In a tall window the stripes seem to move up; in a wide window they seem to move sideways. Their real motion is diagonal and the same in both.
Try this
- Change Window shape between tall, wide and square.
- Tick Remove the window to see the true diagonal motion.
Why it happens
A moving line seen through a window is ambiguous: your eye can only tell how it moves across its own length, not along it. The visual system resolves the ambiguity using the ends of the lines where they meet the window, and a long window has more ends along its long sides, so motion along the window wins. Nicola Fisher and Johannes Zanker studied how the perceived direction depends on the stripes and the window in 2001.
Did you know?
The same effect makes a real barber pole seem to spiral upward while it only turns.
References
- Fisher N., Zanker J. (2001). The Directional Tuning of the Barber-Pole Illusion. Perception, 30, 1321-1336. doi:10.1068/p3156
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Which way are the stripes really moving?
Diagonally. Remove the window to see it.
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