Object recognition using shading - PowerPoint PPT Presentation

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Object recognition using shading

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Illumination models. Sunny day (Horn 70, ... viewpoint (floor, ceiling) 512 trials (64 x 8 conditions) Results (linear regression) ... – PowerPoint PPT presentation

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Title: Object recognition using shading


1
Object recognition using shading

2
Object recognition using shading
  • Strong claim 1 to recognize an object using
    shading, we reconstruct the objects 3D shape
    and then we recognize this 3D shape.

3
Object recognition using shading
  • Strong claim 2 shape from shading is not
    used in object recognition

4
What is the role of shape from shading in object
recognition ?
  • To what extent do humans perceive 3D shape from
    shading ?
  • To what extent do humans use 3D shape perception
    to recognize objects ?
  • We need to answer Q1 before we can answer Q2.

5
Human perception of local shape from shading
under variable lighting
  • Michael S. Langer Heinrich H. Bülthoff
  • Max-Planck-Institute for Biological
    CyberneticsTübingen, Germany
  • McGill University, Montreal, Canada

6
Illumination models
  • Sunny day (Horn 70, )
  • Cloudy day (Langer and Zucker ICCV 93,
    Stewart and Langer CVPR 97)

7
Overview
  • Experiment 1 SFS on a sunny day
  • Experiment 2 SFS on a cloudy day

8
SFS on a Sunny Day
L
N(x)
I(x) r N(x) L
9
Depth-reversal ambiguity on a sunny day
hill
valley
10
  • Hollow Mask Illusion (Luckiesh, 1916)

11
Hollow mask illusion is due to two
factors(Johnston et al 92 , Hill and Bruce
94)
  • face vs. hollow mask
  • familiarity global
    convexity
  • (recognition)

12
Our experiments use unfamiliar surfaces
  • convex
    concave
  • face
    mask

13
Procedure

14

15
Task hill or valley ?

16
Three factors were tested
  • light source direction (above gt below)
  • global shape (convex gt
    concave)
  • viewpoint (above gt
    below)

17
1. light source direction
  • light
  • from
  • above
  • light
  • from
  • below


18
2. global shape
  • convex
    concave

19
3. Viewpoint (Reichel Todd 1990)
  • from above from
    below
  • (floor)
    (ceiling)

20
Factor 3 viewpoint
  • view from above


  • view from below


21
Factor 3 viewpoint
  • view from below


  • view from above


22
Design
  • three factors (2 x 2 x 2)
  • - light direction (above, below)
  • - global shape (convex, concave)
  • - viewpoint (floor, ceiling)
  • 512 trials (64 x 8 conditions)

23
Results (linear regression)
  • percent correct (hill or valley ?)
  • 51 10 light direction
  • 13 global shape
  • 11 viewpoint
  • (Each factor had value of 1 or
    1)

24
Conclusions Experiment 1
  • many factors play a role in resolving the
    depth-reversal ambiguity
  • (light direction, global shape, viewpoint,
    .)

25
Conclusions Experiment 1
  • many factors play a role in resolving the
    depth-reversal ambiguity
  • (light direction, global shape, viewpoint,
    .)
  • observers often ignore available image
    information (perspective, occluding contours,
    shadows)

26
Experiment 2 Shape from shading on a cloudy day
27
Shading on a cloudy day
?
(x)
28
Shading on a Cloudy Day
?
?
I(x)
N(x)
L d L
?
(x)
?
?(x) angle of visible light source
29
Shading in a valley on a cloudy day
local intensity maxima
30
Local intensity maxima in valleys
31
Experiment 2
How well do humans perceive shape from shading in
the presence of these local intensity maxima ?
32
Experiment 2
How well do humans perceive shape from shading in
the presence of these local intensity maxima
? Hypothesis (shape from shading
skeptic) Humans use dark means deep model.
33
Procedure
34
(No Transcript)
35
Task Which is higher ?
36
Two conditions
???intensity
correlated
anti-correlated
_

???height
_

anti-correlated
correlated
37
Which is higher ?
N17
overall score 65 (above chance)
percent correct
_

38
Conclusion Experiment 2
  • Dark means deep is too simple a model to
    explain human perception of shape from shading
    on a cloudy day.

39
Big Picture What role does 3D shape perception
play in 3D object recognition?
40
Big Picture What role does 3D shape perception
play in 3D object recognition?
  • To what extent do we perceive 3D shape ? Were
    on the way to answering this question.

41
Big Picture What role does 3D shape perception
play in 3D object recognition?
  • To what extent do we perceive 3D shape ? Were
    on the way to answering this question.
  • To what extent do we use these 3D shape percepts
    to recognize objects ?
  • The answer to Q2 depends on Q1.

42
Computer vision psychophysics
I(x) q(x)

(Langer and Zucker 93)
?
I(x) N(x) L d L
?
?(x)
(Stewart and Langer 96)
43
PsychophysicsHuman vs. Computer
1
0.8
q q, N
0.6
percent correct
0.4
0.2
-
0

-

-

human computer
44
Conclusions Experiment 2
  • Dark means deep is too simple to explain human
    perception of shape from shading on a cloudy day
  • Computing local shape in valleys is an inherently
    difficult computational problem on a cloudy
    day

45
Which point is brighter ?
height
N10
brightness
percent correct
_

Correlation
46
Examples

87 (best)


15 (worst)
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