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Footsteps Illusion
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In the figure above, the blue and yellow rectangles appear to be moving at alternating speeds, like footsteps, but are actually moving at exactly the same speed. Try staring at the bottom of the figure to make the illusion strong. You can confirm that the two rectangles are moving at the same speed by switching to a plain background.

ReferenceReferences

[1] S. M. Anstis, "Footsteps and inchworms: illusions show that contrast affects apparent speed," Perception, vol. 30, no. 7, pp. 785-794, 2001.
[2] S. M. Anstis, "Moving objects appear to slow down at low contrasts," Neural Networks, vol. 16, pp. 923-938, 2003.
[3] S. M. Anstis, "Factors affecting footsteps: contrast can change the apparent speed, amplitude and direction of motion," Vision Research, vol. 44, pp. 2171-2178, 2004.