Straightness, structure, and shadows. VSS 2001 Edward Adelson David Somers MIT

Illusions can inform us about the operation of the lightness system. Same gray

To see cool lightness demos (& paper): go to or

The snake illusion Diamonds are same gray. Illusion is enhanced Same diamonds, same backgrounds Illusion is slightly reduced Same diamonds, same backgrounds What about this reduction? Do wiggles interfere with discounting?

Example 1: folding the plaid adds junctions, 3D cues. Illusion is enhanced Illusion is slightly reduced Alternately look at +

Example 2: blur the bars to give penumbra Illusion is enhanced Illusion is slightly reduced + + +

Crooked strips enhance or reduce illusion Proposal: crooked vs. straight is a cue for the discounting process in lightness perception, just like penumbra or junctions. Enhanced Reduced + + +

Amplitude of wiggles Big wiggles: Big effect. Small wiggles: Smaller effect but still there

Rough texture works + +

Combine cues to build strong illusions. Articulation, biased statistics Smooth gradients Straightness of contour

Ripple strip

Snake cylinder

Summary Straightness & structure influence lightness illusions. Visual system may take structure as a cue to distinguish reflectance from illumination (atmosphere). I.e., visual system may “believe” that shadows are soft and straight, paint is crisp and crooked. There’s life left in plaids. For related paper and demos see

Checkerboard works, too. (Note that the checkerboard strips can be seen as transparent, but the illusion behaves as if the plain strips, not the checkerboards, were transparent.) + +

Effect of texture

Misalignment: do psi junctions matter?

Pinstripes

Wiggle styles

Lightness illusions For papers and demos see www-bcs.mit.edu/people/adelson

More to try Dark vs light articulation Artic in dark areas vs light areas Scission connection Add overall backgrnd texture Amplitude of sine Combine blur & straight, measure tradeoff Color Stereo Motion Figure ground

Observations and inferences Experimental observation: Straightness of contour effects strength of illusions. Big effects. Possible interpretation: The lightness system “assumes” that shadows/transparency/atmospheres tend to be straight, whereas reflectance/paint tends to be crooked. (Another observation: the illusion strength effects are not always predicted by subjective transparency & scission). (Another question: how to rephrase in terms of groups and frameworks?)

The basic intuition for using the plaid discounting decreases illusion discounting increases illusion If horizontal strip is transparentIf vertical strip is transparent

The basic intuition for using the plaid discounting decreases illusion discounting increases illusion If horizontal strip is transparentIf vertical strip is transparent

Frequency of wiggles 1 w/cycle 16 w/cycle It works for a wide range of frequencies.