Module center_line_exact

Exact centre-line offsetting (ADR 0056).

center_line.rs offsets the FLATTENED path, which is correct for the tessellating pipeline but bakes a chord budget into the result. The exact extruder cannot accept that: flattening here would produce a solid that closes, audits clean, and is not the requested shape.

What can be offset exactly

Only curve kinds whose offset is the SAME kind, measured rather than assumed:

• a line offsets to a parallel line;
• a circular arc offsets to a CONCENTRIC arc of radius r -/+ d, keeping its centre, frame and parameter domain, so endpoints correspond at equal parameters;
• an ellipse does NOT offset to an ellipse (best-fit residual 0.074 for a 3:1 ellipse at distance 0.3), and neither do splines in general.

The last group is refused. Sampling it into chords is exactly the silent approximation this path exists to avoid.

Functions

center_line_contour

Resolve a centre-line profile into an exact closed contour.