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."
Source: W. Hogarth, 1697-1764 Trustees of the British Museum
Theories of Geometrical
Illusions
• Eye-movement  perceived
length
• Perspective cues
• Transactionalist approach
• Adaptation-level 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
Müller-Lyer Lines
• Eye-movement theory:
Arrowheads influence extent of
eye movements
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
Müller-Lyer Lines
Revisited
• The same illusion through
perspective cues
• Oculomotor
Macropsia/Micropsia
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
Old Man
Hamlet (Act III, Scene II)
• Change our way of looking 
Perception will change
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
• 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
• 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
• 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
• Because of the way everything
we see is projected onto the
retina, there is a great deal of
ambiguity
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, 1903
• Although there may be an
obvious ‘best’ interpretation,
once can easily be persuaded to
accept an alternate one!
Of Ambiguous Figures
and Depth Reversals
• Necker cube
• Mach Book
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
• Cochran’s photo of his ‘freemish’
crate (1966).
How did he do that???
Any guesses?
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
• Stage scenery – gives
impression of greater depth
• The Ames Room
The Ames Room
Of Giants and Dwarves?
• Of course not!
• But how?
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?
• 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 experiences
not relevant
But what about
the people?
• A split between perception &
expectation
• Apparent cubic perspective overrides
sense of size constancy
• Stanford psychologist
Robert Shepherd – use
background &
relationship to the horizon
to judge size
Retinal Size != Apparent
Size
• Distance cues: relative size of
elements, separation, density,
clarity, background
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
• 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
• Apparent distance theory –
appears farther away  larger
– Size-distance paradox
• Distance, visual angle,
& linear size illusions
work together
– Oculomotor micropsia
/ macropsia  visual angle
– Distance cues  macropsia for
horizon moon
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 effects
as much as 3x
• Application to
pilots
A new perspective on
seeing
• Many theories, none are allencompassing 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