Haha - I was wrong. It is a linear interpolation of the start and end position. I did not have enough to reveal the difference in the past.

But it's not a simple blend of two frames, either.

Notice that the shadow is not blurred - just the spinning white board.

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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)