A preference for global convexity in local shape perception

1
A preference for global convexity
in local shape perception

• Michael S. Langer Heinrich H. Bülthoff
• Max-Planck-Institute for Biological
  CyberneticsTübingen, Germany

2
Shape from Shading

3
Depth-reversal ambiguity in shape-from-
shading
hill
valley
4

• Hollow Mask Illusion (Luckiesh, 1916)

5
Hollow mask illusion is the sum of two factors
(Johnston et a. 92, Hill Bruce 94)

• face
• familiarity global
  convexity

6
Global shape discrimination is easy

• convex
  concave
• face
  mask

7
Procedure

8
Fixation Mark (1 sec.)

9
Task hill or valley ?

10
Three prior assumptions were tested

• light source direction (Rittenhouse 1786,..)
• viewpoint direction (Reichel Todd 1990,
  
  
  Mamassian Landy 1998)
• global shape (Johnston et. al
  1992, Hill Bruce 1994 )

11
Example in which all three priors assumptions are
met

• 1. light from above 2.
  viewpoint from above
• 3. shape is convex

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Example in which all three prior assumptions fail

• shape is concave viewpoint
  from below
• light from below

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light source direction (collimated source)

• light
• from
• above
• light
• from
• below
• 
  

14
viewing direction (Reichel and Todd 1990)

• view from above
• 
  
  
• view from below
• 
  

15
viewing direction (globally concave surface)

• view from below
• 
  
  
• view from above
• 
  

16
Design

• three factors
• - light direction
• - viewpoint
• - global shape
• 2 x 2 x 2 within observer
• 512 trials (64 per condition)

17
ANOVA Results (12 naïve observers)

• Main effects
• light direction F(1,11) 6.8, p
  .025
• viewpoint F(1,11) 9.6, p
  .01
• global shape F(1,11) 46.1, p lt
  .001

18
Linear Regression

• percent correct
• 51 10 light source direction
  
• 11 viewing direction
• 13 global shape
• (Each factor had value of 1 or 1)

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Examples

87 (best)
15 (worst)
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Conclusion

• The prior for global convexity is used in local
  shape from shading.
• The global convexity prior had roughly the same
  strength as the light-from-above and
  viewpoint-from-above priors.

21
Open questions

• What are the spatial scales over which shading
  information is analyzed?
• How does the prior on global shape depend on
  visual angle?