Transcript Humera Iqbal (PowerPoint presentation)

PsyC54 Lecture Jan. 18: Part I
Color and Motion in
Shape-from-shadow Pictures
Humera Iqbal, John Kennedy & Juan Bai
Hering’s Umbra and Penumbra

Loose shadow
perception when
penumbra is covered.
WHY?
Penumbra
penumbra
Umbra
umbra
Shape-from-shadow perception
Number-of-Contours Account
Contours of a line play a role in shape-fromshadow perception and too many contours
prevent shape from shadow perception
Shape-from-shadow perception
Belongingness Hypothesis
Dark-to-light shadow border has to belong
to both the shadowed and the illuminated
areas to allow perception of a continuous
face bearing shadow
Shape-from-shadow perception
Border-Polarity Hypothesis
For shape-from-shadow perception the
luminosity of the shadowed region and the
shadow border must be lower than the
luminosity of the illuminated region
All Possible Visual Borders

Static or Motion

Luminance or Spectral

Monocular or Binocular
Manipulate each possibility
2x2x2=8
All Possible Visual Borders
Luminance Difference
(B/W)
Monocular
Binocular
Spectral Difference
(colored)
Static
Motion
Static
Motion
B/W
drawings
B/W
motion
Colored
painting
Colored
motion
Colored
stereo
illustrations
Colored
stereo
movies
B/W stereo B/W stereo
illustrations
movie
Number of Contours
John Kennedy (1993)
Too many contours,
prevents shape-fromshadow perception
In the black-line image
the line has two
contours: from grey-toblack and from blackto-white
Belongingness Hypothesis…
Monocular and
Binocular depth
perception
Looking through a
stereoscope, Ss.
May see the small
dotted line only
belonging to
shadowed region or
illuminated region
Border-Polarity Hypothesis
Luminosity of shadow
region and shadow
border must be lower
than the luminosity of
non-shadowed
region.
(i.e. positive border
polarity)
All Possible Visual Borders
original table from Kennedy et al. (2003)
Experiment 1
In a static image,
changed the
luminosity of the
shadowed region
while maintaining
the luminosity of
the red shadow
border
Experiment 1
Created nine images with border polarity changing
gradually from positive to negative and region
polarity kept positive except for the last image
Red Border
Hue = 239
Saturation = 240
Black Shadow
Hue = 0
Saturation = 0
Participants asked to rate each image on a scale
from 1-to-10, where 10 was “easy to recognize
the old person” 1 was “difficult to recognize”
Experiment 1
Border polarity changed from positive to negative
Experiment 1
Results
The face was more evident in frame 1, but
started to become difficult by frame 5, and
was totally gone by frame 9…gradation effect
Conclusion
Shape-from-shadow perception becomes
difficult as luminosity of the shadowed region
becomes higher than the luminosity of the
shadow border
Experiment 2

Using “Polka Dots” instead of solid region

Changed luminosity of the shadow border and
shadow region by changing number of dots in a
square area

Created motion of the shadow border by
changing positions of the polka dots
Experiment 2
1st Test
Black-dot border
Black-dot shadow
Negative border polarity
2nd Test
Black-dot border
Black-dot shadow
Positive border polarity
Experiment 2
3rd Test
Red-dot border
Black-dot shadow
Negative border polarity
4th Test
Red-dot border
Black-dot shadow
Positive border polarity
Predictions
difficult
easy