Cafe wall illusion. This illusion is so named because it was first noticed on a tiled wall in a Bristol cafe. The pattern in the image appears to show sloping grey lines between the black and white tiles", but in reality the lines are parallel. This effect is due to the compromises that both the brain and the light receptors on the retina (back of the eye) have to make when image processing. There is no reason for the tiles to be black and white for the illusion to be achieve - it is their relative intensity which affects the outcome, not their colour."