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Visual Illusions. Playing with Perspective. CS99D Final Project By: Jason Anderson Professor Marc Levoy. William Hogarth. 1754 - "Whoever makes a DESIGN without the knowledge of PERSPECTIVE will be liable to such Absurdities as are shown in this Frontispiece.".

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visual illusions

Visual Illusions

Playing with Perspective

CS99D Final Project

By: Jason Anderson

Professor Marc Levoy

william hogarth
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.

theories of geometrical illusions
Theories of Geometrical Illusions
  • Eye-movement  perceived length
  • Perspective cues
  • Transactionalist approach
  • Adaptation-level theory
eye movement theory
Eye-Movement Theory
  • Line length  eye movement
  • Testable, but usually fails – initial perception, eyes are stable
  • Finding an index of eye movements a problem
m ller lyer lines
Müller-Lyer Lines
  • Eye-movement theory: Arrowheads influence extent of eye movements
perspective cues
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
m ller lyer lines revisited
Müller-Lyer Lines Revisited
  • The same illusion through perspective cues
  • Oculomotor Macropsia/Micropsia
transactionalist theory
Transactionalist Theory
  • The world is a product of perception, not a cause of it
    • Hamlet: Do you see yonder cloud that’s 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

adaptation level theory
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
depth cues on a flat surface
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
retinal disparity
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

the acceptance of perspective
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
depth ambiguity
Depth Ambiguity
  • Because of the way everything we see is projected onto the retina, there is a great deal of ambiguity
wundt s crosses
Wundt’s crosses
  • Hering (1879) & Wundt (1898)
  • Most ambiguous of all figures
  • Infinite number of interpretations, but perceptual system tries to settle with a ‘best’ one
sanford s figure
Sanford’s figure
  • Sanford, 1903
  • Although there may be an obvious ‘best’ interpretation, once can easily be persuaded to accept an alternate one!
of ambiguous figures depth reversals 2
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
the freemish crate
The ‘freemish’ crate
  • Cochran’s photo of his ‘freemish’ crate (1966).
viewing from a single special perspective
Viewing from a single, special perspective
  • Viewing the image from a misleading perspective
  • Viewing from another angle wrecks the effect
  • Monocular viewing required
  • Occlusion
misleading depth cues
Misleading depth cues
  • Stage scenery – gives impression of greater depth
  • The Ames Room
of giants and dwarves
Of Giants and Dwarves?
  • Of course not!
  • But how?
what s going on here
What’s 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
so how does it work
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 & Klarke:Past experiencesnot relevant
but what about the people
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
retinal size apparent size
Retinal Size != Apparent Size
  • Distance cues: relative size of elements, separation, density, clarity, background
but is the ames room necessary
But is the Ames Room necessary?
  • Seckel and Klarke: only charm
  • An apparent horizontal path is all that’s necessary
  • Richard Gregory: same effect, ambiguous background
the moon illusion
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” 
theories
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
the mystery spot
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
a new perspective on seeing
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