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 center of the set of stars. In the default example, the innermost star is white, the outermost star is black, and the gradient from the center to the outside star has 20 luminance steps. The illusory folds that radiate from the center 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.
To do: Use the sliders below the star to vary the parameters. When you invert the contrast of the stars, the perceived sign of the illusory folds (bright or dark) also inverts. The illusory folds are strongest when the range of contrast between the innermost and outermost star is maximized (i.e. when the “contrast” slider is all the way to the left or to the right). Increasing the number of steps between the innermost and outermost star also tends to increase the strength of the illusory folds.
To notice: When you change the angle of the corner (“corner at star tips” slider) the strength of the illusory folds varies. Sharp corner angles result in strong illusory folds, and shallow corner angles result in weak illusory folds. We call this effect Corner Angle Salience Variation.
Vasarely’s nested squares: The ABS illusion is closely related to Vasarely’s “nested squares”: several decades ago, the op-artist Victor Vasarely discovered that nested squares of increasing or decreasing luminance result in illusory diagonals, which appear either brighter or darker than the rest of the squares. This illusion is sometimes called Vasarely’s pyramid, or Vasarely’s illusion. The interactive ABS illusion on this site can be easily transformed into Vasarely’s nested squares. Set the number of points to 4, and the corner at star tips to 90º, and there you go!
Significance: The ABS illusion demonstrates that sharp corners appear more salient to our visual system than shallow corners or straight edges.
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