Displays the 0-level of cos((x^2+y^2)/r^2), based on the values computed on the pixel grid.
Start with this video as an introduction, to see the funny phenomenon (when r decreases, instead of being a set of concentric circles as expected, the function becomes quasi-periodic).
(to see correctly, don't resize but display the video with its natural size -- by the way, videos are quite big [e.g. 20MB], and it's interesting to hit 'pause' to watch the frames)
