Description
This illusion is created when offset rows of alternating dark and light tiles are surrounded by a visible line of mortar. Ideally, the mortar is a shade somewhere between the two tile colors. When the tiles are offset by half a tile width, the horizontal lines appear to slant diagonally, creating the appearance of wedges. The illusionary effect is affected by both the position of the tiles as well as the thickness and color of the grout in between them. If grout lines are removed, there is no longer any illusion of diagonal lines.
Explanation of the Cafe wall illusion
Diagonal lines are perceived because of the way neurons in the brain interact. Different types of neurons react to the perception of dark and light colors, and because of the placement of the dark and light tiles, different parts of the grout lines are dimmed or brightened in the retina. Where there is a brightness contrast across the grout line, a small scale asymmetry occurs whereby half the dark and light tiles move toward each other forming small wedges. These little wedges are then integrated into long wedges with the brain interpreting the grout line as a sloping line.