Structure from motion at equiluminance


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On the right you see a swarm of red moving balls. On closer inspection they organise themselves into a rotating sphere. This sphere may rotate left- or rightwards, and may even switch spontaneously, but that's another illusion ;-).


Now change the relative brightness of the red balls versus the green background by clicking on the buttons at the colour ↑↓ arrows. Start by pressing ‘↑↑’ repeatedly, so the green becomes very dark. A sphere composed of the red balls is clearly apparent. Now pressing ‘↑’ a number of times brings you into the range of equiluminance where the red balls, while differing in hue from the green background, do not differ in brightness (techically: luminance) any more. [The r:g-values where this occurs depend strongly on your screen settings and your colour vision.]


Here –at equiluminance– a number of interesting effects can be observed, namely: (1) the 3-dimensional percept loses salience; the sphere becomes nearly flat. (2) Some balls seem to flicker, especially those with large steps from frame to frame. I assume that the φ-phenomenon breaks down at equiluminance (this may be an original observation, I’m not aware of any publication on this).


This demonstration may be taxing your computer power (in other words: appear jerky), a faster & more realistic C++ with OpenGL version is available on request.



Created: 2002-08-27

Last update: 2013-10-04