kernel::sew refused a solid with a thin planar face away from the origin: a slab
30 x 2 mm and 0.00013 mm thick with a corner at (-300, -200, 5), turned 0.45 rad
about x, failed with "sew: face 1: cannot attach a plane to a wire that is not on
one plane", though its faces are flat to 1e-14 mm.
truck's plane fit sums the cross products of a boundary's control points about a
centre and takes the points' sum for that centre (it divides by the number of wires,
not of points): eight times as far from the origin as a rectangle of four lines. The
sum is the same about any centre, but its rounding grows with the square of the
centre's distance, and on a thin face, whose normal is short, it turns the normal far
enough for truck's own 1e-6 flatness test to fail: truck finds the slab's four thin
faces 3.0e-7 to 2.1e-6 mm out of the planes it fitted.
A planar face keeps truck's plane wherever truck gives one. Where truck refuses, the
same fit is made on a copy of the boundary moved so that the mean of its control
points is the origin, and the plane is moved back; a boundary that leaves its plane
or has no area is refused as before.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
