Yes, then no. Yes, 3DS objects have triangles with consistently
outward-pointing normals, although they also have a "double-sided"
material attribute for rendering randomly oriented triangles.
No, there are certainly examples of popular 3D programs that
use many-sided polygons: Wavefront, Softimage, Vertigo, Prisms,
Sense8 and LightWave come to mind, and the list is much longer.
>Quick question: How does InterChange handle conversion from a format
>which supports non-convex polygons to one which only supports convex
>ones? Say from Open Inventor to 3D Studio? Do you decompose them?
We only export Open Inventor, but in other situations InterChange
does decompose concave polygons by chipping off triangles until
the base polygon becomes convex, when a destination format requires it.