The Alternating Brightness Star (ABS) illusion is made of concentric stars of graded luminance. The physical luminance of each individual star remains constant at all points; however the corners of the stars are perceptually more salient than the straight edges, forming illusory folds that irradiate from the very centre of the set of stars. In the default example, the innermost star is white, the outermost star is black, and the gradient from the centre to the outside star has 20 luminance steps. The illusory folds that radiate from the centre of the set of stars appear light or dark depending on the polarity of the corner angle and on the direction of the luminance gradient (black-to-white or white-to-black). In the default example, the illusory folds leading to the tips of the star appear bright, whereas the illusory folds leading to the valleys in between the tips of the star appear dark. However, all illusory folds (dark or bright) are physically identical to each other in luminance. We call this effect Corner Angle Brightness Reversal. Design by Susana Martinez-Conde