What to see
Watch the patterns evolving above. Almost hypnotic.
What to do
Obviously, there are many parameters you can play around with. The most intriguing variant is the “Marroquin Pattern” (Marroquin 1976) which arises when the three grids are aranged in ±60° angles (select from the pop-up menu instead of “Default pattern”). A self-organising structure appears, circles at various sizes, squares, crosses, etc, which dynamically change while you observe them (Wilson et al. 2000).
Comments
Technically, this is a Moiré pattern arising from rotation (see here for moirés from translation). Basically, here are three regular grids of dots. One is stationary, the other two rotate with speeds differing by a factor of two. The relationship between the periodic patterns gives rise to regular neighbourhood relationships, which are immediately picked up by our visual system. This effect is often studied in “Glass Patterns” (Glass 1969, Dakin 1997). I thank José L. Marroquin for making me aware of his eponymous pattern.
Sources
Dakin SC (1997) The Detection of Structure in Glass Patterns: Psychophysics and Computational Model. Vision Res 37:2227–2246
Glass L (1969) Moiré Effect from Random Dots. Nature 223:578–580
Marroquin JL (1976) Human Visual Perception of Structure (MIT, thesis)
Wilson HR, Krupa B, Wilkinson F (2000) Dynamics of perceptual oscillations in form vision. Nature neurosci 3:170–176