There can be several causes, but the most likely one (together with the pattern generating algorithm) would be that the Gamma of the display is slightly different from 2.2 . On LCD displays one may see these effects come and go as one changes the (often vertical) viewing angle towards the display. It often reminds me to recalibrate my display when I see it occurring more often than before.By the way, I notice some "hyperbolic" art well outside even the white circle. I have conjectured that this might be due to the fact that the luminance pattern across a radial is not perfectly sinusoidal. (It couldn't be, as that would require infinite precision in the underlying numerical construction of the function.) So perhaps the fundamental frequency of the circumferential variation is accompanied (at low amplitude) by some harmonics (likely mostly odd). In the region of interest, those frequencies would fall outside the Nyquist limit and thus create their own hyperbolic artwork.
Is that a credible conjecture?
I apologize for being a factor in the sleep deprivation.I wrote that piece after, at about 4:45 am, I awoke and couldn't go back to sleep. After it was done and posted, I went back to bed and slept until 9:30 am!
You're welcome. Inquiring minds just want to know how stuff like that works, my mind is no exception. And when I figure it out, I might as well share the findings, sometimes in the form of a tool.Thanks again for your wonderful work in bringing to us tools for measuring the performance of our toys, and for working with me as I try to reconcile basic and idealized theoretical concepts with actual behavior.