You wrote on your site (http://www.polytope.net/hedrondude/polychora.htm):
Just wanted to check about that comment re filling. This is only for non-orientable polychora, right? In such cases I feel that there is no such thing as a density 2 region, so the mod-2 filling method is always used for such models, which does indeed add more holes. I don't see that there's a reliable alternative to this.Robert Webb, the designer of Stella software, has recently created Stella4D, which shows sections of all of the uniform polychora, although his software uses a different "filling method" than I use in my section pictures (Stella4D renders may have more holes and tunnels in them).
Consider this case in 3D:
(The yellow faces are crossed-quads and the red faces are hexagons).
A cross-section taken half way down the 5-fold symmetry axis yields a pentagram, but clearly that pentagram must be empty in this case. This only happens for non-orientable polyhedra (such as this one). So to avoid possibly filling a region that is actually external, I fill cross-sections for non-orientables using mod-2, even when those cross-sections are themselves orientable.
On the other hand, I've noticed as you say that some more complex polychora end up being mostly hollow, and sometimes elements align so that some odd-density regions are pinched down to nothing leaving large unfilled regions (even-density) connected to other unfilled regions.
So I don't know how to improve the situation. Do you have any suggestions?