What you describe is possible with a concave geometry mirror, or a flat one with a carefully designed normal map applied. Since you didn't mention a normal map, your surface must not be flat.

If the inversion happens around 20 feet away then the radius of curvature is 20 feet. A mirror about 4 feet in height with an indentation of about 1.3 inches would produce such an effect.

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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)