sphere","circular pattern","rectangular pattern","hole","join cylinder","join box","cut sphere"],"unsupported":2,"volume":13139.658749},
   "82": {"area":3195.487112,"bodies":1,"edges":30,"error":null,"faces":13,"outcome":"ok","rejected":0,"shells":1,"steps":["box","hole","circular pattern","cut box"],"unsupported":0,"volume":5902.162954},
-  "83": {"area":5125.0,"bodies":1,"edges":28,"error":"offset: a corner whose faces don't meet the same way after t","faces":12,"outcome":"failed","rejected":0,"shells":1,"steps":["box","join box","rectangular pattern","shell"],"unsupported":0,"volume":19250.0},
+  "83": {"area":8489.059183,"bodies":1,"edges":97,"error":null,"faces":35,"outcome":"ok","rejected":0,"shells":2,"steps":["box","join box","rectangular pattern","shell","fillet","cut sphere","join box"],"unsupported":1,"volume":6217.394627},
   "84": {"area":4906.192237,"bodies":1,"edges":67,"error":null,"faces":25,"outcome":"ok","rejected":2,"shells":1,"steps":["extruded profile","fillet","hole","hole","cut cylinder","cut box","fillet"],"unsupported":0,"volume":14219.865875},
   "85": {"area":3309.210936,"bodies":1,"edges":57,"error":null,"faces":24,"outcome":"ok","rejected":1,"shells":2,"steps":["revolved section","fillet","cut sphere","circular pattern","join cylinder","hole","join box"],"unsupported":0,"volume":6904.455565},
   "86": {"area":7885.256491,"bodies":1,"edges":66,"error":"offset: a corner whose faces don't meet the same way after t","faces":28,"outcome":"failed","rejected":0,"shells":3,"steps":["extruded profile","join cylinder","join cylinder","circular pattern","rectangular pattern","shell"],"unsupported":2,"volume":21498.10695},
@@ -122,6 +122,7 @@
   {"reason":"#391 after merging main with #1213: seed 41 now reaches its second fillet (built since #1213), and the end-snap fix again drops a sliver edge the cuts left (94 to 93 edges); its steps, outcome, faces and shells are unchanged, volume moves by 0.005 mm³ (5185.215737 to 5185.221068) and area by 0.0014 mm²","seeds":[41]},
   {"reason":"the chamfer bug is on main too, see #1390","seeds":[57]},
   {"reason":"#1390: a chamfer of a convex edge that would grow the part is refused (TooLarge). Seed 57's 0.5 mm chamfer on the sphere cut's rim, which grew the part since #391 reordered its steps, is now refused, so the run goes on to its later steps and ends ok (16017.21 mm³, 24 faces)","seeds":[57]},
+  {"reason":"Seed 83 builds: a shell splits a saddle corner (convex and concave edges in turn) into two corners","seeds":[83]},
   {"reason":"seed 15 now cuts around a sealed void","seeds":[15]},
   {"reason":"#1490: when the last-resort nudge fails both ways, two more screw axes are tried; seed 63's final box join (its face 0.00005 mm off the half-ring's cut face) now builds instead of being refused with an implausible volume, and the design goes on to its fillet","seeds":[63]}
  ]