Week 5B: Depth perception and motion perception and eye movements

11 Depth cues

Occlusion, retinal size, perspective, familiarity, height in picture, blur, brightness, colour, shadow, motion parallax, stereo vision (disparity)

If objects of known size have different retinal sizes then they must be at (…). Familiarity and size.

Different distances

Explain the Ames room based on the depth cues

perspective (lines look parallel), relative size (left person looks smaller than the right one), and occlusion (the real shapes are hidden because of illusion)

Lens accommodation: For far objects the ciliary muscles are (…) and the lens is (…)

Relaxed, thin

Lens accommodation: For near objects the ciliary muscles are (…) and the lens is (…)

Tensed, thick

The state of the extraocular muscles offers a cue to vergence angle (gamma), and hence fixation distance. Large distance: (…) vergence angle

Small

Binocular disparity

Slight difference in the images seen by the left and right eyes due to their horizontal separation

When does crossed disparity happen

when an object is closer than the fixation point (The image of the object falls to the outer side (temporal side) of each retina, so to align them mentally, the brain has to "cross" the lines of sight inward.)

When does uncrossed disparity happen

when an object is farther than the fixation point (The image lands on the inner side (nasal side) of each retina, so the brain "uncrosses" the lines of sight outward to fuse them)

Vergence angle formula (binocular parallax)

beta - alpha

Version angle formula (binocular lattitude)

(beta + alpha)/2

Vieth-Muller circle

The curve of constant vergence angle is a circle through the fixation point and the nodal points of the eyes

How is stereo vision caused

Each eye having a slightly different image (horizontal disparity)

Why does stereo vision cause head-ache and eye-strain?

Your eyes are tricked into converging as if objects are at different depths, but the actual object is at a fixed distance, so accommodation doesn't match. This conflict between cues confuses the visual system and makes the eye muscles work harder to reconcile the mismatch

What is vergence

the inward or outward turning of the eyes to focus on objects at different distances.

How does a stereo viewer prevent eye-strain and headaches

Aligning the eye correctly to look at the different images by blocking the incorrect half-image

Random Dot Stereogram

A subset of dots in one image is shifted in position relative to the other images. The disparity between the images induces a sensation of depth.

What does a Random Dot Stereogram show

Disparity alone is sufficient to support an impression of depth and shape

What is the correspondence problem in stereopsis

Each point in one image can be matched in many ways with points in the other image

Cue integration

In stereograms, virtual reality cue conflicts occur. This is used in psychophysical experiments to determine how the human brain combines depth cues

Cue combination

The visual system combines cues to derive robust, reliable depth estimates

Monocular cues are available using (…) eye(s), while binocular cues require (…) eye(s).

one, two

Cue magnitude generally varies lawfully with (…) and varies most at depths (…)

a distance from fixation, close to fixation