Straight lines along the edges of the diamonds: every edge is straight.
Straight, bent or tilted?
Warping cushion
Are the slanting edges between the diamonds straight, or do they bend like the edges of a puffed-up cushion?
Straight: every edge between the diamonds is a perfectly straight line, and every stripe is straight up and down.
Why? Where a black stripe meets a gray one at an edge, the step from one to the next seems to lean a little. Your brain joins all the small leans along an edge into a curve, so the edges bow and the whole picture seems to swell and sink like a cushion.
Picture: Drawn by Science Playground