What to observe
Above you see a movie, and something moves, obviously, but doesn't really advance, does it?
By the way: You see a scene in the Swiss Alpes (near Flims) which I took while snowboarding.
Speed can be adjusted by the inter-frame delay in multiples of 17 ms.
The movie consists of altogether four frames. Two frames are consecuitve frames from a movie, the next two frames repeat the first two, but in the negative. The reason for the seemingly continuous forward motion is that our motion detectors are sign-dependent. Simultanous inversion of contrast and motion direction does not reverse perceived motion direction. This effect is also known as “four stroke motion”.
Anstis S (1970) Phi movement as a subtraction process. Vision Res 10:1411–1430
Anstis SM, Rogers BJ (1975) Illusory reversals of visual depth and movement during changes in contrast. Vision Res 15:957–961
Anstis SM, Rogers BJ (1986) Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception. Perception 15:627–640
George Mather (several motion demos)