Oh wow! I dont do animations, so for me its particularly impressive.
I think it behaves in the somewhat unexpected manner because of the way the greyscale bit map looks. I noticed some small distortions in my experiments too. Lines of equal greyness arnt precisely radial, they seem to have a spiral component. I dont know how to fix that because my 2d
 image editing skills are so minimal :)
Good luck in your next attempts, and keep posting!

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
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