Without the background, the red lines are plainly straight.
Straight, bent or tilted?
Wundt illusion
Are the two red lines straight?
Yes, though this time they seem to bend inward: the opposite of the Hering illusion.
Why? The slanting lines in the background pull the red lines the other way.
Picture: Fibonacci, CC BY-SA 3.0 · Wikipedia(opens in a new tab)