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 lengthEye-movement  perceived length Perspective cuesPerspective cues Transactionalist approachTransactionalist approach Adaptation-level theoryAdaptation-level theory

4. Eye-Movement Theory Line length  eye movementLine length  eye movement Testable, but usually fails – initial perception, eyes are stableTestable, but usually fails – initial perception, eyes are stable Finding an index of eye movements a problemFinding an index of eye movements a problem

5. Müller-Lyer Lines Eye-movement theory: Arrowheads influence extent of eye movementsEye-movement theory: Arrowheads influence extent of eye movements

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

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

8. Transactionalist Theory The world is a product of perception, not a cause of itThe 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 changeChange our way of looking  Perception will change Old Man

9. Adaptation-Level Theory Helson, 1964 – “spatial pooling”Helson, 1964 – “spatial pooling” Green & Stacey, 1966 applied to illusionsGreen & Stacey, 1966 applied to illusions Past stimulation  current stimulationPast stimulation  current stimulation “stored norms”“stored norms” “Top-down processing”“Top-down processing” Some flaws – Ames roomSome flaws – Ames room

10. Depth Cues on a Flat Surface 1967, R.L. Gregory – all pictures are “impossible objects”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 presentedConflicting 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 surfaceNo retinal disparity on a flat surface As a viewer of an image, we choose to suppress the cue of retinal disparityAs 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 conceptsWe 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 gradientsPictorial cues: interposition (occlusion), relative size, linear perspective & texture gradients Ambiguous dimensional cues can lend themselves to be great visual illusionsAmbiguous 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 ambiguityBecause of the way everything we see is projected onto the retina, there is a great deal of ambiguity

14. Wundt’s crosses Hering (1879) & Wundt (1898)Hering (1879) & Wundt (1898) Most ambiguous of all figuresMost ambiguous of all figures Infinite number of interpretations, but perceptual system tries to settle with a ‘best’ oneInfinite number of interpretations, but perceptual system tries to settle with a ‘best’ one

15. Sanford’s figure Sanford, 1903Sanford, 1903 Although there may be an obvious ‘best’ interpretation, once can easily be persuaded to accept an alternate one!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 cubeNecker cube Mach BookMach Book

17. Of Ambiguous Figures & Depth Reversals 2 Not enough information in the image to make a decision as to the “best” interpretationNot enough information in the image to make a decision as to the “best” interpretation Taken advantage of to create “impossible” figuresTaken advantage of to create “impossible” figures

18. The ‘freemish’ crate Cochran’s photo of his ‘freemish’ crate (1966).Cochran’s 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 perspectiveViewing the image from a misleading perspective Viewing from another angle wrecks the effectViewing from another angle wrecks the effect Monocular viewing requiredMonocular viewing required OcclusionOcclusion

21. Misleading depth cues Stage scenery – gives impression of greater depthStage scenery – gives impression of greater depth The Ames RoomThe Ames Room

22. The Ames Room

23. Of Giants and Dwarves? Of course not!Of course not! But how?But how?

24. What’s going on here? Adelbert Ames, Jr. (1946) – concept by HelmoltzAdelbert Ames, Jr. (1946) – concept by Helmoltz Special viewpoint – monocularSpecial viewpoint – monocular Floor, ceiling, some walls, & windows are trapezoidalFloor, ceiling, some walls, & windows are trapezoidal Inclined floorInclined floor Appears as a normal cubic roomAppears as a normal cubic room

25. So how does it work? Peephole removes stereopsisPeephole removes stereopsis Forms an identical image of a cubic room on your retinaForms an identical image of a cubic room on your retina Both corners of the room subtend the same visual angle to your eye – appear equidistantBoth corners of the room subtend the same visual angle to your eye – appear equidistant Seckel & Klarke: Past experiences not relevantSeckel & Klarke: Past experiences not relevant

26. But what about the people? A split between perception & expectationA split between perception & expectation Apparent cubic perspective overrides sense of size constancyApparent cubic perspective overrides sense of size constancy Stanford psychologist Robert Shepherd – use background & relationship to the horizon to judge sizeStanford psychologist Robert Shepherd – use background & relationship to the horizon to judge size

27. Retinal Size != Apparent Size Distance cues: relative size of elements, separation, density, clarity, backgroundDistance cues: relative size of elements, separation, density, clarity, background

28. But is the Ames Room necessary? Seckel and Klarke: only charmSeckel and Klarke: only charm An apparent horizontal path is all that’s necessaryAn apparent horizontal path is all that’s necessary Richard Gregory: same effect, ambiguous backgroundRichard Gregory: same effect, ambiguous background

29. The Moon Illusion Perceived distance, visual angle, & linear size != physical valuesPerceived distance, visual angle, & linear size != physical values Illusion from comparison of perceived values at the horizon & at the zenithIllusion from comparison of perceived values at the horizon & at the zenith Subtends.5º in the eye no matter whatSubtends.5º in the eye no matter what Not atmosphericNot atmospheric Illusion disappears in a “mooning position”Illusion disappears in a “mooning position”

30. Theories Apparent distance theory – appears farther away  largerApparent distance theory – appears farther away  larger –Size-distance paradox Distance, visual angle, & linear size illusions work togetherDistance, visual angle, & linear size illusions work together –Oculomotor micropsia / macropsia  visual angle –Distance cues  macropsia for horizon moon

31. The Mystery Spot Tilted houseTilted house No visible horizon – assumed horizon with internal reference frame of houseNo visible horizon – assumed horizon with internal reference frame of house Your body is on a tilt as well – enhances effects as much as 3xYour body is on a tilt as well – enhances effects as much as 3x Application to pilotsApplication to pilots

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