Visual Perception

1
Visual Perception

• Cecilia R. Aragon
• IEOR 170
• UC Berkeley
• Spring 2006

2
Acknowledgments

• Thanks to slides and publications by Pat
  Hanrahan, Christopher Healey, Maneesh Agrawala,
  and Lawrence Anderson-Huang.

3
Visual perception

• Structure of the Retina
• Preattentive Processing
• Detection
• Estimating Magnitude
• Change Blindness
• Multiple Attributes
• Gestalt

4
Visual perception and psychophysics

• Psychophysics is concerned with establishing
  quantitative relations between physical
  stimulation and perceptual events.

5
Structure of the Retina
6
Structure of the Retina

• The retina is not a camera!
• Network of photo-receptorcells (rods and cones)
  andtheir connections

Anderson-Huang, L. http//www.physics.utoledo.edu
/lsa/_color/18_retina.htm
7
Photo-transduction

• When a photon enters a receptor cell (e.g. a rod
  or cone), it is absorbed by a molecule called
  11-cis-retinal and convertedto trans form.
• The different shapecauses it to
  ultimatelyreduce the electricalconductivity of
  thephoto-receptor cell.

Anderson-Huang, L. http//www.physics.utoledo.edu
/lsa/_color/18_retina.htm
8
Electric currents from photo-receptors

• Photo-receptors generate an electrical current in
  the dark.
• Light shuts off the current.
• Each doubling of light causes roughly the same
  reduction of current (3 picoAmps for cones, 6 for
  rods).
• Rods more sensitive, recover more slowly.
• Cones recover faster, overshoot.
• Geometrical response in scaling laws of
  perception.

Anderson-Huang, L. http//www.physics.utoledo.ed
u/lsa/_color/18_retina.htm
9
Preattentive Processing
10
How many 5s?

• 385720939823728196837293827
• 382912358383492730122894839
• 909020102032893759273091428
• 938309762965817431869241024

Slide adapted from Joanna McGrenere
http//www.cs.ubc.ca/joanna/
11
How many 5s?

• 385720939823728196837293827
• 382912358383492730122894839
• 909020102032893759273091428
• 938309762965817431869241024

12
Preattentive Processing

• Certain basic visual properties are detected
  immediately by low-level visual system
• Pop-out vs. serial search
• Tasks that can be performed in less than 200 to
  250 milliseconds on a complex display
• Eye movements take at least 200 msec to initiate

13
Color (hue) is preattentive

• Detection of red circle in group of blue circles
  is preattentive

image from Healey 2005
14
Form (curvature) is preattentive

• Curved form pops out of display

image from Healey 2005
15
Conjunction of attributes

• Conjunction target generally cannot be detected
  preattentively (red circle in sea of red square
  and blue circle distractors)

image from Healey 2005
16
Healey appleton preattentive processing

• http//www.csc.ncsu.edu/faculty/healey/PP/index.ht
  ml

17
Preattentive Visual Features
closure color (hue) intensity flicker direction
of motion stereoscopic depth 3D depth cues

• line orientation
• length
• width
• size
• curvature
• number
• terminators
• intersection

18
Cockpit dials

• Detection of a slanted line in a sea of vertical
  lines is preattentive

19
Detection
20
Just-Noticeable Difference

• Which is brighter?

21
Just-Noticeable Difference

• Which is brighter?

(130, 130, 130)
(140, 140, 140)
22
Webers Law

• In the 1830s, Weber made measurements of the
  just-noticeable differences (JNDs) in the
  perception of weight and other sensations.
• He found that for a range of stimuli, the ratio
  of the JND ?S to the initial stimulus S was
  relatively constant
• ?S / S k

23
Webers Law

• Ratios more important than magnitude in stimulus
  detection
• For example we detect the presence of a change
  from 100 cm to 101 cm with the same probability
  as we detect the presence of a change from 1 to
  1.01 cm, even though the discrepancy is 1 cm in
  the first case and only .01 cm in the second.

24
Webers Law

• Most continuous variations in magnitude are
  perceived as discrete steps
• Examples contour maps, font sizes

25
Estimating Magnitude
26
Stevens Power Law

• Compare area of circles

27
Stevens Power Law

• s(x) axb
• s is the sensation
• x is the intensity of the attribute
• a is a multiplicative constant
• b is the power
• b gt 1 overestimate
• b lt 1 underestimate

graph from Wilkinson 99
28
Stevens Power Law

• Stevens 1961

29
Stevens Power Law

• Experimental results for b
• Length .9 to 1.1
• Area .6 to .9
• Volume .5 to .8
• Heuristic b 1/sqrt(dimensionality)

30
Stevens Power Law

• Apparent magnitude scaling

Cartography Thematic Map Design, p. 170, Dent,
96 S 0.98A0.87 J. J. Flannery, The
relative effectiveness of some graduated point
symbols in the presentation of quantitative data,
Canadian Geographer, 8(2), pp. 96-109, 1971
slide from Pat Hanrahan
31
Relative Magnitude Estimation

• Most accurate
• Least accurate

• Position (common) scale
• Position (non-aligned) scale
• Length
• Slope
• Angle
• Area
• Volume
• Color (hue/saturation/value)

32
Change Blindness
33
Change Blindness

• An interruption in what is being seen causes us
  to miss significant changes that occur in the
  scene during the interruption.
• Demo from Ron Rensink http//www.psych.ubc.ca/re
  nsink/flicker/

34
Possible Causes of Change Blindness
Simons, D. J. (2000), Current approaches to
change blindness, Visual Cognition, 7, 1-16.
35
Multiple Visual Attributes
36
The Game of Set

• Color
• Symbol
• Number
• Shading
• A set is 3 cards such that each feature is EITHER
  the same on each card OR is different on each
  card.

Set applet by Adrien Treuille,
http//www.cs.washington.edu/homes/treuille/resc/
set/
37
Multiple Visual Attributes

• Integral vs. separable
• Integral dimensions
• two or more attributes of an object are perceived
  holistically (e.g.width and height of rectangle).
  
• Separable dimensions
• judged separately, or through analytic processing
  (e.g. diameter and color of ball).
• Separable dimensions are orthogonal.
• For example, position is highly separable from
  color. In contrast, red and green hue perceptions
  tend to interfere with each other.

38
Integral vs. Separable Dimensions

• Integral
• Separable

Ware 2000
39
Gestalt
40
Gestalt Principles

• figure/ground
• proximity
• similarity
• symmetry
• connectedness
• continuity
• closure
• common fate
• transparency

41
Examples
Proximity
Connectedness from Ware 2004

• Figure/Ground

http//www.aber.ac.uk/media/Modules/MC10220/vispe
r07.html
42
Conclusion

• What is currently known about visual perception
  can aid the design process.
• Understanding low-level mechanisms of the visual
  processing system and using that knowledge can
  result in improved displays.