Enum SectionPlacement 

pub enum SectionPlacement {
    Transplant,
    Rigid,
}

HOW a section is placed at each station — the two things “sweep a profile along a path” can mean, and the reason they are not the same shape.

Transplant MOVES the profile onto the path: the anchor origin lands on the station point and the profile plane becomes the station’s NORMAL plane. The profile’s own position and its angle to the path are both discarded — every sweep comes out centred on the path and square to it. That is the classic swept-surface reading, and it is what SWP builds.

Rigid CARRIES the profile: the section at station k is the profile moved by the same rigid motion the PATH undergoes between its start and station k,

    section_k(x) = P_k + R_k · (x − P_0)

where R_k is the rotation taking the start frame (T_0, r_0, s_0) to the station’s (T_k, r_k, s_k). The profile keeps its drawn POSITION and its drawn ANGLE to the path; what changes along the sweep is only what the path itself does. Two consequences are the whole point of the mode:

• a STRAIGHT path makes every R_k the identity, so the section merely translates and the result is the oblique prism extrude_profile_brep builds from the same profile and vector — pathAlign and translate agree on a straight path rather than merely having the same volume;
• a circular ARC makes R_k the rotation about the ARC’S OWN CENTRE AXIS (for a planar path the RMF’s transport rotation IS that rotation), so the profile is carried round the pivot and the end cap arrives at the angle the start cap had to the tangent. A full-circle path degenerates to exactly a revolve of the profile about that axis;
• a HELIX makes R_k the SCREW about the helix’s own axis — but only when the path says it is one (SweepPath::screw_axes), because the frame that realizes it is not the RMF. A helix is carried into itself by the screw, which spins the tangent frame at the helix’s torsion; the RMF has no spin about the tangent by construction, so it rolls away from the screw by ∫τ ds (169.75° over three turns of a radius-5, pitch-5 coil). A helix segment’s stations are therefore framed off its axis (screw_reference), and every other segment keeps the RMF — which on a chain of lines and arcs in ANY planes is still exactly each arc’s turn about its own centre, composed. The two hand over at a joint without a roll.

Why not one frame for every curve: the Frenet frame is the screw on a helix but spins where a spatial curve nearly straightens (a planar S-cubic lifted 1e-3 out of its plane carries a 180° roll through the inflection, which 32 stations read as 0.4°), and one axis fitted to the whole run is the screw on a helix but rolls a two-arc spatial chain by up to 256° against its arcs.

R_k = F_k · F_0ᵀ does NOT depend on which perpendicular r_0 the RMF was seeded with: re-seeding rolls F_0 and every F_k by the same angle about the tangent, and the two rolls cancel in the product. That matters because Transplant maps pu → r_k directly, so under IT the profile’s roll about the path is whatever tangents[0].perpendicular() happened to return — an arbitrary orientation no caller can predict. Rigid has no such freedom.

Variants

Transplant

Move the profile onto the path, square to it (the classic sweep).

Rigid

Carry the profile by the path’s own rigid motion (see above).

Trait Implementations

impl Clone for SectionPlacement

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for SectionPlacement

impl Debug for SectionPlacement

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for SectionPlacement

impl PartialEq for SectionPlacement

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for SectionPlacement