What to see
The pattern at the right is stationary – yet it seems to expand, especially if you don't look directly at it, for instance when reading this text.
Not everybody perceives this illusion, for as yet ill-understood reasons.
What to do
This is one of the many variants of the “Snake Illusion” as made famous by Akiyoshi Kitaoka. So what is special here? The controls in the present demonstration allow you to explore the figural components to optimise the illusion.
First thing to recognise: colour is nice, but not necessary. So for ease of control the figure switches to grey when adjusting the brightness.
The upper slider controls the relative brightness (technically: the luminance) of the medium bright stripes. As the diagram at the bottom right shows in a cross-section of part of the figure: there are narrow very dark (bluish) and very bright (yellowish) stripes, which I will call anchor stripes. In between these are wider stripes with a darker and a lighter grey. The double arrows indicate that these levels can be adjusted (with said slider). When they are equal (slider in the middle), there is no illusionary motion. When their brightness is exchanged, the illusory expansion changes to contraction.
The number stepper left of the sliders adjusts the numerosity of the star figure. As you may see, this does not play a major role. Below are color pickers where you can change the colours to your heart's content. If you're lost, click on the “reset” button.
Clément Depoutre points out that he has a similar design (scroll to bottom). I may have seen it (a case of “source agnosia”?) when I programmed the above demonstration. His design from June 2007 has been “adopted” by many internet sites.
More literature on the companion page