Dynamic illusory waves generated by relative motion in periodic displays

We describe a new display involving what we are calling ‘dynamic illusory waves’. The display brings together a number of previously documented illusory effects and perceptual mechanisms to generate non-rigidly deforming phantom surfaces that take even experienced observers by surprise. There are only two physically drawn components to the display: a shape with a sinewave-modulated contour and a regular ‘pinstripe’ background grating. When the shape is superimposed on the grating, and relative motion is added, a third element immediately emerges. This is a dynamic illusory wave (diwa): a coherent, deforming illusory surface that often appears as a transparent layer partially occluding the two physically drawn components. The nature of the illusory wave is primarily a function of the spatial frequency ratio between the periodic patterning of the physical shape and the grating, but changing the relative speed, amplitude, or contrast of the drawn components also modulates the nature of the perceived surface. We discuss and make available an online interactive tool so that readers can further create and explore their own examples of such dynamic illusory waves.