Below: All the diamonds are identical. However, each row of diamonds looks darker than the row above it. This illusion is related to Craik-O’Brien-Cornsweet edges.
Cumulative Cornsweet effect. Each individual diamond on the left appears to have fairly uniform lightness which then increases from darker for the bottom diamonds to lighter for the upper ones (Watanabe, Cavanagh, & Anstis, 1995). This global lightness shift is opposite in direction to the actual luminance gradients within each diamond. In addition, the cumulative Cornsweet effect is more evident on the left for the pointy diamonds than on the right for the squat diamonds.
The diamond stimulus can be seen as a combination of a set of diamonds with uniform reflectance (left image), stepping up in reflectance from left to right, viewed under an illumination gradient that gets darker from left to right (right image). The result, on the right, can be a set of identical diamonds with the same mean luminance and the same internal gradients. The visual system then decomposes this into uniform reflectances increasing from left to right seen under a gradient of illumination.
A set of identical spiky bars each having the same luminance gradient also produces a cumulative lightness increase as the mean luminance across the borders increases along the border, as it does in the diamond pattern.