solidcore: a curvature fillet is refused until solidcore has the equal-area Gⁿ section

SMLib d01d7b0's capi builds the curvature section as the Gⁿ generator's order 2 at the
equal-area thumbweight (fork 6148773), taking off (1 - π/4) r² at a square corner. Our capi
port still built solidcore's blend-curve quintic (113/756 r²): a body every check passes,
off the closed form (862's box edge 7892.38 against 7845.49). solidcore has no Gⁿ generator
yet (solidcore#2250, port in progress as #2260), so Blend::Curvature now gives
Error::Unported, "not supported yet" with the issue, rather than another section. The
refusal comes off when the pin carries #2260 and 6148773 with fillet_gn_section_generator on.

The C++ closed-form case takes the equal-area volume; its solidcore twins are ignored until then.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SdcgEu4Lr3moCCisdZShKz

crates/kernel/src/blend_smlib.rs

@@ -445,6 +445,7 @@ mod imp {
                      run on tangentially into each other, or meet in a way no blend corner builds)"
                 )),
                 SmError::NoRoom => KernelError::TooLarge(format!("the {what} is larger than the faces beside its edges allow")),
+                SmError::Unported(why) => KernelError::Invalid(format!("not supported yet: {why}")),
                 e => failed(what, "operation", e),
             })?;
             // Checked here unless the evaluation's validation mode leaves it to the feature's end

crates/smlib/src/core/blend.rs

@@ -235,10 +235,6 @@ fn chamfer_radius(k: &Kernel, b: &mut Brep, e: EdgeId, distance: f64) -> solidco
 #[derive(Clone, Copy, PartialEq)]
 enum Section<'a> {
     Round,
-    /// The curvature-continuous section (`SM_FSG_BLEND_CURVE`, continuity 2), the quintic whose
-    /// inner control points lie a third and two thirds of the way from each contact to the corner
-    /// its tangents make (`SetBlendCurveAtThirds`, fork fd3b6a7).
-    Curvature,
     Chord,
     /// A chamfer, with each edge's offset of its second surface (`chamfer_offsets`' `second`;
     /// the sizes are the first's).
@@ -334,7 +330,6 @@ fn blend_edges_stopping(
     let before = face_set(b)?;
     let done = {
         let mut s = FilletSession::new(k, b)?;
-        s.set_blend_curve_at_thirds(matches!(section, Section::Curvature));
         // `SetFilletParameters(b, es, sections, solvers, values, laws, -1, -1, -1, -1,
         // continuity 1, thumbweight 1)`: the session sets the executive's global merge first, as
         // `SmApiFilletEdges` does, and `SetFilletParameters` resets it, as in the capi.
@@ -345,7 +340,6 @@ fn blend_edges_stopping(
                 Some(&size) => s.prepare_variable_radius_edges(k, edges, size, law, 1, 1),
                 None => return Ok(Err(Err(Error::Argument))),
             },
-            (Section::Curvature, _) => s.prepare_per_edge_constant_radius(k, edges, sizes, &[FilletSurfaceGeneratorType::BlendCurve], 2, 1.0),
             (Section::Chord, _) => s.prepare_per_edge_constant_distance(k, edges, sizes, &[FilletSurfaceGeneratorType::Circular], 1, 1.0),
             (Section::Chamfer(second), _) => chamfer_parameters(k, &mut s, edges, sizes, second),
         };
@@ -658,6 +652,14 @@ fn untagged(mut b: Brep) -> Out<Brep> {
     Ok(b)
 }
 
+/// Why [`Blend::Curvature`] is refused. The capi's curvature section is the Gⁿ generator's order 2
+/// at the equal-area thumbweight (`SMLIB_BLEND_THUMBWEIGHT`, fork 6148773), which solidcore does
+/// not have yet; the blend-curve quintic it does have takes off another area, so it is not built
+/// in its place. The refusal comes off when the solidcore pin carries #2260 and 6148773 and its
+/// `fillet_gn_section_generator` flag is on.
+pub(crate) const CURVATURE_SECTION: &str = "a curvature-continuous fillet on solidcore: the curvature section's equal-area Gⁿ \
+generator is not in solidcore yet (solidcore#2250, port in progress as #2260)";
+
 /// [`blend_holes`] without its holes.
 #[allow(clippy::too_many_arguments)] // the capi's blend_call
 pub(crate) fn blend(
@@ -749,7 +751,7 @@ fn blend_call(
         let solid_in = brep_p2::is_manifold_solid(&w)?;
         let section = match kind {
             Blend::Round => Section::Round,
-            Blend::Curvature => Section::Curvature,
+            Blend::Curvature => return Ok(Err(Error::Unported(CURVATURE_SECTION))),
             Blend::Chord => Section::Chord,
             // `chamfer_edges`: each edge's offsets from its setback at its midpoint.
             Blend::Chamfer => {

crates/smlib/src/core/blend_tests.rs

@@ -107,11 +107,11 @@ fn case(name: &str) -> Case {
             }
             c(Op::Blend(BLOCK, Blend::Round, edges, r), Expect::Volume(v))
         }
-        // Each section of a curvature-continuous round is the quintic from contact to contact
-        // whose inner control points lie a third and two thirds of the way to the corner the
-        // contacts' tangents make (fork fd3b6a7). On the unit square's sides it takes
-        // ½∮(x dy - y dx) = 113/756 from the corner: abc - 113/756 r² L.
-        "fillet_curvature_one_edge" => c(Op::Blend(BLOCK, Blend::Curvature, vec![TOP_FRONT], r), Expect::Volume(ABC - 113.0 / 756.0 * r * r * 30.0)),
+        // The curvature section's default thumbweight is equal area (fork 6148773): on the
+        // round's rails it takes off what the ball's circle does, abc - (1 - π/4) r² L.
+        "fillet_curvature_one_edge" => {
+            c(Op::Blend(BLOCK, Blend::Curvature, vec![TOP_FRONT], r), Expect::Volume(ABC - (1.0 - PI / 4.0) * r * r * 30.0))
+        }
         // abc - d² L / 2.
         "chamfer_one_edge" => c(Op::Blend(BLOCK, Blend::Chamfer, vec![TOP_FRONT], r), Expect::Volume(ABC - r * r * 30.0 / 2.0)),
         // abc - d1 d2 L / 2: 2 back along the top (the face leaning most along +z), 3 down the
@@ -387,7 +387,7 @@ macro_rules! cases {
 cases!(
     fillet_one_edge,
     fillet_all_edges,
-    fillet_curvature_one_edge,
+    fillet_curvature_one_edge: ignore = "solidcore refuses the curvature section until it has the equal-area Gⁿ generator (solidcore#2250)",
     chamfer_one_edge,
     chamfer_two_distances,
     chamfer_distance_angle,
@@ -662,6 +662,21 @@ fn a_setback_corner_left_open_returns_its_hole() {
     assert_eq!(refused, Some(Error::CornerRefused { corner: 0 }));
 }
 
+/// A curvature-continuous round is refused on solidcore, whose only curvature section is the
+/// blend-curve quintic (113/756 r² off a square corner, not the equal-area (1 - π/4) r²), until
+/// its pin has the Gⁿ section generator (solidcore#2250).
+#[test]
+fn a_curvature_round_is_refused_until_solidcore_has_the_equal_area_section() {
+    let b = BLOCK.core();
+    let r = blend(&b, Blend::Curvature, &[TOP_FRONT], &[2.0], TOL, &[], &[], Corners::Filled).err();
+    assert_eq!(r, Some(Error::Unported(super::blend::CURVATURE_SECTION)));
+    assert!(r.is_some_and(|e| e.to_string().contains("solidcore#2250")));
+    let set_back = blend(&b, Blend::Curvature, &[TOP_FRONT], &[2.0], TOL, &[[0.0, 0.0, 10.0]], &[3.0], Corners::Filled).err();
+    assert_eq!(set_back, Some(Error::Unported(super::blend::CURVATURE_SECTION)));
+    // A pick that misses is still named first.
+    assert_eq!(blend(&b, Blend::Curvature, &[[90.0; 3]], &[2.0], TOL, &[], &[], Corners::Filled).err(), Some(Error::NoMatch { point: 0 }));
+}
+
 /// The capi's argument checks, before any kernel call.
 #[test]
 fn hostile_arguments_are_refused() {

crates/smlib/src/lib.rs

@@ -85,6 +85,9 @@ pub enum Error {
     Solidcore(String),
     /// solidcore panicked; the guard caught it (the call, then the panic's message).
     Internal(String),
+    /// A step of the C++ kernel solidcore has not ported yet, refused rather than built another
+    /// way: what is missing, and the solidcore issue that ports it.
+    Unported(&'static str),
 }
 
 impl std::fmt::Display for Error {
@@ -124,6 +127,7 @@ impl std::fmt::Display for Error {
             Error::Record(u) => write!(f, "{u}"),
             Error::Solidcore(e) => write!(f, "solidcore: {e}"),
             Error::Internal(e) => write!(f, "solidcore failed internally in {e}"),
+            Error::Unported(what) => write!(f, "not supported yet: {what}"),
         }
     }
 }