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step: a swept surface's circle or ellipse profile is its NURBS form

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A surface of linear extrusion or of revolution over a circle or ellipse built its profile with the
reader's own conic spline, on a parameter of its own; the profile is now `form::curve`, the form
its edges carry, so the surface's parameter along the profile is theirs and their pcurves are
straight. The reader's conic spline is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BnQawySUFx57iaJZq7z1Fm
This commit is contained in:
jyviko
2026-10-23 18:20:36 -04:00
parent 69702833ff
commit 819928205a
1 changed files with 11 additions and 29 deletions
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crates/step/src/topology/swept.rs
+11 -29
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@@ -24,8 +24,6 @@ use crate::geom::{Axis1, Curve, P3, Placement, Surface};
/// Margin around a face's parameters on a profile, as a fraction of their span.
const MARGIN: f64 = 0.1;
/// Most sweep of one rational quadratic conic piece.
const CONIC_PIECE: f64 = FRAC_PI_2;
/// Most parameter step of one rational quadratic hyperbola piece (its weight `cosh(step / 2)`
/// stays near 1).
const HYPERBOLA_STEP: f64 = 1.0;
@@ -45,31 +43,6 @@ fn p3(v: Vec3) -> P3 {
[v.x, v.y, v.z]
}
/// The ellipse of `radii` in `p` from angle `t0` over `sweep` > 0 as rational quadratic pieces
/// whose knots are the pieces' end angles.
pub(super) fn conic_spline(p: &Placement, radii: [f64; 2], t0: f64, sweep: f64) -> BCurve {
let n = ((sweep / CONIC_PIECE).ceil() as usize).clamp(1, 8);
let dt = sweep / n as f64;
let w = (0.5 * dt).cos();
let at = |c: f64, s: f64| p.at(radii[0] * c, radii[1] * s, 0.0);
let mut knots = vec![t0; 3];
let first = at(t0.cos(), t0.sin());
let mut homogeneous = vec![[first[0], first[1], first[2], 1.0]];
for i in 0..n {
let ta = t0 + dt * i as f64;
let (tm, tb) = (ta + 0.5 * dt, ta + dt);
// The meeting point of the end tangents, weighted cos of the half sweep.
let m = at(tm.cos() / w, tm.sin() / w);
homogeneous.push([m[0] * w, m[1] * w, m[2] * w, w]);
let b = at(tb.cos(), tb.sin());
homogeneous.push([b[0], b[1], b[2], 1.0]);
let k = if i + 1 == n { t0 + sweep } else { tb };
knots.extend([k, k]);
}
knots.push(t0 + sweep);
BCurve::RationalSpline { knots, homogeneous }
}
/// The hyperbola branch `a cosh t X + b sinh t Y` from `t0` to `t1` (t0 < t1) as rational
/// quadratic pieces whose knots are the pieces' end parameters.
pub(super) fn hyperbola_spline(p: &Placement, a: f64, b: f64, t0: f64, t1: f64) -> BCurve {
@@ -212,6 +185,11 @@ fn span(ts: &[f64], seam: &[f64], closed: bool) -> R<Span> {
Ok(Span::Open { inner: (lo, hi), outer: (lo - m, hi + m) })
}
/// A conic's NURBS form over `[lo, hi]` in angle.
fn form(c: BCurve, lo: f64, hi: f64) -> R<BCurve> {
Ok(solidcraft_geom::brep::form::curve(&c, [lo, hi]).ok_or_else(|| unsupported("a conic profile of more than one turn"))?.0)
}
/// The profile over its span as an exact spline (B-spline and polyline profiles whole).
fn bounded(c: &Curve, s: Span) -> R<BCurve> {
let (lo, hi) = match s {
@@ -223,8 +201,12 @@ fn bounded(c: &Curve, s: Span) -> R<BCurve> {
}
Ok(match c {
Curve::Line { origin, direction, magnitude } => line_spline(*origin, *direction, *magnitude, lo, hi),
Curve::Circle { position, radius } => conic_spline(position, [*radius; 2], lo, hi - lo),
Curve::Ellipse { position, semi_axis_1, semi_axis_2 } => conic_spline(position, [*semi_axis_1, *semi_axis_2], lo, hi - lo),
// A circle or ellipse as its NURBS form (`form::curve`), the one its edges carry: the swept
// surface's parameter along the profile is then theirs, and their pcurves are straight.
Curve::Circle { position, radius } => form(BCurve::Circle { frame: super::frame(position), radius: *radius }, lo, hi)?,
Curve::Ellipse { position, semi_axis_1, semi_axis_2 } => {
form(BCurve::Ellipse { frame: super::frame(position), radii: [*semi_axis_1, *semi_axis_2] }, lo, hi)?
}
Curve::Hyperbola { position, semi_axis, semi_imag_axis } => {
if hi - lo > HYPERBOLA_STEP * MAX_PIECES as f64 {
return Err(unsupported("a hyperbola profile that long"));