I've been trying to generate a better 3D model of the galaxy NGC 4622 by creating a CMOD model with its surface height displaced on a 100x100 grid proportionately to the brightness of regions of NASA's picture of the galaxy.
Since Nebula models aren't depth sorted at all, I've been working on the model while viewing as if it were a moon.
In the screenshot below, the upper image is viewing the model while using a translucent PNG surface texture. the lower image is viewing the model while using an opaque JPG surface texture image.
(this links to a much larger picture)
When seen from one side, the model with the translucent surface texture looks OK. When seen from the "bad" side, however, it looks like the nearest surface is not being drawn in those locations where it should be hiding the far side of the model.
(The model has the same surface texture image drawn on both sides of both surfaces, so one could, in principle, go inside and look around.)
The model that uses the opaque JPG surface texture image looks fine from both sides.
I've put together an 'Addon' which demonstrates this problem:
http://www.lepp.cornell.edu/~seb/celestia/ngc4622-3d.zip
Unfortunately, it's a rather large 7.3 MB. This is because the archive includes two copies of the 4MB ASCII CMOD model. The two models are identical except for the name of the surface texture images specified within them.