Just an illusion: still images that move

Earlier this week I found myself on a long-haul flight to the SIGGRAPH computer graphics conference in Los Angeles. To pass the time I decided to catch up on some of the papers that are being presented here. But I had to stop when it came to the mind-warping, travel-sickness-inducing images that littered a paper by researchers from National Cheng Kung University in Taiwan and the Chinese University of Hong Kong.


For the full effect click the image above to enlarge or here.

Ming-Te Chi and colleagues have analysed a number of hand drawn examples of these 'self-animating images', such as the image above - "Rotating Snake" by Akiyoshi Kitaoka. Even though the viewer knows they are static the images unnervingly seem to move anyway. Chi's team is trying to find out why.

They've identified a number of important factors. The pictures appear to creep because of the arrangement of colour bands in the small repeated asymmetric patterns (dubbed RAPs). Certain combinations seem to give the impression of it creeping in a particular direction, although that effect is relatively weak. The illusion is strengthened if a ribbon of RAPs that appears to flow to the right is placed next to one that appears to flow to the left.