imaging science fundamentalschester f. carlson center for imaging science the human visual system...

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

The Human Visual SystemThe Human Visual SystemPart 2: Perception


Visual PerceptionVisual Perception

How one visually interprets a scene

4 forms of perception to be studied: Depth Color Temporal Motion


How does one determine how far away an object is located?

Depth PerceptionDepth Perception

HOW FAR ?



Monocular Cues Require only 1 eye to perceive depth;

Cyclops.

http://www.shag-art.com/cyclops.html



Binocular Cues Require 2 eyes to perceive depth.


Monocular Cue #1Monocular Cue #1

Interposition (Overlap) An object that is

partially covered by another object is farther away.



Familiar Size Previous knowledge of

object sizes aid in judging distance.

Which object appears closer?Which object appears closer?



Linear Perspective The farther away

an object is the smaller it appears to be.



Atmospheric Perspective Objects farther off in

the distance appear less saturated and less sharp (fuzzier) than those nearby.

The more atmospheric particles between the viewer and a distant object the more light that is scattered.



Motion Parallax Stationary objects

that are physically closer to a moving viewer appear to shift faster than those farther away.

Example 1 Driving by in a car

looking at objects near and far (animation).



Example 2 Close one eye. Hold your left thumb

upward at arm’s length. Hold your right thumb

upward at half arm’s length.

Position the thumbs so the right thumb blocks the left and move your head to one side.

Example 3 Close one eye. Hold both index fingers

pointing toward each other.

Circle the fingers in a bike pedaling motion.

Stop them at eye level and move them inward to make them meet forming a straight line.

Observe that the background did not move, but the thumbs appeared to move relative to each other and the background.

Try again, but this time move your head side-to-side to tell the distance between your index fingers.


Monocular Cue #6Monocular Cue #6 Shading

Perception of light falling on an object from a certain angle gives form and depth to an object.

Shadows cast by an object aid in locating it.



Patterns Use contour lines to infer depth.



Accommodation The change of shape

performed by the eye lens to focus on an object aids the brain in determining the object’s distance.

Thick lens - object is near

Thin lens - object is far


Binocular Cue #1Binocular Cue #1

Convergence The angle between the line of sight of

each eye is larger as an object moves closer.

This works for nearby objects (with accommodation)

45° 20°


Binocular Cue #2Binocular Cue #2

Retinal Disparity Each eye receives a

slightly different view of a scene.

The two views are used to determine the ratio of distances between nearby objects.

Example Close one eye and

position your thumbs so that one blocks the other with ~1 cm distance between them.

Switch your viewing eye. Open both eyes.

Threading a needle utilizes retinal disparity.


Color PerceptionColor Perception

Trichromats Humans have three cones that correspond to

three ranges of the visible light in the areas of red, green, and blue light.

400 460 530 650600 700500

Wavelength (nm)

Rela

tive r

esp

on

se

Blue Cyan Green Red

490

I LS


Human Color Vision DeficienciesHuman Color Vision Deficiencies

Normal Vision Viewer uses 3 basic

colors (one for each cone) to match all colors in the spectrum.

91% Males ~ 99% Females

Anomalous Trichromacy Also uses 3 basic

colors to match all colors, but the ratios of those three basic colors differ from a person with normal vision.

~ 6% Males



Monochromacy Sensitivity to

only one color (or no color sensitivity at all)

Two types: Cone

monochromats: Only one type of cone (very small % of population)

Rod monochromats: Only the rods respond (.003% males)

Dichromacy Sensitivity to only two

colors; 8-10% of Caucasion males

Four types: Protanopes: No L cones

(1% males) Deuteranopes: No I cones

(1% males) Tritanopes: No S cones

(very small % pop.)


Temporal PerceptionTemporal Perception

Negative Afterimages A viewer stares at an image for a period of

time. The cones become desensitized. Upon looking at a plain white surface, the

viewer perceives the previous image with complementary colors.


Temporal PerceptionTemporal Perception

Positive Afterimages - Persistence of Vision An intense flash of light allows a viewer to

see a scene. When the light goes out the signal from the

cones persist. The viewer still perceives the image.


Motion PerceptionMotion Perception

Real Movement An object

physically moves.

Induced Movement The background

moves behind an object causing the perception that the object moved.


Motion PerceptionMotion Perception

Stroboscopic Movement Quick, sequential

flashes of light / images that imply motion.

e.g. television, film, monitor

Autokinetic Movement A still spot of light

appears to move in an unlit background.

e.g. star in the night sky

imaging science fundamentalschester f. carlson center for imaging science the human visual system...

Documents

imaging science u

u example

u hold

u close

u linear perspective

u interposition overlap

scene u

u shadows