ifc-alignment

IFC4X3 linear positioning: IfcAlignment with its horizontal, vertical and cant layouts, referents and stationing, and linear placement. Alignment curves are lowered exactly into the format-neutral Axiolid geometry model; nothing is tessellated or numerically integrated. Transition spirals are curvature laws, a CUBIC is its cubic parabola trimmed by arc length, a vertical arc is the circle itself, and cant makes the centreline a banked curve. A form the file does not determine (a vertical CLOTHOID, whose segment states no curvature) is a typed refusal, and the open gaps are listed on the capabilities page.

cargo add ifc-alignment

The openbim-ifc facade also provides it behind its alignment feature.

• API documentation: docs.rs/ifc-alignment
• Reference page: openbimrs.github.io/ifc
• Source and issues: github.com/openbimrs/ifc

Design notes

• A vertical profile read from a file is checked at the seam tolerance the file declares: IfcGeometricRepresentationContext.Precision (3D, coarsest, in the project length unit, capped at 1 mm). Exporters that round stations and heights to their stated precision are accepted; a step beyond it is refused. Without a declared precision only floating-point rounding is tolerated (SeamTolerance, vertical_profile_law).

• A grade break at a height-continuous vertical seam is legal IFC4.3 and accepted; the piecewise elevation law carries it exactly, and VerticalLayout::seams reports it. require_tangential asks for a tangent profile explicitly.

• The zero-length segment IFC4.3 requires at the end of every layout adds no geometry; its start is checked against the layout's end like any seam. A zero-length segment anywhere else is refused.

• Cant is exact data per station (CantLayout::frame_at_distance: rail heights, cant, bank angle arcsin(D / b), rotation-point elevation and the section frame), and the cant-carrying centreline (IfcSegmentedReferenceCurve) is a Curve3::Banked (lower_segmented_reference_curve): the gradient curve, the cant law D = left - right and the pivot (left + right) / 2, one piece per cant segment in the IFC4.3 base formula, rotating the section about the 3D tangent by arcsin(D / b) (BankConvention::TangentRotation, the angle reading IFC4.3 states). A pivot that moves through a Viennese bend is refused: it would follow the bank angle, which a height-form pivot law cannot carry.

• A multi-segment horizontal layout elevates as one exact plan curve: the first segment's start frame plus one curvature piece per segment, or an arc-length chain when it holds a CUBIC. Seams are checked in closed form: a heading kink after a segment with a curvature law, or a position gap after a line or arc, is refused; the position after a transition spiral, and the position and heading after a CUBIC, are reported as authored rather than verified.

• A CUBIC is IFC4.3's y = x^3 / (6 R L) leaving a straight, with SegmentLength along the curve: an exact cubic Bezier trimmed at TrimSelector::ArcLength, the inverse left to the kernel. A CUBIC that starts curved has no formula in the standard and is refused.

• A vertical CIRCULARARC is ElevationLaw::CircularArc from StartHeight, StartGradient and the signed RadiusOfCurvature (positive is a sag), not the EN 13803 parabola that approximates it.

• This crate is the geometric bridge for alignments, not a road or rail application. Product workflows (corridors, cross-sections, track design) and rendering policy stay out of it and belong to the application that composes it.