Notes on the Illusions - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 331

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Illusions of Lightness
An illusion of lightness is an image in which we misperceive the lightness (or luminance ) of some
portion of the image. The simplest lightness illusion is the Simultaneous Contrast illusion , in which
areas of identical lightness appear lighter or darker depending on the background against which
they're displayed. This effect has been known since ancient times. It is demonstrated by the images
on pages 163 and 165. In both of these images, the central rectangle and the second frame from the
outside are the same shade of gray throughout, but they appear to get lighter as the surrounding
frame gets darker.
The checkerboard pattern on page 23 is based loosely on the Craik-O'Brien-Cornsweet illusion . The
top and bottom squares appear lighter than the left and right squares, but all four squares are
identical. The lighter corners of the top and bottom squares point inward, while the lighter corners
of the left and right squares point outward. Changes in lightness at the edges where the squares
meet influence your perception of the relative lightness of the squares.
Logvinenko's illusion (page 189) was devised by vision scientist Alexander Logvinenko
[Logvinenko99] . Strange as it may seem, the horizontal cube faces are all the same shade of gray. If
you think the horizontal faces in the first and third rows are lighter than the ones in the second and
fourth, cover the vertical faces with a mask and prepare to be surprised. The effect underlying this
illusion is closely related to the Craik-O'Brien-Cornsweet illusion.
A similar effect is exploited in Todorovic's Gradient Chessboard illusion (page 149), devised by
vision scientist Dejan Todorovic [Todorovic97] . Though the disks on the checkerboard appear to be
of three different shades of gray, they are all the same.
The Vasarely illusion , devised by op artist Victor Vasarely, is shown on pages 228 and 229. There
appears to be a dark cross on the diagonals of the light version (page 228) and a light cross on the
diagonals of the dark version (page 229), but no such crosses exist.
The drawing on page 211 is based on White's effect [White79] . The gray bars separated by white
bars appear lighter than the gray bars separated by black bars, but all the gray bars are the same.
The drawing on page 135 is based on Adelson's Illusion of Haze [Adelson99] . The central diamond,
which appears clear, is exactly the same shade of gray as the two flanking diamonds, which appear
hazy.
The drawing on page 127 is based loosely on Kitaoka's Light of Chrysanthemums (2005), which
uses Zavagno's Glare effect [Zavagno99] . The center of the flower appears unnaturally bright and
an illusory fog appears between the petals.
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