1. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science The Human Visual System Part 2: Perception

2. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Visual Perception u How one visually interprets a scene u 4 forms of perception to be studied: u Depth u Color u Temporal u Motion

3. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science u How does one determine how far away an object is located? Depth Perception HOW FAR ?

4. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Depth Perception u Monocular Cues u Require only 1 eye to perceive depth; Cyclops. http://www.shag-art.com/cyclops.html

5. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Depth Perception u Binocular Cues u Require 2 eyes to perceive depth.

6. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #1 u Interposition (Overlap) u An object that is partially covered by another object is farther away.

7. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #2 u Familiar Size u Previous knowledge of object sizes aid in judging distance. Which object appears closer?

8. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #3 u Linear Perspective u The farther away an object is the smaller it appears to be.

9. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #4 u Atmospheric Perspective u Objects farther off in the distance appear less saturated and less sharp (fuzzier) than those nearby. u The more atmospheric particles between the viewer and a distant object the more light that is scattered.

10. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #5 u Motion Parallax u Stationary objects that are physically closer to a moving viewer appear to shift faster than those farther away. u Example 1 u Driving by in a car looking at objects near and far (animation).

11. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #5 u Example 2 u Close one eye. u Hold your left thumb upward at arm’s length. u Hold your right thumb upward at half arm’s length. u Position the thumbs so the right thumb blocks the left and move your head to one side. u Example 3 u Close one eye. u Hold both index fingers pointing toward each other. u Circle the fingers in a bike pedaling motion. u 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.

12. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #6 u Shading u Perception of light falling on an object from a certain angle gives form and depth to an object. u Shadows cast by an object aid in locating it.

13. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #7 u Patterns u Use contour lines to infer depth.

14. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Monocular Cue #8 u Accommodation u 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

15. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Binocular Cue #1 u Convergence u The angle between the line of sight of each eye is larger as an object moves closer. u This works for nearby objects (with accommodation) 45° 20°

16. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Binocular Cue #2 u Retinal Disparity u Each eye receives a slightly different view of a scene. u The two views are used to determine the ratio of distances between nearby objects. u Example u Close one eye and position your thumbs so that one blocks the other with ~1 cm distance between them. u Switch your viewing eye. u Open both eyes. Threading a needle utilizes retinal disparity.

17. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Color Perception u Trichromats u Humans have three cones that correspond to three ranges of the visible light in the areas of red, green, and blue light. 400460530650600700500 Wavelength (nm) Relative response BlueCyanGreenRed 490 ILS

18. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Human Color Vision Deficiencies u Normal Vision u Viewer uses 3 basic colors (one for each cone) to match all colors in the spectrum. u 91% Males u ~ 99% Females u Anomalous Trichromacy u Also uses 3 basic colors to match all colors, but the ratios of those three basic colors differ from a person with normal vision. u ~ 6% Males

19. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Human Color Vision Deficiencies u Monochromacy u Sensitivity to only one color (or no color sensitivity at all) u Two types: u Cone monochromats: Only one type of cone (very small % of population) u Rod monochromats: Only the rods respond (.003% males) u Dichromacy u Sensitivity to only two colors; 8-10% of Caucasion males u Four types: u Protanopes: No L cones (1% males) u Deuteranopes: No I cones (1% males) u Tritanopes: No S cones (very small % pop.)

20. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Human Color Vision Deficiencies

21. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Temporal Perception u Negative Afterimages u A viewer stares at an image for a period of time. u The cones become desensitized. u Upon looking at a plain white surface, the viewer perceives the previous image with complementary colors.

22. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Temporal Perception u Positive Afterimages - Persistence of Vision u An intense flash of light allows a viewer to see a scene. u When the light goes out the signal from the cones persist. u The viewer still perceives the image.

23. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Motion Perception u Real Movement u An object physically moves. u Induced Movement u The background moves behind an object causing the perception that the object moved.

24. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Motion Perception u Stroboscopic Movement u Quick, sequential flashes of light / images that imply motion. u e.g. television, film, monitor u Autokinetic Movement u A still spot of light appears to move in an unlit background. u e.g. star in the night sky

25. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Aftereffects of Movement u Waterfall Illusion u After staring at water falling, look at a still object, and it will appear to move upward. u Spiral Aftereffect u After staring at a rotating spiral: u a still spiral appears to move in the opposite direction. u an object will appear to deform in the spiral motion opposite that initially observed.

26. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Consequences of Movement u Perception of Causality u One event is seen after another, thus it may be interpreted that the 1st event caused the 2nd. u Personification u Human / animal characteristics given to inanimate objects. u Perception of Location u Object 1 is seen going behind object 2, thus causing the viewer to extrapolate the location of object 1. u Linking Successive Shots u e.g. television, film

27. Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Summary u Depth Perception u Interposition; Familiar Size; Linear Perspective; Atmospheric Perspective; Motion Parallax; Shading; Patterns; Accommodation u Convergence; Retinal Disparity u Color Perception u Trichromacy u Temporal Perception u Desensitization: negative afterimage u Persistence of Vision: positive afterimage u Motion Perception u Real; Induced u Stroboscopic; Autokinetic u Waterfall/Spiral Effects