Visual Illusions Rizwan
William Hogarth 1754 - "Whoever makes a DESIGN without the knowledge of PERSPECTIVE will be liable to such Absurditie...
Theories of Geometrical Illusions <ul><li>Eye-movement    perceived length </li></ul><ul><li>Perspective cues </li></ul><...
Eye-Movement Theory <ul><li>Line length    eye movement </li></ul><ul><li>Testable, but usually fails – initial percepti...
Müller-Lyer Lines <ul><li>Eye-movement theory: Arrowheads influence extent of eye movements </li></ul>
Perspective Cues <ul><li>Pictures converted in our brain from 2-dimensional drawings to represent 3-dimensional scenes </l...
Müller-Lyer Lines Revisited <ul><li>The same illusion through perspective cues </li></ul><ul><li>Oculomotor Macropsia/Micr...
Transactionalist Theory <ul><li>The world is a product of perception, not a cause of it </li></ul><ul><ul><li>Hamlet: Do y...
Adaptation-Level Theory <ul><li>Helson, 1964 – “spatial pooling” </li></ul><ul><li>Green & Stacey, 1966 applied to illusio...
Depth Cues on a Flat Surface <ul><li>1967, R.L. Gregory – all pictures are “impossible objects” </li></ul><ul><li>Conflict...
Retinal Disparity <ul><li>No retinal disparity on a flat surface </li></ul><ul><li>As a viewer of an image, we choose to s...
The Acceptance of Perspective <ul><li>We have come to accept that although we are seeing a flat surface, that the objects ...
Depth Ambiguity <ul><li>Because of the way everything we see is projected onto the retina, there is a great deal of ambigu...
Wundt’s crosses <ul><li>Hering (1879) & Wundt (1898) </li></ul><ul><li>Most ambiguous of all figures </li></ul><ul><li>Inf...
Sanford’s figure <ul><li>Sanford, 1903 </li></ul><ul><li>Although there may be an obvious ‘best’ interpretation, once can ...
Of Ambiguous Figures and Depth Reversals <ul><li>Necker cube </li></ul><ul><li>Mach Book </li></ul>
Of Ambiguous Figures & Depth Reversals 2 <ul><li>Not enough information in the image to make a decision as to the “best” i...
The ‘freemish’ crate <ul><li>Cochran’s photo of his ‘freemish’ crate (1966).  </li></ul>
How did he do that??? Any guesses?
Viewing from a single, special perspective <ul><li>Viewing the image from a misleading perspective </li></ul><ul><li>Viewi...
Misleading depth cues <ul><li>Stage scenery – gives impression of greater depth </li></ul><ul><li>The Ames Room </li></ul>
The Ames Room
Of Giants and Dwarves? <ul><li>Of course not! </li></ul><ul><li>But how? </li></ul>
What’s going on here? <ul><li>Adelbert Ames, Jr. (1946) – concept by Helmoltz </li></ul><ul><li>Special viewpoint – monocu...
So how does it work? <ul><li>Peephole removes stereopsis </li></ul><ul><li>Forms an identical image of a cubic room on you...
But what about  the people? <ul><li>A split between perception & expectation </li></ul><ul><li>Apparent cubic perspective ...
Retinal Size != Apparent Size <ul><li>Distance cues: relative size of elements, separation, density, clarity, background <...
But is the Ames Room necessary? <ul><li>Seckel and Klarke: only charm </li></ul><ul><li>An apparent horizontal path is all...
The Moon Illusion <ul><li>Perceived distance, visual angle, & linear size != physical values </li></ul><ul><li>Illusion fr...
Theories <ul><li>Apparent distance theory – appears farther away    larger </li></ul><ul><ul><li>Size-distance paradox </...
The Mystery Spot <ul><li>Tilted house </li></ul><ul><li>No visible horizon – assumed horizon with internal reference frame...
A new perspective on seeing <ul><li>Many theories, none are all-encompassing  yet </li></ul><ul><li>New ways to see things...
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Visual illusions

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Visual illusions

  1. 1. Visual Illusions Rizwan
  2. 2. William Hogarth 1754 - &quot;Whoever makes a DESIGN without the knowledge of PERSPECTIVE will be liable to such Absurdities as are shown in this Frontispiece.&quot;     
  3. 3. Theories of Geometrical Illusions <ul><li>Eye-movement  perceived length </li></ul><ul><li>Perspective cues </li></ul><ul><li>Transactionalist approach </li></ul><ul><li>Adaptation-level theory </li></ul>
  4. 4. Eye-Movement Theory <ul><li>Line length  eye movement </li></ul><ul><li>Testable, but usually fails – initial perception, eyes are stable </li></ul><ul><li>Finding an index of eye movements a problem </li></ul>
  5. 5. Müller-Lyer Lines <ul><li>Eye-movement theory: Arrowheads influence extent of eye movements </li></ul>
  6. 6. Perspective Cues <ul><li>Pictures converted in our brain from 2-dimensional drawings to represent 3-dimensional scenes </li></ul><ul><li>Different level of explanation – does not propose a mechanism for perception </li></ul><ul><li>Well established, although some ‘loopholes’ have been found </li></ul>
  7. 7. Müller-Lyer Lines Revisited <ul><li>The same illusion through perspective cues </li></ul><ul><li>Oculomotor Macropsia/Micropsia </li></ul>
  8. 8. Transactionalist Theory <ul><li>The world is a product of perception, not a cause of it </li></ul><ul><ul><li>Hamlet: Do you see yonder cloud that’s almost in shape of a camel? </li></ul></ul><ul><ul><li>Polonius: By the Mass, and ‘tis like a camel indeed. </li></ul></ul><ul><ul><li>Hamlet: Methinks it is like a weasel </li></ul></ul><ul><ul><li>Polonius: It is backed like a weasel </li></ul></ul><ul><ul><li>Hamlet: Or like a whale? </li></ul></ul><ul><ul><li>Polonius: Very like a whale </li></ul></ul><ul><ul><li>Hamlet (Act III, Scene II) </li></ul></ul><ul><li>Change our way of looking  Perception will change </li></ul>Old Man
  9. 9. Adaptation-Level Theory <ul><li>Helson, 1964 – “spatial pooling” </li></ul><ul><li>Green & Stacey, 1966 applied to illusions </li></ul><ul><li>Past stimulation  current stimulation </li></ul><ul><li>“ stored norms” </li></ul><ul><li>“ Top-down processing” </li></ul><ul><li>Some flaws – Ames room </li></ul>
  10. 10. Depth Cues on a Flat Surface <ul><li>1967, R.L. Gregory – all pictures are “impossible objects” </li></ul><ul><li>Conflicting depth cues in the content of the picture with the flat surface on which it is presented </li></ul>
  11. 11. Retinal Disparity <ul><li>No retinal disparity on a flat surface </li></ul><ul><li>As a viewer of an image, we choose to suppress the cue of retinal disparity </li></ul>Source: http://frank.mtsu.edu/~pyskip/splec6.htm
  12. 12. The Acceptance of Perspective <ul><li>We have come to accept that although we are seeing a flat surface, that the objects on it represent 3 dimensional concepts </li></ul><ul><li>Pictorial cues: interposition (occlusion), relative size, linear perspective & texture gradients </li></ul><ul><li>Ambiguous dimensional cues can lend themselves to be great visual illusions </li></ul>
  13. 13. Depth Ambiguity <ul><li>Because of the way everything we see is projected onto the retina, there is a great deal of ambiguity </li></ul>
  14. 14. Wundt’s crosses <ul><li>Hering (1879) & Wundt (1898) </li></ul><ul><li>Most ambiguous of all figures </li></ul><ul><li>Infinite number of interpretations, but perceptual system tries to settle with a ‘best’ one </li></ul>
  15. 15. Sanford’s figure <ul><li>Sanford, 1903 </li></ul><ul><li>Although there may be an obvious ‘best’ interpretation, once can easily be persuaded to accept an alternate one! </li></ul>
  16. 16. Of Ambiguous Figures and Depth Reversals <ul><li>Necker cube </li></ul><ul><li>Mach Book </li></ul>
  17. 17. Of Ambiguous Figures & Depth Reversals 2 <ul><li>Not enough information in the image to make a decision as to the “best” interpretation </li></ul><ul><li>Taken advantage of to create “impossible” figures </li></ul>
  18. 18. The ‘freemish’ crate <ul><li>Cochran’s photo of his ‘freemish’ crate (1966). </li></ul>
  19. 19. How did he do that??? Any guesses?
  20. 20. Viewing from a single, special perspective <ul><li>Viewing the image from a misleading perspective </li></ul><ul><li>Viewing from another angle wrecks the effect </li></ul><ul><li>Monocular viewing required </li></ul><ul><li>Occlusion </li></ul>
  21. 21. Misleading depth cues <ul><li>Stage scenery – gives impression of greater depth </li></ul><ul><li>The Ames Room </li></ul>
  22. 22. The Ames Room
  23. 23. Of Giants and Dwarves? <ul><li>Of course not! </li></ul><ul><li>But how? </li></ul>
  24. 24. What’s going on here? <ul><li>Adelbert Ames, Jr. (1946) – concept by Helmoltz </li></ul><ul><li>Special viewpoint – monocular </li></ul><ul><li>Floor, ceiling, some walls, & windows are trapezoidal </li></ul><ul><li>Inclined floor </li></ul><ul><li>Appears as a normal cubic room </li></ul>
  25. 25. So how does it work? <ul><li>Peephole removes stereopsis </li></ul><ul><li>Forms an identical image of a cubic room on your retina </li></ul><ul><li>Both corners of the room subtend the same visual angle to your eye – appear equidistant </li></ul><ul><li>Seckel & Klarke: Past experiences not relevant </li></ul>
  26. 26. But what about the people? <ul><li>A split between perception & expectation </li></ul><ul><li>Apparent cubic perspective overrides sense of size constancy </li></ul><ul><li>Stanford psychologist Robert Shepherd – use background & relationship to the horizon to judge size </li></ul>
  27. 27. Retinal Size != Apparent Size <ul><li>Distance cues: relative size of elements, separation, density, clarity, background </li></ul>
  28. 28. But is the Ames Room necessary? <ul><li>Seckel and Klarke: only charm </li></ul><ul><li>An apparent horizontal path is all that’s necessary </li></ul><ul><li>Richard Gregory: same effect, ambiguous background </li></ul>
  29. 29. The Moon Illusion <ul><li>Perceived distance, visual angle, & linear size != physical values </li></ul><ul><li>Illusion from comparison of perceived values at the horizon & at the zenith </li></ul><ul><li>Subtends .5º in the eye no matter what </li></ul><ul><li>Not atmospheric </li></ul><ul><li>Illusion disappears in a “mooning position”  </li></ul>
  30. 30. Theories <ul><li>Apparent distance theory – appears farther away  larger </li></ul><ul><ul><li>Size-distance paradox </li></ul></ul><ul><li>Distance, visual angle, & linear size illusions work together </li></ul><ul><ul><li>Oculomotor micropsia / macropsia  visual angle </li></ul></ul><ul><ul><li>Distance cues  macropsia for horizon moon </li></ul></ul>
  31. 31. The Mystery Spot <ul><li>Tilted house </li></ul><ul><li>No visible horizon – assumed horizon with internal reference frame of house </li></ul><ul><li>Your body is on a tilt as well – enhances effects as much as 3x </li></ul><ul><li>Application to pilots </li></ul>
  32. 32. A new perspective on seeing <ul><li>Many theories, none are all-encompassing yet </li></ul><ul><li>New ways to see things – become more aware of space through witnessing these illusions </li></ul><ul><li>Perspective is a powerful tool – in ‘imitating’ reality, it can also deceive </li></ul><ul><li>Seeing is believing  Perceiving is believing </li></ul>

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