I like to build simple structures in Poser Pro 2010 by combining different prop shapes. That is, exporting a group of props as an OBJ then importing it as a single prop. Normally this works just fine.

But when adding props to a more complex structure in this way, some  polys may disappear from the added sections. I don't mean they are transparent, I mean they are gone. Just a few scattered polys remain from the prop(s) I have combined into the structure.

I cannot detect a rhyme or reason for why it happens, other than generally it seems to happen with a complex prop I am trying to add to. It is not consistent. Some props with many more polys and vertices may be unaffected, while a smaller (but still complex) prop has the problem.

Has anyone heard of this happening when merging props, and know of a way to avoid it?

would you please post the obj of the combined prop ?

You can also try setting the "normals forward"  option and see if that helps .

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maybe you could try display setting to wireframe (control shift 3) or hidden line (control shift 4) and might reveal if vertices are there or shape of certain faces.

"normals forward" sets the normals to face the camera during rendering. It has no effect in preview mode.

The poly's aren't just invisible or reversed, they are gone, missing, deleted. I have verified this with the Group tool. 

A strange thing: When I export the group of props as an OBJ, and I open that OBJ in a modeling app, all the polys are there. It is when it is imported back into Poser that some of the polys are deleted. (Not an entire former individual prop, but pieces of it.) Even when resaved as a new OBJ in the modeling app, the same thing happens when pulled back into Poser.

Somehow it is the combining of props in Poser that is screwing up the polys. And it happens when the construction has become pretty complex (but not always). Also, upon trying to reproduce it while merging the same group of props, sometimes different polys are removed, but still in the same former individual props. Rebuilding those props from scratch and using them instead doesn't solve the problem; once the overall structure reaches a certain level or type of complexity, almost anything that is merged into it will have missing polys. I am not sure what that state is, or how to avoid it.

The OBJ file is almost 2 MB, or I would have posted it. I was hoping someone else had experienced this and figured out what was causing it.

Probably the mesh has two sided polygons and are deleted during the export/import process. Poser doesn't like two sided faces that have common vertices.

No two-sided polys. Perhaps it would help if I illustrated the problem. This image shows the prop(s) before combining, and after. The two rounded shapes in the foreground are the props combined into the larger prop. You can see one of them is gone altogether, the other one has most of its polys deleted. Again, they are not invisible or reversed, they are gone.

Have you tried combing only one of the round shapes instead of both together?  If not, try combining with one shape, and then import the object again and combine the other round shape.

When you import the obj into Poser uncheck the option weld identical vertices and tell me if there is any difference so I can have a clue.

I tried adding one prop at a time instead of two. It did not matter.

I re-imported the combined prop with no options checked, including weld identical vertices. No difference. 

Note that I can combine these props with a different, less complex prop without problems. When I try to combine them with a different, equally complex prop, the same vanishing-polys thing happens. When I say complex, I mean less than 50,000 vertices, which is really not all that complex.

I'm starting to think the only solution might be to do it as a figure, with all new added pieces being body parts. I don't really like that solution because I try to have as few figures in my scenes as possible. More than a couple figures and Poser becomes slow and jerky.

I don't know what can be. Upload somewhere the mesh so I can examin it.

The problem turned out to be n-gons, that is, polygons with more than 4 sides. Poser likes 4-sided polys best, 3-sided ones most of the time (unless they are long and narrow), and n-gons not very much at all.

When adding new geometry to a model, i.e. by combining props, n-gons in any of the parts will cause polys to be deleted from the new parts. This doesn't happen 100% of the time, but often enough that it pays to make sure there are no n-gons before exporting anything from a 3D modeler for use in Poser. Too bad, because n-gons allow for more interesting and efficient modeling. N-gons are often created when beveling and smoothing corners in 3rd-party apps.

Subdividing every n-gon can create an ugly patchwork mesh that is more difficilt to work with, say with Poser's Groupiing Tool. But it must be done so Poser can handle it.

Another wonderful undocumented "feature" of Poser.