Notes on the Illusions - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 332

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Compound Illusions
Compound illusions combine two or more illusory effects. The drawing on page 160 is based on
Ehrenstein's figure , discovered by Walter Ehrenstein in 1941. It combines subjective contours with
an illusion of lightness. Your mind perceives circles where the grid lines converge, and the circles
are unnaturally bright.
The Subjective Necker Cube (pages 33 and 224) was devised by Bradley and Petry [Bradley77] . It
combines subjective contours with an ambiguous figure. You can see two cubes, one at a time, but
the figure contains none. It merely suggests the edges that comprise these cubes.
The drawing on page 57, based on an illusion by vision scientist Nicholas Wade, combines
subjective contours with an illusion of lightness: Your mind perceives concentric circles at the radii
where the arcs cross, and the circles appear brighter than the surrounding page.
The drawings on pages 48 and 49 are based on the Neon Square illusion of Marc Albert and Donald
Hoffman [Albert99] . This illusion combines subjective contours with the Neon Spreading effect
[van Tuijl75] . Not only do you perceive squares where none exist, but the squares have illusory
color. The illusory square on page 48 appears hazy and the one on page 49 appears dark.
The drawing on page 125 uses an effect similar to the one that Kitaoka used in cushion, bulge, and
checkered flag. The drawing looks like a spiral, but it consists of concentric circles. If you move
towards the drawing or away from it, it appears to rotate.
The drawing on page 92 combines the "out-of-focus" effect found in Kitaoka's Earthquake (2001)
with MacKay's squares. The frame appears to float above the defocused grid, and the grid appears
to vibrate.
Finally, the front cover illustration combines the Peripheral Drift illusion, the Scintillating Grid, and
a third unnamed illusion. The outer ring of fishes appears to rotate clockwise, the inner ring appears
to rotate counterclockwise, the white disks at the intersections of the grid lines appear to sparkle,
and the grid inset appears to move slightly relative to the fishes.
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