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