Quote - And with the center polygon un-welded from the others.

I see what you're showing me, and acknowledge that the steps you're taking to alter the mesh result in it being ... altered. But that's not the essence of my question.

What I was asking boils down to this: When you render a rectangle in whatever program it is you're using, does it look like a rectangle, or does it get rounded like you're showing here?

When you render four rectangles arranged in the three ways I uploaded above, do they look like my render or do they not look like rectangles anymore?

I am suspicious that my understand of the term Catmull Clark is incorrect. But if it is incorrect, and a CC-enabled renderer will round the corners off any un-welded edge, including the outer edges of the simple 3x3 group you showed, then I don't understand how people get rectangles to show up in such renderers. To get a simple rectangle, do you have to use edge loops?

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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)