OK, Found it! I couldn't see it until I remembered an old example of visual perception shown in almost every psychology text book. When I looked at it that way, it happened immediately.
Edit: incidentally, the psychology text example of a visual phenomena never impressed me as something important. It was like the writers of the text just put it there to make the book seem more interesting. I never understood how it affected my life or my thinking. I'm sure there was more of a point to it than that, but I never understood the importance of the point.