## Skyscrapers and Clouds

Sandro Bettella, Clara Casco and Sergio Roncato

This movie requires Flash Player 9

Here is a novel illusion that everyone can experience when the jagged contours of skyscrapers appear against the cloudy sky: with the clouds still behind, the skyscrapers contours appear to bulge out and the effect magnifies when the clouds move.
Most illusory distortions of parallel lines disappear if contours are jagged, but not our new illusion: we experience the “Skyscrapers and clouds” illusion because the visual system relies on local luminance contrast to code local tilts and positions along the contour bordered by a thin outline.

Skyscrapers and Clouds

## Pinball Wizard

Michael Pickard

Sunderland University, UK

The interesting thing about the Pinball Wizard illusion is that it breaks the ‘rules’. Whilst the classic Rubin Vase illusion demonstrates how we automatically segregate foreground and background in an image, in this illusion a single image is seen acting simultaneously as both, giving rise to an illusory sense of rotation.
Using visual cues to create an impression of depth and carefully chosen colour values, a static screen is combined with an animation of horizontally traversing spheres. The screen appears simultaneously as background and as foreground surface on the spheres – inducing a sense of rotation as the spheres move.

Pinball Wizard

## The Mutually Interfering Shapes Illusion (The MISillusion)

Maarten Wijntjes, Robert Volcic & Tomas Knapen

Utrecht University, The Netherlands

A circle’s a circle and a square’s a square, right? Wrong! Just look at the center of our MISillusion display and you’ll see why – Two dots are moving around bouncing off one another. Do you see a square and a circle? They really are! The illusion becomes even stronger when tracing the inner square closely: you’ll see the outer dot moving along four arcs instead of one circle. If you now follow the outer dot, the square suddenly seems curved inward!
So watch out next time you track a baseball pitch, your brain might just throw you a curveball!

The Mutually Interfering Shapes Illusion (The MISillusion)

## It’s a Circle, Honest!

David Whitaker

The illusion in the figure on the left consists of two sinusoidal gratings at 45° and 135° which combine to form a plaid. The contrast of this plaid is windowed by a perfect circle. Despite this, the percept is far from circular – rather, it appears octagonal with distinct sides. The percept is generated by attraction and repulsion of the circular envelope in the orientation domain by the sinusoidal carrier gratings. It relies upon the sharp transition between Fraser illusion (attraction) and Zöllner illusion (repulsion) at the knee-points of the octagon.
Whilst the illusion is scale-invariant in that it does not change with viewing distance, if the scale of the carrier grating is lowered (Figure on the right) relative to the circle, the percept changes from an octagon to a diamond. This is well-predicted by the variation in the strength of the Fraser and Zöllner illusions as the relative spatial scale of carrier and envelope is varied (Skillen et al. (2002) Vision Research 42, 2447-2455).

It’s a Circle, Honest!

## Coffer Illusion

Anthony Norcia

Smith-Kettlewell Eye Research Institute, USA

First time viewers of this display invariably do not see the 16 circles segmented from the background. Rather, they see a series of rectangles that they frequently describe as “door panels”. The illusion pits segmentation cues against what appears to be a very strong prior to interpret the image as a series of 3-D structures “coffers” with closed boundaries. (A coffer is a decorative sunken panel.) It appears that the prior involves both closure and shape-from shading assumptions. The Coffer Illusion is a variation on Gianni Sarcone’s “Op Art illusion”.

Coffer Illusion

## Clones and Donors Have Opposite Inclinations (in Vision)

Oronzo Parlangeli & Sergio Roncato

Fig. 1 and Fig. 2 show the same inducing pattern.
However, if the blue lines in fig 1 are seen as donors, we can easily appreciate that when they give birth to two clones as in fig 2, these clones grow apart and they show a diametrically opposed inclination: convex lines become concave and concave lines becomes convex.
A clone for each donor is sufficient to produce the effect.