Visual IllusionsVisual Illusions

Playing with PerspectivePlaying with Perspective

CS99D Final ProjectCS99D Final Project

By: Jason AndersonBy: Jason Anderson

Professor Marc LevoyProfessor Marc Levoy

William HogarthWilliam 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 Theories of Geometrical IllusionsIllusions

• Eye-movement Eye-movement perceived length perceived length• Perspective cuesPerspective cues• Transactionalist approachTransactionalist approach• Adaptation-level theoryAdaptation-level theory

Eye-Movement TheoryEye-Movement Theory

• Line length Line length eye movement eye movement• Testable, but usually fails – initial Testable, but usually fails – initial

perception, eyes are stableperception, eyes are stable• Finding an index of eye movements Finding an index of eye movements

a problema problem

Müller-Lyer LinesMüller-Lyer Lines

• Eye-movement theory: Arrowheads Eye-movement theory: Arrowheads influence extent of eye movementsinfluence extent of eye movements

Perspective CuesPerspective Cues

• Pictures converted in our brain Pictures converted in our brain from 2-dimensional drawings to from 2-dimensional drawings to represent 3-dimensional scenesrepresent 3-dimensional scenes

• Different level of explanation – Different level of explanation – does not propose a mechanism for does not propose a mechanism for perceptionperception

• Well established, although some Well established, although some ‘loopholes’ have been found‘loopholes’ have been found

Müller-Lyer Lines RevisitedMüller-Lyer Lines Revisited

• The same illusion through The same illusion through perspective cuesperspective cues

• Oculomotor Macropsia/MicropsiaOculomotor Macropsia/Micropsia

Transactionalist TheoryTransactionalist Theory

• The world is a product of perception, not a The world is a product of perception, not a cause of itcause of it– Hamlet: Do you see yonder cloud that’s almost in Hamlet: Do you see yonder cloud that’s almost in

shape of a camel?shape of a camel?– Polonius: By the Mass, and ‘tis like a camel indeed.Polonius: By the Mass, and ‘tis like a camel indeed.– Hamlet: Methinks it is like a weaselHamlet: Methinks it is like a weasel– Polonius: It is backed like a weaselPolonius: It is backed like a weasel– Hamlet: Or like a whale?Hamlet: Or like a whale?– Polonius: Very like a whalePolonius: Very like a whale

Hamlet (Act III, Scene II)Hamlet (Act III, Scene II)

• Change our way of looking Change our way of looking Perception will Perception will changechange

Old Man

Adaptation-Level TheoryAdaptation-Level Theory

• Helson, 1964 – “spatial pooling”Helson, 1964 – “spatial pooling”• Green & Stacey, 1966 applied to Green & Stacey, 1966 applied to

illusionsillusions• Past stimulation Past stimulation current current

stimulationstimulation• ““stored norms”stored norms”• ““Top-down processing”Top-down processing”• Some flaws – Ames roomSome flaws – Ames room

Depth Cues on a Flat Depth Cues on a Flat SurfaceSurface

• 1967, R.L. Gregory – all pictures 1967, R.L. Gregory – all pictures are “impossible objects”are “impossible objects”

• Conflicting depth cues in the Conflicting depth cues in the content of the picture with the flat content of the picture with the flat surface on which it is presentedsurface on which it is presented

Retinal DisparityRetinal Disparity

• No retinal No retinal disparity on a flat disparity on a flat surfacesurface

• As a viewer of an As a viewer of an image, we choose image, we choose to suppress the to suppress the cue of retinal cue of retinal disparitydisparity

Source: http://frank.mtsu.edu/~pyskip/splec6.htm

The Acceptance of The Acceptance of PerspectivePerspective

• We have come to accept that although We have come to accept that although we are seeing a flat surface, that the we are seeing a flat surface, that the objects on it represent 3 dimensional objects on it represent 3 dimensional conceptsconcepts

• Pictorial cues: interposition (occlusion), Pictorial cues: interposition (occlusion), relative size, linear perspective & relative size, linear perspective & texture gradientstexture gradients

• Ambiguous dimensional cues can lend Ambiguous dimensional cues can lend themselves to be great visual illusionsthemselves to be great visual illusions

Depth AmbiguityDepth Ambiguity

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

Wundt’s crossesWundt’s crosses

• Hering (1879) & Wundt (1898)Hering (1879) & Wundt (1898)• Most ambiguous of all figuresMost ambiguous of all figures• Infinite number of interpretations, Infinite number of interpretations,

but perceptual system tries to but perceptual system tries to settle with a ‘best’ onesettle with a ‘best’ one

Sanford’s figureSanford’s figure

• Sanford, 1903Sanford, 1903• Although there may be an obvious Although there may be an obvious

‘best’ interpretation, once can ‘best’ interpretation, once can easily be persuaded to accept an easily be persuaded to accept an alternate one!alternate one!

Of Ambiguous Figures and Of Ambiguous Figures and Depth ReversalsDepth Reversals

• Necker cubeNecker cube

• Mach BookMach Book

Of Ambiguous Figures & Of Ambiguous Figures & Depth Reversals 2Depth Reversals 2

• Not enough information in the Not enough information in the image to make a decision as to the image to make a decision as to the “best” interpretation“best” interpretation

• Taken advantage of to create Taken advantage of to create “impossible” figures“impossible” figures

The ‘freemish’ crateThe ‘freemish’ crate

• Cochran’s photo of his ‘freemish’ Cochran’s photo of his ‘freemish’ crate (1966). crate (1966).

How did he do that???How did he do that???

Any guesses?Any guesses?

Viewing from a single, Viewing from a single, special perspectivespecial perspective

• Viewing the image from a Viewing the image from a misleading perspectivemisleading perspective

• Viewing from another angle wrecks Viewing from another angle wrecks the effectthe effect

• Monocular Monocular viewing viewing requiredrequired

• OcclusionOcclusion

Misleading depth cuesMisleading depth cues

• Stage scenery – gives impression of Stage scenery – gives impression of greater depthgreater depth

• The Ames RoomThe Ames Room

The Ames RoomThe Ames Room

Of Giants and Dwarves?Of Giants and Dwarves?

• Of course not!Of course not!

• But how?But how?

What’s going on here?What’s going on here?

• Adelbert Ames, Jr. (1946) – concept Adelbert Ames, Jr. (1946) – concept by Helmoltzby Helmoltz

• Special viewpoint – monocularSpecial viewpoint – monocular• Floor, ceiling, some walls, & Floor, ceiling, some walls, &

windows are trapezoidalwindows are trapezoidal• Inclined floorInclined floor• Appears as a normal Appears as a normal

cubic roomcubic room

So how does it work?So how does it work?

• Peephole removes stereopsisPeephole removes stereopsis• Forms an identical image of a cubic room on Forms an identical image of a cubic room on

your retinayour retina• Both corners of the room subtend the same Both corners of the room subtend the same

visual angle to visual angle to your eye – appear your eye – appear equidistantequidistant

• Seckel & Klarke:Seckel & Klarke:Past experiencesPast experiencesnot relevantnot relevant

But what about But what about the people?the people?

• A split between perception & A split between perception & expectationexpectation

• Apparent cubic perspective overrides Apparent cubic perspective overrides sense of size constancysense of size constancy

• Stanford psychologistStanford psychologistRobert Shepherd – use Robert Shepherd – use background & background & relationship to the horizon relationship to the horizon to judge sizeto judge size

Retinal Size != Apparent Retinal Size != Apparent SizeSize

• Distance cues: relative size of Distance cues: relative size of elements, separation, density, elements, separation, density, clarity, backgroundclarity, background

But is the Ames Room But is the Ames Room necessary?necessary?

• Seckel and Klarke: only charmSeckel and Klarke: only charm• An apparent horizontal path is all An apparent horizontal path is all

that’s necessarythat’s necessary• Richard Gregory: same effect, Richard Gregory: same effect,

ambiguous backgroundambiguous background

The Moon IllusionThe Moon Illusion

• Perceived distance, visual Perceived distance, visual angle, & linear size != angle, & linear size != physical valuesphysical values

• Illusion from comparison of Illusion from comparison of perceived values at the perceived values at the horizon & at the zenithhorizon & at the zenith

• Subtends .5º in the eye no Subtends .5º in the eye no matter whatmatter what

• Not atmosphericNot atmospheric• Illusion disappears in a Illusion disappears in a

“mooning position” “mooning position”

TheoriesTheories

• Apparent distance theory – appears Apparent distance theory – appears farther away farther away larger larger– Size-distance paradoxSize-distance paradox

• Distance, visual angle,Distance, visual angle,& linear size illusions& linear size illusionswork togetherwork together– Oculomotor micropsia Oculomotor micropsia

/ macropsia / macropsia visual angle visual angle– Distance cues Distance cues macropsia for macropsia for

horizon moonhorizon moon

The Mystery SpotThe Mystery Spot

• Tilted houseTilted house• No visible horizon – assumed horizon No visible horizon – assumed horizon

with internal reference frame of housewith internal reference frame of house• Your body is on a Your body is on a

tilt as well – tilt as well – enhances effectsenhances effectsas much as 3xas much as 3x

• Application toApplication topilotspilots

A new perspective on A new perspective on seeingseeing

• Many theories, none are all-Many theories, none are all-encompassing encompassing yetyet

• New ways to see things – become more New ways to see things – become more aware of space through witnessing these aware of space through witnessing these illusionsillusions

• Perspective is a powerful tool – in Perspective is a powerful tool – in ‘imitating’ reality, it can also deceive‘imitating’ reality, it can also deceive

• Seeing is believing Seeing is believing Perceiving is Perceiving is believingbelieving