This document provides an introduction to the field of computer vision. It discusses (1) the goals of understanding and modeling visual processing in humans and building artificial vision systems, (2) how the human retina represents images and the brain processes them to recognize faces and objects, and (3) psychophysical experiments used to test hypotheses about visual processing, including inattentional blindness and change blindness. Figure-ground segregation is also introduced, where only one side of a contour is perceived as the figure based on cues like symmetry, convexity, and region.

1. Introduction
to
Computer Vision
ashishkhare@jkinstitute.org

24. goals of field of vision
• understand how animals represent and
process information carried by light, by
– measuring and modeling visual performance
in humans and other animals
– finding ways to build artificial visual systems
– characterizing neural mechanisms that
• implement visual systems
– apply this understanding to obtain medical,
technological advances

25. processing of images in humans
• as a first approximation, rods and cones
(sensory cells in the retina) represent image as
large 2D array of light intensities
– about 126 million sensory cells!
• this image representation is processed by brain
enabling complex cognitive functions
– recognize a familiar face or scene
– disambiguate overlapping objects
– read sloppy handwriting
• how does the brain do all of this? how might
image processing be partitioned into subtasks?

27. image processing tasks of brain
• possible tasks:
– extraction of contour (e.g. sharp light intensity
changes in the image)
– extraction of motion
– identification of object parts
• still unclear: how are these integrated to
enable us to extract meaning from what
we see?

28. psychophysical experiments
• used to test hypotheses about how the
brain/mind processes optical information

29. Examples
• Inattentional Blindness
• Change Blindness
• Figure-Ground Segregation

30. Inattentional Blindness
Mack & Rock (1998)
• Definition: the failure to see consciously, caused by
lack of attention
• We can miss perceiving very obvious changes if we
are not attending. Subjects do not consciously
perceive features of the visual scene that they do
not attend to.
• Subjects were engaged in tasks that demanded a
high degree of attention, such as looking at a cross
and trying to determine which arm is longer.

31. Trials 1

32. Trials 2

33. Trials 3

34. Inattention Trial

35. Recognition Test

36. Inattentional Blindness
• 25% of subjects failed to see the square when it was
presented in the parafovea (2° from fixation).
• But 65% failed to see it when it was at fixation!
• What is missed?
– Sad or Neutral face
– A word (priming for the word is present)
• What is not missed?
– Name
– Smiling face

37. Change Blindness
• Change Blindness is the phenomena were
we fail to perceive large changes, in our
surroundings as well as in experimental
conditions.
• Change could be in existence, properties,
semantic identity and spatial layout.
• Attention is required to perceive change,
and in the absence of localized motion
signals, attention is directed by high level
of interest (Rensink et al, 1997).

38. Flicker paradigm
• Basically, alternate an
original image A with a
modified image A’, with brief
blank fields placed between
successive images

39. Change Blindness
• “Visual perception of
change in an object
occurs only when that
object is given focused
attention”
• “In the absence of
such attention, the
contents of visual
memory are simply
overwritten by
subsequent stimuli,
and so cannot be used
to make comparisons”

40. CB – Another Example

41. Why CB?
• Change blindness could be due to –
– Poor representation of pre- and post change
scene or
– Pre change representation gets over-written
by post change representation or
– Capacity to retrieve and compare information
is limited (Hollingworth, 2003).
• Color change detection with multi-element
displays

42. Figure-Ground Segregation
• Discovered by Edgar
Rubin (Fig.1, 1921).
• Only one side of the
contour is seen as figure.
• Has shape, appears
closer.
• Background appears
Fig.1 (Faces/vase display)
behind the figure and has
no shape.

43. Background
• Configural cues:
– symmetry
– convexity
– area ………….. Gestalt psychologists
• Lower region
• lower region of a display will be seen as figure than the upper
region. (Vecera et al. 2002)
• Top-bottom polarity
• Stimuli having wide base and narrow top were perceived as
figures than the ones which had narrow base and wide top.
• (Hulleman and Humphreys, 2004)
• Higher cognitive processes
» Object memory (Mary Peterson et al. 1991)
» Attention ( Vecera et al. 2004)

44. Motivation
• Palmer and his colleagues conducted a study in which
they used temporal frequency (flicker) and manipulated
edge synchrony with the two regions (left and right).
• They concluded from their studies that edge plays the
key role in determining figure-ground segregation. In
their study they found that the region with which the
edge synchronizes will be seen as figure irrespective of
whether the region is flickering or not.
• Wong and Weisstein (1987) demonstrated that spatial
and temporal frequencies play a major role in assigning
figural status to a region.

45. Reference Books
• Fundamentals of Digital Image Processing:
Anil K. Jain
• Digital Image Processing: Gonzalez & Woods
• The Image Processing Handbook: J.C. Russ
• Digital Image Processing: B. Jahne
• Image Processing, Analysis and Machine
Vision: M. Sonka, V. Hlavac, R. Boyle
• Computer Vision Handbook: B. Jahne
• Computer Vision: M. Brady