mesh: projection divisions from the mean edge length of each class, thin-layer tests (#418)

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

crates/engine/src/cmd/project/tests.rs

@@ -151,7 +151,7 @@ fn cup_params(size: f64, inset: Value) -> Value {
 fn cup_with_a_handle_at(size: f64) {
     let (_d, mut s) = cup_session();
     let r = s.execute("mesh.hex.project", &cup_params(size, json!(0.5))).unwrap();
-    assert!(sj(&r) >= 0.3, "{r}");
+    assert_thin_layers_hold(&mut s, &r);
     let (v, body) = (r["volume"].as_f64().unwrap(), r["body_volume"].as_f64().unwrap());
     assert!((v - body).abs() < 0.01 * body, "{r}");
     assert!(r["max_gap"].as_f64().unwrap() < 1e-6, "{r}");
@@ -168,14 +168,22 @@ fn cup_with_a_handle() {
 #[ignore = "heavy: cargo xtask test --heavy (CI heavy job)"]
 fn cup_with_a_handle_refined() {
     cup_with_a_handle_at(1.0);
+    cup_with_a_handle_at(0.75);
+}
+
+/// Every element valid, the scaled Jacobian at least 0.5 and no edge ratio above 10 (#418).
+fn assert_thin_layers_hold(s: &mut Session, r: &Value) {
+    assert!(sj(r) >= 0.5, "{r}");
+    let q = s.execute("mesh.quality", &json!({})).unwrap();
+    assert_eq!(q["valid"].as_u64(), Some(s.model.elements().len() as u64), "{q}");
+    assert!(q["metrics"]["scaled_jacobian"]["min"].as_f64().unwrap() >= 0.5, "{q}");
+    assert!(q["metrics"]["edge_ratio"]["max"].as_f64().unwrap() <= 10.0, "{q}");
 }
 
 /// Without the split, the ring block under the upper arm is squeezed between the petal's edge
-/// and the window-top plane and folds: the mesher refuses the mesh and names the twisted
-/// elements' block and the block junction they touch. (Smoothing now unfolds this grid to a
-/// scaled Jacobian of 0.22, so the check runs without it.)
+/// and the window-top plane: the mesher refuses the mesh and names the worst element's block.
 #[test]
-fn twisted_junction_elements_are_reported_not_meshed() {
+fn squeezed_junction_elements_are_reported_not_meshed() {
     let (_d, mut s) = cup_session();
     let mut p = cup_params(2.0, json!([0.5, 0.3, 0.3]));
     p["operations"] = json!([
@@ -189,47 +197,39 @@ fn twisted_junction_elements_are_reported_not_meshed() {
         {"op": "connect", "from": [45, "+z"], "to": [47, "-z"], "layers": 2},
         {"op": "remove", "blocks": [1, 2, 3, 4, 5]}
     ]);
-    p["smoothing"] = json!("none");
     let model = s.model.clone();
     let e = s.execute("mesh.hex.project", &p).unwrap_err().to_string();
-    assert!(e.contains("twisted") && e.contains("where blocks 5, 8, 12, 29, 30, 33 meet"), "{e}");
+    assert!(e.contains("smallest scaled Jacobian") && e.contains("worst in block 34"), "{e}");
     assert_eq!(s.model, model);
 }
 
 /// The cup without its handle: the floor's O-grid inside the wall's butterfly, cavity carved.
-#[test]
-fn cup_floor_double_o_grid() {
+/// The wall (4 mm) shares its radial divisions with the floor's longer radial edges; counted
+/// from the longest, the wall was cut into slivers that sheared at the cavity's rim, and
+/// smoothing collapsed their edges (edge ratio 172 at size 1, #418).
+fn cup_floor_double_o_grid_at(size: f64) {
     let (_d, mut s) = session(json!([
         {"command": "solid.cylinder", "params": {"radius": 20, "height": 30, "body_name": "Cup"}},
         {"command": "solid.cylinder", "params": {"base": [0, 0, 4], "radius": 16, "height": 27, "operation": "cut", "targets": ["Cup"]}}
     ]));
-    let mut p = cup_params(2.0, json!(0.5));
+    let mut p = cup_params(size, json!(0.5));
     p["operations"] = json!([{"op": "o_grid", "block": 0, "open": [1, 2], "inset": 0.6}, {"op": "remove", "blocks": [1, 2, 3, 4, 5]}]);
     let r = s.execute("mesh.hex.project", &p).unwrap();
-    assert!(sj(&r) >= 0.5, "{r}");
+    assert_thin_layers_hold(&mut s, &r);
     let (v, body) = (r["volume"].as_f64().unwrap(), r["body_volume"].as_f64().unwrap());
     assert!((v - body).abs() < 0.02 * body, "{r}");
 }
 
-/// Every element valid and no edge ratio above 10: smoothing must not collapse edges (#418).
-fn assert_no_collapsed_edges(s: &mut Session) {
-    let q = s.execute("mesh.quality", &json!({})).unwrap();
-    assert_eq!(q["valid"].as_u64(), Some(s.model.elements().len() as u64), "{q}");
-    assert!(q["metrics"]["edge_ratio"]["max"].as_f64().unwrap() <= 10.0, "{q}");
+#[test]
+fn cup_floor_double_o_grid() {
+    cup_floor_double_o_grid_at(2.0);
+    cup_floor_double_o_grid_at(1.0);
 }
 
-/// The handle-less cup at size 1: smoothing collapsed edges at the cavity's rim to an edge ratio
-/// of 172 (#418).
 #[test]
-fn cup_floor_smoothing_keeps_edges() {
-    let (_d, mut s) = session(json!([
-        {"command": "solid.cylinder", "params": {"radius": 20, "height": 30, "body_name": "Cup"}},
-        {"command": "solid.cylinder", "params": {"base": [0, 0, 4], "radius": 16, "height": 27, "operation": "cut", "targets": ["Cup"]}}
-    ]));
-    let mut p = cup_params(1.0, json!(0.5));
-    p["operations"] = json!([{"op": "o_grid", "block": 0, "open": [1, 2], "inset": 0.6}, {"op": "remove", "blocks": [1, 2, 3, 4, 5]}]);
-    s.execute("mesh.hex.project", &p).unwrap();
-    assert_no_collapsed_edges(&mut s);
+#[ignore = "heavy: cargo xtask test --heavy (CI heavy job)"]
+fn cup_floor_double_o_grid_refined() {
+    cup_floor_double_o_grid_at(0.75);
 }
 
 /// Thin layers like a sealed enclosure's: a 1.6 mm floor slab and 1 mm walls round a pocket.
@@ -248,7 +248,7 @@ fn thin_enclosure_layers() {
                 &json!({"body": "Case", "size": size, "grid": {"planes": {"x": [0, 1, 39, 40], "y": [0, 1, 29, 30], "z": [0, 1.6, 12]}, "carve": [{"i": [1, 1], "j": [1, 1], "k": [1, 1]}]}}),
             )
             .unwrap();
-        assert_no_collapsed_edges(&mut s);
+        assert_thin_layers_hold(&mut s, &r);
         assert!(sj(&r) > 0.99, "{r}");
     }
 }

crates/mesh/src/project/mod.rs

@@ -62,11 +62,16 @@ pub const DEFAULT_MAX_ARC_ANGLE: f64 = std::f64::consts::PI / 6.0;
 /// Samples per side of an outer block face when measuring how much its target turns.
 const CURVATURE_SAMPLES: usize = 32;
 
+/// The longest an element edge may be, in element sizes, when a class of parallel block edges
+/// takes its divisions from the mean of its lengths.
+const MAX_STRETCH: f64 = 2.0;
+
 /// How to project.
 #[derive(Debug, Clone, PartialEq)]
 pub struct ProjectOptions {
     /// The element size: each class of parallel block edges gets `round(length / size)`
-    /// divisions (at least one), from its longest projected edge.
+    /// divisions (at least one), `length` the geometric mean of its projected edges' lengths,
+    /// and enough that no element is longer than twice the size.
     pub size: f64,
     /// The largest angle (radians) the surface may turn through across one element on a curved
     /// target face: a class takes enough divisions that its elements are no longer than the
@@ -360,9 +365,16 @@ pub fn project_mesh(t: &Topology, surface: &BoundarySurface, g: &dyn Geometry, o
             }
         }
     }
-    // 2. Divisions per class of parallel edges, from the longest projected edge.
+    // 2. Divisions per class of parallel edges: the count whose element length is nearest the
+    // size, in ratio, over all the class's edges (the geometric mean of their projected
+    // lengths). A class runs through every block its edges are parallel in, so its edges can
+    // differ a lot in length (a cup's wall, 4 mm thick, and its floor's radial edges, twice
+    // that); the longest edge alone would cut the thin blocks into flat slivers. No element is
+    // longer than [`MAX_STRETCH`] sizes, so a coarse size still follows the geometry.
     let (classes, nclass) = edge_classes(t);
-    let mut class_len = vec![0.0f64; nclass];
+    let mut class_log = vec![(0.0f64, 0usize); nclass];
+    let mut class_max = vec![0.0f64; nclass];
+    let mut seen: BTreeSet<[u32; 2]> = BTreeSet::new();
     for (block, cls) in t.blocks.iter().zip(&classes) {
         for d in 0..3 {
             for (k, bits) in CORNERS.iter().enumerate() {
@@ -372,6 +384,9 @@ pub fn project_mesh(t: &Topology, surface: &BoundarySurface, g: &dyn Geometry, o
                 let mut e = *bits;
                 e[d] = 1;
                 let (a, b) = (block[k], block[corner_at(e)]);
+                if !seen.insert(edge_key(a, b)) {
+                    continue;
+                }
                 let (pa, pb) = (vpos.get(a as usize).copied().unwrap_or_default(), vpos.get(b as usize).copied().unwrap_or_default());
                 let c = edge_c(edge_key(a, b));
                 let mut len = 0.0;
@@ -381,8 +396,12 @@ pub fn project_mesh(t: &Topology, surface: &BoundarySurface, g: &dyn Geometry, o
                     len += dist(prev, x);
                     prev = x;
                 }
-                if let Some(l) = class_len.get_mut(cls[d]) {
-                    *l = l.max(len);
+                if let Some(m) = class_max.get_mut(cls[d]) {
+                    *m = m.max(len);
+                }
+                if let Some(l) = class_log.get_mut(cls[d]).filter(|_| len > tol) {
+                    l.0 += (len / opts.size).ln();
+                    l.1 += 1;
                 }
             }
         }
@@ -441,8 +460,15 @@ pub fn project_mesh(t: &Topology, surface: &BoundarySurface, g: &dyn Geometry, o
             }
         }
     }
-    let mut class_n: Vec<usize> =
-        class_len.iter().zip(&class_arc).map(|(l, &arc)| ((l / opts.size).round() as usize).max(arc).clamp(1, crate::block::MAX_DIVISIONS)).collect();
+    let mut class_n: Vec<usize> = class_log
+        .iter()
+        .zip(&class_max)
+        .zip(&class_arc)
+        .map(|((&(sum, k), &longest), &arc)| {
+            let mean = if k == 0 { 1.0 } else { (sum / k as f64).exp().round() };
+            (mean.max((longest / (MAX_STRETCH * opts.size)).ceil()) as usize).max(arc).clamp(1, crate::block::MAX_DIVISIONS)
+        })
+        .collect();
     for &(b, d, n) in &opts.divisions {
         let cls = classes.get(b).and_then(|c| c.get(d)).ok_or_else(|| Error::Invalid(format!("no direction {d} of block {b}")))?;
         if !(1..=crate::block::MAX_DIVISIONS).contains(&n) {

crates/mesh/src/project/tests.rs

@@ -79,7 +79,7 @@ fn seven_block_sphere_converges_without_losing_quality() {
     assert_eq!(topo.blocks.len(), 7);
     let exact = 4.0 / 3.0 * PI * r * r * r;
     let mut last_error = f64::INFINITY;
-    for size in [4.0, 2.0, 1.0] {
+    for size in [3.0, 1.5, 0.75] {
         let m = project_mesh(&topo, &s, &t, &options(size)).unwrap();
         let rep = &m.report;
         assert!(rep.min_scaled_jacobian >= 0.5, "size {size}: {rep:?}");