Now we're likely to speak past each other as you and I make differing assumptions and use words we both know but inadvertently do not agree on the meaning.

There is no doubt that the set of points directly between two vertices is a straight line.

Where we disagree is in our understanding of what Poser creates for the interior points of a polygon.

Poser will not render the edges of a polygon as straight, but rather as a curve, if smoothing is on and the adjacent polygons are not coplanar. There is also no doubt that the surface normal of a polygon will differ from a truly flat one if the adjacent polygons are not coplanar and below the crease angle, regardless of whether smoothing is on or off. The notion of a concave polygon is quite literally real in Poser, even if you don't directly model it.

I can show you curved polygons quite easily.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)