Straightness, structure, and shadows.

VSS 2001

Edward AdelsonDavid Somers

MIT

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

Same gray Same gray

To see cool lightness demos (& paper):go to http://www-bcs.mit.edu/gaz

or

www.tedadelson.org

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

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Example 2: blur the bars to give penumbra

Illusion is enhanced

Illusion is slightly reduced

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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

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Amplitude of wiggles

Big wiggles:Big effect.

Small wiggles:Smaller effectbut still there.

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Rough texture works

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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 seewww.tedadelson.org

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.)

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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 transparent If vertical strip is transparent

The basic intuition for using the plaid

discounting decreases illusion

discounting increases illusion

If horizontal strip is transparent If vertical strip is transparent

Frequency of wiggles

1 w/cycle

16 w/cycle

It works for a wide range of frequencies.