Module seam 

Seams and poles for periodic faces that arrive without them.

A STEP face on a cylinder may be a band between two rings that each go round once, with no seam edge: one ring a whole circle, the other a notched run of arcs and rulings. A face on a sphere may be a cap bounded by one circle, an octant whose two meridians meet at the pole with no edge along the pole’s row, or a hemisphere bounded by one great circle through both poles. Each is a valid solid’s boundary and meshes, but in the surface’s chart the boundary does not close (or, for the great circle, has no exact image), and anything that works in the chart (a boolean’s arrangement) cannot use it.

The band gets a seam at a column where each ring passes once: a vertex of the notched ring, with the whole circle re-anchored there when it has no vertex of its own on that column. The cap gets a pole vertex, a degenerate pole edge and a seam down to it. The octant gets a degenerate edge along the pole’s row between the two meridians. The great circle is split at the poles into half meridians, each a straight line in the chart, with a degenerate edge along the pole’s row between them.

Functions

seam_periodic_faces

Give every seamless periodic face of a solid the seam or pole edge its chart needs to close.