the radial lines appear to rotate in counter-clockwise direction,
whereas when receding from it, they appear to rotate clockwise.
It presumably represents the simplest form of apparent motion illusions where self-motion is required for perceiving circular rotation.
Increase your distance from the chart until it is readable ...
Look at the picture above. Is it a flat door? Yes it is a flat door. The artist paint the door to make it looks bulged, convex or concave.
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