I’ve a pretty good idea what non-manifold geometry is, however, there is something i would like to have clarified. To help out i will provide two examples of simple meshes for you to look at.
My First example is a coat i modeled for a character of mine. Notice at the end of the sleeve, all i have done is extrude and fold the geometry in to create the appearance of volume/ thickness, the same was not done for the opening at the front of the coat. The other sleeve is missing because my character wont need it. A mechanical arm will be placed there.
In my second sample i have created a Jersey. Here i have done the same for the openings, extruded and folded the geometry inward.
Would either or both of these meshes be considered non-manifold?
My reason is because i have run into problems with displacement maps from zbrush in maya. I have read that the issue is caused by non-manifold geometry.
Would it be a good idea to model an inner wall to such a mesh like the ones i have? Or would they be fine as they are?