Straightness, structure, and shadows.

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

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

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

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

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

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

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

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

9. Rough texture works + +

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

11. Ripple strip

12. Snake cylinder

13. 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 www.tedadelson.org

14. 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.)

15. Effect of texture

16. Misalignment: do psi junctions matter?

17. Pinstripes

18. Wiggle styles

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

20. 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 More to try

21. 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?)

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

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

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