Some simple geometric patterns and designs, when repeated, can induce strong illusory shape distortions. Thus, visual misperceptions are very common in the op art world.
If dark and clear rectangular tiles are arranged in a checkered fashion, as shown below, there is obviously no illusion, but wait, just add at the intersection of these tiles some transparent dark and clear square patches, et voilà, magic appears!
So, transparencies can produce apparent linear distortions. The parallel red lines in this example look like being convergent / divergent.
We can continue the experiment with dark and light circles… Each circle is crossed by a diagonal segment of the same color and assembled as illustrated below.
Are the horizontal bars containing the circular designs parallel to one another? Yes, they are, and they also appear to slightly move / grow.
And what if we alternate some black and white X-shaped designs as shown in the example below?
Yes, the alignments of Xs look very slanted even though they are parallel in reality!