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Visual Illusions

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Visual Illusions Playing with Perspective CS99D Final Project By: Jason Anderson Professor Marc Levoy William Hogarth Theories of Geometrical Illusions Eye-movement ... – PowerPoint PPT presentation

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Title: Visual Illusions


1
Visual Illusions
  • Playing with Perspective

CS99D Final Project By Jason Anderson Professor
Marc Levoy
2
William Hogarth
1754 - "Whoever makes a DESIGN without the
knowledge of PERSPECTIVE will be liable to such
Absurdities as are shown in this
Frontispiece."    
Source W. Hogarth, 1697-1764 Trustees of the
British Museum.
3
Theories of Geometrical Illusions
  • Eye-movement ? perceived length
  • Perspective cues
  • Transactionalist approach
  • Adaptation-level theory

4
Eye-Movement Theory
  • Line length ?? eye movement
  • Testable, but usually fails initial perception,
    eyes are stable
  • Finding an index of eye movements a problem

5
Müller-Lyer Lines
  • Eye-movement theory Arrowheads influence extent
    of eye movements

6
Perspective Cues
  • Pictures converted in our brain from
    2-dimensional drawings to represent 3-dimensional
    scenes
  • Different level of explanation does not propose
    a mechanism for perception
  • Well established, although some loopholes have
    been found

7
Müller-Lyer Lines Revisited
  • The same illusion through perspective cues
  • Oculomotor Macropsia/Micropsia

8
Transactionalist Theory
  • The world is a product of perception, not a cause
    of it
  • Hamlet Do you see yonder cloud thats almost in
    shape of a camel?
  • Polonius By the Mass, and tis like a camel
    indeed.
  • Hamlet Methinks it is like a weasel
  • Polonius It is backed like a weasel
  • Hamlet Or like a whale?
  • Polonius Very like a whale
  • Hamlet (Act III, Scene II)
  • Change our way of looking ? Perception will change

Old Man
9
Adaptation-Level Theory
  • Helson, 1964 spatial pooling
  • Green Stacey, 1966 applied to illusions
  • Past stimulation ? current stimulation
  • stored norms
  • Top-down processing
  • Some flaws Ames room

10
Depth Cues on a Flat Surface
  • 1967, R.L. Gregory all pictures are impossible
    objects
  • Conflicting depth cues in the content of the
    picture with the flat surface on which it is
    presented

11
Retinal Disparity
  • No retinal disparity on a flat surface
  • As a viewer of an image, we choose to suppress
    the cue of retinal disparity

Source http//frank.mtsu.edu/pyskip/splec6.htm
12
The Acceptance of Perspective
  • We have come to accept that although we are
    seeing a flat surface, that the objects on it
    represent 3 dimensional concepts
  • Pictorial cues interposition (occlusion),
    relative size, linear perspective texture
    gradients
  • Ambiguous dimensional cues can lend themselves to
    be great visual illusions

13
Depth Ambiguity
  • Because of the way everything we see is projected
    onto the retina, there is a great deal of
    ambiguity

14
Wundts crosses
  • Hering (1879) Wundt (1898)
  • Most ambiguous of all figures
  • Infinite number of interpretations, but
    perceptual system tries to settle with a best
    one

15
Sanfords figure
  • Sanford, 1903
  • Although there may be an obvious best
    interpretation, once can easily be persuaded to
    accept an alternate one!

16
Of Ambiguous Figures and Depth Reversals
  • Necker cube
  • Mach Book

17
Of Ambiguous Figures Depth Reversals 2
  • Not enough information in the image to make a
    decision as to the best interpretation
  • Taken advantage of to create impossible figures

18
The freemish crate
  • Cochrans photo of his freemish crate (1966).

19
How did he do that???
  • Any guesses?

20
Viewing from a single, special perspective
  • Viewing the image from a misleading perspective
  • Viewing from another angle wrecks the effect
  • Monocular viewing required
  • Occlusion

21
Misleading depth cues
  • Stage scenery gives impression of greater depth
  • The Ames Room

22
The Ames Room
23
Of Giants and Dwarves?
  • Of course not!
  • But how?

24
Whats going on here?
  • Adelbert Ames, Jr. (1946) concept by Helmoltz
  • Special viewpoint monocular
  • Floor, ceiling, some walls, windows are
    trapezoidal
  • Inclined floor
  • Appears as a normal cubic room

25
So how does it work?
  • Peephole removes stereopsis
  • Forms an identical image of a cubic room on your
    retina
  • Both corners of the room subtend the same visual
    angle to your eye appear equidistant
  • Seckel KlarkePast experiencesnot relevant

26
But what about the people?
  • A split between perception expectation
  • Apparent cubic perspective overrides sense of
    size constancy
  • Stanford psychologistRobert Shepherd use
    background relationship to the horizon to
    judge size

27
Retinal Size ! Apparent Size
  • Distance cues relative size of elements,
    separation, density, clarity, background

28
But is the Ames Room necessary?
  • Seckel and Klarke only charm
  • An apparent horizontal path is all thats
    necessary
  • Richard Gregory same effect, ambiguous background

29
The Moon Illusion
  • Perceived distance, visual angle, linear size
    ! physical values
  • Illusion from comparison of perceived values at
    the horizon at the zenith
  • Subtends .5º in the eye no matter what
  • Not atmospheric
  • Illusion disappears in a mooning position ?

30
Theories
  • Apparent distance theory appears farther away ?
    larger
  • Size-distance paradox
  • Distance, visual angle, linear size
    illusionswork together
  • Oculomotor micropsia / macropsia ? visual angle
  • Distance cues ? macropsia forhorizon moon

31
The Mystery Spot
  • Tilted house
  • No visible horizon assumed horizon with
    internal reference frame of house
  • Your body is on a tilt as well enhances
    effectsas much as 3x
  • Application topilots

32
A new perspective on seeing
  • Many theories, none are all-encompassing yet
  • New ways to see things become more aware of
    space through witnessing these illusions
  • Perspective is a powerful tool in imitating
    reality, it can also deceive
  • Seeing is believing ? Perceiving is believing
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