Title: Visual Perception
1Visual Perception
- Cecilia R. Aragon
- IEOR 170
- UC Berkeley
- Spring 2006
2Acknowledgments
- Thanks to slides and publications by Pat
Hanrahan, Christopher Healey, Maneesh Agrawala,
and Lawrence Anderson-Huang.
3Visual perception
- Structure of the Retina
- Preattentive Processing
- Detection
- Estimating Magnitude
- Change Blindness
- Multiple Attributes
- Gestalt
4Visual perception and psychophysics
- Psychophysics is concerned with establishing
quantitative relations between physical
stimulation and perceptual events.
5Structure of the Retina
6Structure 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
7Photo-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
8Electric 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
9Preattentive Processing
10How many 5s?
- 385720939823728196837293827
- 382912358383492730122894839
- 909020102032893759273091428
- 938309762965817431869241024
Slide adapted from Joanna McGrenere
http//www.cs.ubc.ca/joanna/
11How many 5s?
- 385720939823728196837293827
- 382912358383492730122894839
- 909020102032893759273091428
- 938309762965817431869241024
12Preattentive 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
13Color (hue) is preattentive
- Detection of red circle in group of blue circles
is preattentive
image from Healey 2005
14Form (curvature) is preattentive
- Curved form pops out of display
image from Healey 2005
15Conjunction of attributes
- Conjunction target generally cannot be detected
preattentively (red circle in sea of red square
and blue circle distractors)
image from Healey 2005
16Healey appleton preattentive processing
- http//www.csc.ncsu.edu/faculty/healey/PP/index.ht
ml
17Preattentive Visual Features
closure color (hue) intensity flicker direction
of motion stereoscopic depth 3D depth cues
- line orientation
- length
- width
- size
- curvature
- number
- terminators
- intersection
18Cockpit dials
- Detection of a slanted line in a sea of vertical
lines is preattentive
19Detection
20Just-Noticeable Difference
21Just-Noticeable Difference
(130, 130, 130)
(140, 140, 140)
22Webers 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
23Webers 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.
24Webers Law
- Most continuous variations in magnitude are
perceived as discrete steps - Examples contour maps, font sizes
25Estimating Magnitude
26Stevens Power Law
27Stevens 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
28Stevens Power Law
29Stevens Power Law
- Experimental results for b
- Length .9 to 1.1
- Area .6 to .9
- Volume .5 to .8
- Heuristic b 1/sqrt(dimensionality)
30Stevens 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
31Relative Magnitude Estimation
- Most accurate
- Least accurate
- Position (common) scale
- Position (non-aligned) scale
- Length
- Slope
- Angle
- Area
- Volume
- Color (hue/saturation/value)
32Change Blindness
33Change 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/
34Possible Causes of Change Blindness
Simons, D. J. (2000), Current approaches to
change blindness, Visual Cognition, 7, 1-16.
35Multiple Visual Attributes
36The 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/
37Multiple 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.
38Integral vs. Separable Dimensions
Ware 2000
39Gestalt
40Gestalt Principles
- figure/ground
- proximity
- similarity
- symmetry
- connectedness
- continuity
- closure
- common fate
- transparency
41Examples
Proximity
Connectedness from Ware 2004
http//www.aber.ac.uk/media/Modules/MC10220/vispe
r07.html
42Conclusion
- 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.