Keep fine meshes whole at collapsed taper boundaries (#906)

The exact surface search could return an arbitrary parameter at a collapsed NURBS boundary, which corrupted the trimmed triangulation domain and lost volume on fine meshes of a fillet tapering to zero. Search results in the boundary projection are now validated, preferring a bounded parameter at a pole and snapping to a finite endpoint only when its transverse tangent has collapsed and the residual improves. Adds four regressions for decreasing chord tolerances, pole parameters, unbounded planar support and provisional preview.

Fixes #796.

vendor/truck-meshalgo/src/tessellation/triangulation.rs

@@ -46,17 +46,47 @@ fn clamped<S: ParametricSurface>(surface: &S, (u, v): (f64, f64)) -> (f64, f64)
     (c(u, ur, surface.u_period()), c(v, vr, surface.v_period()))
 }
 
-/// SolidCraft: a nearest-parameter search can run off a B-spline's domain and extrapolate to
-/// thousands of parameter units (meshing that grid never finishes). Outside the nominal domain
-/// keep the result only if it is really closer to the point than the clamped one (planes report
-/// a nominal unit range but extend for ever).
-fn sane<S: RobustMeshableSurface>(surface: &S, point: Point3, uv: (f64, f64)) -> (f64, f64) {
-    if in_domain(surface, uv) {
-        return uv;
-    }
+/// SolidCraft: exact and nearest searches can run off a B-spline's domain, especially at a
+/// collapsed edge. Prefer a bounded equivalent within the existing spatial search tolerance;
+/// otherwise retain extrapolation only when it is closer (planes have a nominal unit range
+/// but extend for ever). At most four boundary candidates are evaluated, with no new search.
+fn sane<S: ParametricSurface3D>(surface: &S, point: Point3, uv: (f64, f64)) -> (f64, f64) {
     let c = clamped(surface, uv);
     let d = |(u, v): (f64, f64)| surface.subs(u, v).distance2(point);
-    if d(uv) <= d(c) { uv } else { c }
+    let (mut best, mut error) = (uv, d(uv));
+    let dc = d(c);
+    let collapsed = |(u, v): (f64, f64), axis| {
+        let tangent = if axis == 0 { surface.vder(u, v) } else { surface.uder(u, v) };
+        tangent.magnitude2() <= TOLERANCE2
+    };
+    if !in_domain(surface, uv)
+        && (dc < error || (dc <= TOLERANCE2 && (collapsed(c, 0) || collapsed(c, 1)))) {
+        (best, error) = (c, dc);
+    }
+    // SolidCraft: at a collapsed boundary an exact search can return arbitrary,
+    // enormous parameters, or stop just inside the pole. Prefer an equivalent
+    // in-domain point and the exact boundary when it improves the spatial fit.
+    // A plane's nominal unit box must not clip genuinely unbounded geometry.
+    let (ur, vr) = surface.try_range_tuple();
+    for (axis, range, period) in [(0, ur, surface.u_period()), (1, vr, surface.v_period())] {
+        if period.is_some() { continue; }
+        if let Some((a, b)) = range {
+            for end in [a, b] {
+                if !end.is_finite() { continue; }
+                let coordinate = if axis == 0 { best.0 } else { best.1 };
+                let other = if end == a { b } else { a };
+                if (coordinate - end).abs() > (coordinate - other).abs() { continue; }
+                let candidate = if axis == 0 { (end, best.1) } else { (best.0, end) };
+                let distance = d(candidate);
+                // A closed spline seam has equivalent endpoints too, but moving
+                // between them changes the trim loop. Only canonicalize poles.
+                if distance <= TOLERANCE2 && distance < error && collapsed(candidate, axis) {
+                    (best, error) = (candidate, distance);
+                }
+            }
+        }
+    }
+    best
 }
 
 pub(super) fn by_search_parameter<S>(
@@ -83,8 +113,8 @@ where
     surface
         .search_parameter(point, hint, 100)
         .or_else(|| surface.search_parameter(point, None, 100))
-        .or_else(|| surface.search_nearest_parameter(point, hint, 100).map(|uv| sane(surface, point, uv)))
-        .or_else(|| surface.search_nearest_parameter(point, None, 100).map(|uv| sane(surface, point, uv)))
+        .or_else(|| surface.search_nearest_parameter(point, hint, 100))
+        .or_else(|| surface.search_nearest_parameter(point, None, 100))
 }
 
 /// Tessellates faces
@@ -393,7 +423,9 @@ impl PolyBoundaryPiece {
             .into_iter()
             .flat_map(|pt| {
                 let (mut u, mut v) = match sp(surface, pt, previous) {
-                    Some(hint) => hint,
+                    // SolidCraft: validate exact and nearest results alike, once per
+                    // boundary point, including the bounded provisional preview.
+                    Some(hint) => sane(surface, pt, hint),
                     None => return vec![None],
                 };
                 if let (Some(up), Some((u0, _))) = (up, previous) {
@@ -704,8 +736,8 @@ pub(super) fn preview_face<S: RobustMeshableSurface>(surface: &S, loops: &[Vec<P
     let search = |surface: &S, point, hint| {
         surface.search_parameter(point, hint, 16)
             .or_else(|| surface.search_parameter(point, None, 16))
-            .or_else(|| surface.search_nearest_parameter(point, hint, 16).map(|uv| sane(surface, point, uv)))
-            .or_else(|| surface.search_nearest_parameter(point, None, 16).map(|uv| sane(surface, point, uv)))
+            .or_else(|| surface.search_nearest_parameter(point, hint, 16))
+            .or_else(|| surface.search_nearest_parameter(point, None, 16))
     };
     let pieces = loops.iter().map(|w| {
         PolyBoundaryPiece::try_new(surface, std::iter::once(PolylineCurve(w.clone())), search)