This illusion is catching flame to be quite interesting. As our beloved illusion contributor, Akiyoshi Kitaoka has shared on his Twitter.
Analyzing these turtle shapes in depth gives my eyes and brain a tease. See if we draw the attention to the shapes, they are all the same. However, it is the color of every alternating one that is giving the illusion of this bend or curved pattern.
Kitaoka flipped the color and contrast of this piece without the lightness and it is another version of this illusion.
Some users are debating that they see dark-gray turtles and light-gray ones because they are NOT the same luminance. if averaged over the area — though they do have identical luminance at their center points. However, it is confirmed that the texture is axis-aligned and the colors are indeed the same.
Still looking for more? Check out other Kitaoka illusions here.