If you're trying to look at 88 degrees and above on a door or something around your house, forget it. Because of the foreshortened viewpoint, you need a very very long slab of glass to be able to see clearly what is happening. You don't have one long enough. In my scene, the top slab at 89 degrees is over 20 feet long.

You also have to stand far away to have a somewhat consistent angle across the whole slab. Otherwise the closest part of the slab will be very much below 89 degrees.

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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)