Class-A surfacing 1/3: the continuity core, a G3 free-form blend and patch crown (#1249)

The continuity core (kernel bridge: Hermite control rows reproducing a support surface's position, tangent, curvature and change of curvature, G0 to G3), surface.blend between two edge chains (#764), and a crown for fitted patches. Existing patches and fillets are unchanged by default (crown 0 is the plain patch bit for bit). SurfaceBlend and Patch.crown are noted under format 22.

crates/kernel/src/spline_conv.rs

@@ -4,6 +4,7 @@
 use solidcraft_geom::bspline::{HPoint, NurbsSurface};
 use truck_modeling as mt;
 
+use crate::body::p3;
 use crate::{KernelError, Result};
 
 fn fail(m: impl std::fmt::Display) -> KernelError {
@@ -34,3 +35,30 @@ pub(crate) fn surface(s: &NurbsSurface) -> Result<mt::Surface> {
     let rows: Vec<Vec<mt::Vector4>> = (0..nu).map(|i| (0..nv).map(|j| h4(s.point(i, j))).collect()).collect();
     nurbs_surface((mt::KnotVec::from(ku.clone()), mt::KnotVec::from(kv.clone())), rows)
 }
+
+/// One face of a whole tensor surface: its four boundary curves as edges, sharing the corners.
+/// This is what a patch built as a single control net (`bridge`, `boundary`, the T-spline sheet)
+/// hands to the B-rep.
+pub(crate) fn tensor_face(s: &NurbsSurface) -> Result<mt::Face> {
+    let [nu, nv] = s.count();
+    if nu < 2 || nv < 2 {
+        return Err(fail("a face needs at least 2 x 2 control points"));
+    }
+    let [ku, kv] = s.knots();
+    let corners = [s.point(0, 0), s.point(nu - 1, 0), s.point(nu - 1, nv - 1), s.point(0, nv - 1)];
+    let mut v: Vec<mt::Vertex> = Vec::with_capacity(4);
+    for c in corners {
+        let p = c.project().ok_or_else(|| fail("a corner has no position"))?;
+        v.push(mt::builder::vertex(p3(p)));
+    }
+    let row = |j: usize| -> Vec<HPoint> { (0..nu).map(|i| s.point(i, j)).collect() };
+    let column = |i: usize| -> Vec<HPoint> { (0..nv).map(|j| s.point(i, j)).collect() };
+    let (Some(v0), Some(v1), Some(v2), Some(v3)) = (v.first(), v.get(1), v.get(2), v.get(3)) else { return Err(fail("corners")) };
+    let edge = |a: &mt::Vertex, b: &mt::Vertex, c: mt::Curve| mt::Edge::try_new(a, b, c).map_err(fail);
+    let bottom = edge(v0, v1, nurbs_curve(mt::KnotVec::from(ku.clone()), &row(0))?)?;
+    let right = edge(v1, v2, nurbs_curve(mt::KnotVec::from(kv.clone()), &column(nu - 1))?)?;
+    let top = edge(v3, v2, nurbs_curve(mt::KnotVec::from(ku.clone()), &row(nv - 1))?)?;
+    let left = edge(v0, v3, nurbs_curve(mt::KnotVec::from(kv.clone()), &column(0))?)?;
+    let wire: mt::Wire = vec![bottom, right, top.inverse(), left.inverse()].into();
+    mt::Face::try_new(vec![wire], surface(s)?).map_err(fail)
+}