I’m using Zbrush to create models I can later use in an game engine(Unity). So, I would like to merge all the subtools like Body, Shirt, Pants together and delte all polygons that are not visible by that(f. ex. all body polygons that are hidden by the shirt should be deleted). Also, I would like to keep the subdivide history in this process, since I need the polygon count as low as possible.
Can Zbrush do that?
retopo is probably the only way to achieve everything you’d like.
You can try merging tools together, and decimating, but that probably won’t give you what you need.
I used the retopology and made a low poly version of the overall character. The only problem is the transfer of the details to the low poly model(I later use displacement and normal maps). Since the whole model has more than one subtool, It could be quite to make everything in one model. How can I do that?
I had the same problem, a fully dressed figure that had zillions of overlapping polys in the subtools, that I wanted to eliminate to lower the poly count. Decimation did not do the job, but this is how I managed it (though it took the better part of an hour). I will try to use the proper terminology here:
First, don’t forget to take a snapshot or save your original version in case you delete something you wish you hadn’t.
I had generated all these clothing and accessories by Mesh Extraction, which makes a double layer, inside and out, of polygons. Besides the hidden parts of the outfit, I wanted to delete the inside layers of the meshes, too.
In Polygroups, “Group by Polypaint”, then in Subtools, “Split Groups”. This gave something likde 256 separate subtools, which I then deleted or saved, one at a time, down the list, until I ended up with the ones I wanted. Slow, but it worked.
The other way I have had success in deleting overlapping sections, is to mask the offending part, click “Ctrl I” to invert the mask, then under “Visibility” click “Hide Pt”. This hides all unmasked areas. Then, in Subtools, click “Delete Hidden”. This is as close to erasing polys as I’ve been able to get, much cleaner than clip masks.
If you have not painted these subtools, you may not be able to split the groups, as the mesh will be one polygroup. Using “Backface Masking” in the Brush menu, to avoid bleed thru, you can paint on masks instead of
polypainting colours, and split the groups that way. I just happened to be working on a full colour model, I seem to be learning everything in reverse here! <LOL>
Good luck.
~Marcia Cash, Traveler in Thyme
The Polygroup trick is really clever. I wouldn’t have found it out myself 
I also tried the Hide-and-delete trick but its a bit hard to handle if you have a complex model. Nevertheless, it’s hard work to merge subtools seamlessly but nothing that’s impossible.
In the end, I created a retopology of the merged character, then deleted all overlapping polygons and everything, that isn’t showed because of the cloth from the original character. Then I tried to transfer the details from the original character to the new character and it worked quite well (but not everything got transfered, a little hand work but it works). Thanks travelerinthyme and beta_channel