Internal variable _slice2_precision controls how slice2 (or slice2x) handles points very close to the slicing plane or surface. PRECISION should be a positive number or zero. Zero PRECISION means to clip exactly to the plane, with points exactly on the plane acting as if they were slightly on the side the normal points toward. Positive PRECISION means that edges are clipped to parallel planes a distance PRECISION on either side of the given plane. (Polygons lying entirely between these planes are completely discarded.)