Stroboscopic Artifacts

from “Visual Phenomena & Optical Illusions

Kinetic Depth Effect, Stereokinetic

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What to do

The colour wheel on the right initially spins quite slowly. The top right buttons select various speeds, the buttons below allow fine adjustments. Can you achieve a seemingly stationary gray disk?

What to observe

For slow speeds, below about 3 rps (rps = rotations/s) there is fairly smooth movement. For higher speeds we see jumbled segments, backward motion etc. At some high speeds the motion disappears entirely, leaving only a bit of flicker.

Comment

Motion demonstrations on a computer screen, especially when fast motion is involved, suffer from stroboscopic artifacts. A grander designation would be “temporal aliasing”. So I decided to make a teaching point out of a nuisance – my original goal was to demonstrate additive colour mixture.

The disk does not rotate smoothly (even if it might look so) but rather is presented in rapidly succeeding still frames, each with a different rotation angle. This produces the percept of smooth motion (within certain limits), the “Phi Phenomenon” of Wertheimer. The current demo is set up with a frame rate of 120 Hz and, initially, an angle increment ∆ of 7° every 20 ms. However, here in your browser another aspect comes into play: the frame rate of your display. Many LCDs have a refresh rate of 60 Hz, CRTs come in rates from 60 to 200 Hz. So what you see exactly depends on the interaction of the two frame rates and the angle increment, and the intermediate effects may not be pretty ;-). Under some conditions you may see a slow backwards rotation. This is well known as the “wagon wheel effect”.

Sources

Bach M, Meigen T, Strasburger H (1997) Raster-scan cathode ray tubes for vision research – limits of resolution in space, time and intensity, and some solutions. Spatial Vision 10:403–414


 

Created: 2004-Jan-30


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Last update 2013-10-04 by Michael Bach (G+)