As you gaze around this figure you might see that some green “streets” appear curved. On direct inspection, however, they are quite straight – but bend again when looking elsewhere.
Quite number of parameters need to be just so to allow this illusory curvature to appear. Many aspects can be tried out using the various controls.
This nice design is from illustrator Lesha Porche (USA). When it appeared, it quickly made the rounds, and I thank Jeremy Hinton of “Lilac Chaser” fame to alert me.
My explanation attempt is as follows: The tiles contain these low-resolution brightness curves, but they are masked by the borders. It can be made more apparent by applying a little blur (4–5). The blur occurs automatically in our visual system, because –largely unnoticed– our visual resolution rapidly falls off from the center of gaze: at 10° eccentricity our visual acuity is 1/10th of the central one. However, the periphery does not “feel” blurred, does it? Its sharpness is “invented” by our visual system, just as our blind spots are “filled in”. A similar effect occurs with Ryota Kanai’s Healing Grid, where also information in the periphery is “invented”. The ill-defined hedge borders are removed by blur, and the low-resolution brightness information takes over. Quite likely this may not be the full story, though.