By James J. Todd and Victor J. PerottiPresented by Samuel Crisanto

THE VISUAL PERCEPTION OF SURFACE ORIENTATION FROM OPTICAL MOTION

What motivates the paper?• Scientists were investigating what information is

available from motion by studying how people interpreted a series of frames in an animation.

• In particular, they were wondering:

●How much information is uniquely specified in the optic array?

●How much of that information do human perceptual systems extract?

• It was shown that:

●You can specify rigid motion from non-rigid motion given○ 2 views (frames)

●You can mathematically determine an object’s 3D structure given

○ 3 distinct views of 4 points

What information is uniquely specified in the optic array?

Do our perceptual systems extract this information?• When you show people a multiple frame sequence:

●They cannot differentiate between structures even though there is sufficient information

●They see a definite 3D shape even if available information is ambiguous

○ They give a “highly reliable answer…[that exhibits] systematic biases”.

● The paper asks: ○ What aspects of image motion determine how we

perceive the orientation of a flat, rotating surface?

The Claim: Our visual perception of 3-D structure from motion is based primarily on first-order temporal derivatives of moving images.

What do we want to know?

First Order Temporal Derivatives?

VELOCITY is first-order information

• Our perceptual system cares about how quickly points move

• Our perceptual system does not care about how quickly points accelerate (this is higher order information)

What information is uniquely specified in the optic array?We can model a surface as

______________________________

Tilt is uniquely specified

Tilt =

______________________________

Slant is not uniquely specified (we need the angular velocity w)

Slant =

The Hypothesis:

● People can accurately judge tilt because it is a ratio of first-order relationships.

● People base their estimation of slant on the local deformation of textures.

(Recall that slant = )

What information do our perceptual systems extract?

The Experimental StimulusPeople used shuttered glasses to view an LCD screen with

a stereogram of a rocky texture

The texture simulated two oriented surfaces

The texture had been mapped to a dihedral angle

● They occluded the edges so observers could not get information from bounding contours

They deformed the texture to simulate rotation

Observers reported perceived tilt and slant

● Observers could toggle back and forth between the deforming texture and a computer model of a dihedral angle

● They were allowed to adjust the model of the angle until it matched what they perceived

● Models a constant flow field: no higher order derivatives. Slant is NOT uniquely specified in a constant flow field.

● 26 different display conditions, with different velocity values.

● The display conditions were presented five times each in a random sequence over a period of two experimental sessions.

Experiment 1:

Experiment 1: Tilt in a Constant Flow FieldObservers are almost perfectly accurate at determining tilt

Perceived slant depends on Vx and Vy, not deformation

Experiment 1: Slant in a Constant Flow Field

A New HypothesisPeople do not judge slant based on deformation. They instead judge slant by

where tau is tilt and alpha is a free parameter.

IMPLICATION: Since slant is underdetermined, our visualsystem uses tilt, something which is uniquely determined.

Support for the HypothesisObserver’s perceptions fit the new hypothesis (alpha = .24)

Experiment 2: Higher Order Relations● The texture no longer deforms with a constant flow field:

higher order relationships were simulated

● We now have a true simulated slant

● Experimenters used two different angular velocities

● If we use higher order relations, then the different values of omega will yield different perceived slant

Experiment 2: Perception of TiltPeople are still very accurate perceivers of tilt.

Experiment 2: Perception of SlantPeople are less accurate at perceiving slant.

Experiment 2:Testing the old hypothesisOur perception of slant is not based purely on the

deformation of a texture.

Using .066 as the free parameter

Experiment 2: Testing the new hypothesis

● The perception of 3-D structure from motion is based primarily on first-order temporal derivatives of moving elements within a visual image.

● People accurately estimate tilt, which is uniquely specified by first order information

● People inaccurately judge slant, even when slant is uniquely determined by higher-order derivatives. ○ People seem to judge slant as a function of first

order derivatives and tilt.

Conclusion