Visual computation of lightness in simple and complex images

1. Visual computationof lightness in simple and complex images Alan Gilchrist National Science Foundation: BCS-9906747Public Health Service: GM 60826-02

2. What is Lightness? Perceivedwhite,gray,or blackshade of a surface.

3. These two squares are identical The problem of lightness constancy: Adelson’s checkered shadow

4. Luminance is ambiguous Any absolute luminancecan appear as any shade of gray

5. 1948 - Wallach’s solution: Relative luminance

7. 5 50 1 10 1948 - Wallach’s solution: Relative luminance Disks appear equal when luminance ratios are equal

8. Near condition Far condition

10. 1 5 This local luminance ratio. . . But, without an anchoring rule, luminance ratios are also ambiguous . . . is consistent with any of these:

11. SELF- LUMINOUS Given: WHITE A range of luminances in the image GRAY BLACK the scale of perceived gray shades THE ANCHORING PROBLEM How to map these onto….

12. Helson, Buchsbaum Gray world assumption Two proposed anchoring rules Wallach, Land & McCann SELF- LUMINOUS WHITE Highest luminance Rule Average luminance Rule GRAY BLACK

13. Another rule SELF- LUMINOUS Koffka,Rock WHITE Bipolar anchoring GRAY BLACK Which rule is correct?

14. Challenge: pit these rules against each other in the simplest possible image

15. What is a simple image? Heinemann:Disk/annulus in a dark room Gilchrist:Disk/annulus is too complex Simplest image: Two surfaces of different gray that fill the entire visual field. Wallach: "Opaque colors which deserve to be called white or gray, in other words ‘surface colors,’ will make their appearance only when two regions of different light intensity are in contact with each other..."

16. 1 2 3

17. 2.5 5.5 Physical Stimulus 4.5 9.5 Li & Gilchrist, 1999 Appearance Anchoring under minimal conditions: Two surfaces fill entire visual field Highest Luminance Rule wins

18. Highest luminance rule: The highest luminance within a framework appears white and darker regions are computed relative to this value.

19. Physical Stimulus Appearance 1. Highest Luminance Rule Highest luminance appears white Physical Stimulus Appearance 2. Area Rule. The larger the lighter 3. Scale Normalization Rule. The perceived range of grays tends toward that between black and white. (30:1) Physical Stimulus Appearance Three rules of anchoring in simple images:

20. Two problems for the highest luminance rule • Self-luminosity perception • Upward induction/downward induction problem

21. What color is the ceiling? Highest luminance rule fails

23. Does the darker one appear to get darker? Or does the lighter one appear to get lighter still? Upward induction/downward induction problem When the luminance difference between two adjacent regions increases: Downward induction Upward induction

24. Downward induction Upward induction

25. The answer lies in relative area

28. Method Nine stimulus domes: Each viewed by a different group of 15 subjects Matches made from immediate memory using a Munsell chart

29. Standard Deviations Perceived Log reflectance White 1.89 1.69 1.49 Gray 1.29 1.09 0.89 0.69 Black 0.49

30. Perceived Log reflectance White 1.85 1.65 1.45 Gray 1.25 1.05 0.85 0.65 Black 0.45 0 50 100 150 200 250 300 350 Degrees of dark gray

31. The area rule: The darker region lightens as it gets larger • As the darker region becomes very large, the lighter region • appears first super-white, and then self-luminous..

33. 2 degree square 7 x 9 degree rectangle

34. % LUMINOSITY REPORTS 1 0 0 8 0 6 0 50% 4 0 2 0 WHITE 0 2 0 1 0 0 3 0 0 TARGET LUMINANCE (cd /m 2 ) GRAY BACKGROUND LIGHTNESS 9 0 BLACK 1 0 3 2 0 1 0 0 3 0 0 TARGET LUMINANCE (cd /m 2 )

35. Theoretical significance: • Inconsistent with inverse optics • Neurally plausible

36. Scale normalization rule: The perceived range of grays within a framework tends toward that between black and white • If the range is truncated (less than 30:1), expansion occurs. • Coefficient of expansion proportional to the degree of truncation. • The expansion shows up at the bottom of the range, • not the top, which is anchored at white. • Similar to MacLeod and Brown’s gamut expansion

37. EXPANSION COMPRESSION Percentage Rescaling 160 140 120 100 4.8 80 60 40 Stimulus: 1 10 40 Disk/Ganzfeld Range full Gilchrist & Bonato (1995)

38. Physical Stimulus Appearance 1. Highest Luminance Rule Highest luminance appears white Physical Stimulus Appearance 2. Area Rule. The larger the lighter 3. Scale Normalization Rule. The perceived range of grays tends toward that between black and white. (30:1) Physical Stimulus Appearance Three rules of anchoring in simple images:

39. What about complex images?

40. What is the relationship between simple and complex images? Contrast era: Findings from simple images can be directly applied to complex images Arend (1994): Disk/annulus displays are too simple to tell us anything useful about lightness perception. Gilchrist: Simple and complex images are related in a systematic way. • Applicability assumption • Co-determination principle

41. The applicability assumption: Rules of lightness computation in simple images can be applied to frameworks embedded within complex images

42. The co-determination principle Lightness is determined by computations both in the relevant framework and in adjacent and/or superordinate frameworks Lajos Kardos ... brilliant but largely-unknown Gestalt psychologist.

43. Applicability assumption: Highest luminance rule 2. Area function 3. Scale normalization

44. A B Corrugated Mondrian (Adelson)

46. Now for a Live Demo!

47. LOG PERCEIVED REFLECTANCE 2 WHITE WHITE GLOBAL 1.8 1.6 1.4 GRAY 1.2 1 LOCAL 0.8 0.6 BLACK 0.4 -1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 LOG T/H

48. Applicability assumption: Highest luminance rule 2. Area function 3. Scale normalization

50. Applicability assumption: Highest luminance rule 2. Area function 3. Scale normalization