The ratio of the values in the add of x * x + y * y can be altered. (Math_Functions_8 in my shader.)

 Here I've used 3 * x * x + y * y. This gives us an ellipse instead of a circle.

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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)