##### 3.2.6.5 Sweep face splitting

Before sending each four-sided body polygon of a `sweep` to the output, `sketch` tests to see if it is roughly planar. Since planarity is necessary for proper functioning of the hidden surface algorithm, “warped” polygons are automatically split into two triangles.

Hole-filling polygons produced by closure-tagged line sweeps are not split. Nor are original polygons in polygon sweeps. It is the user's responsibility to ensure these are planar.