The Human Visual System

1. The Human Visual System Part 2: Perception

2. Visual Perception • How one visually interprets a scene • 4 forms of perception to be studied: • Depth • Color • Temporal • Motion

3. HOW FAR ? Depth Perception • How does one determine how far away an object is located?

4. Depth Perception • Monocular Cues • Require only 1 eye to perceive depth; Cyclops. http://www.shag-art.com/cyclops.html

5. Depth Perception • Binocular Cues • Require 2 eyes to perceive depth.

6. Interposition (Overlap) An object that is partially covered by another object is farther away. Monocular Cue #1

7. Familiar Size Previous knowledge of object sizes aid in judging distance. Monocular Cue #2 Which object appears closer?

8. Linear Perspective The farther away an object is the smaller it appears to be. Monocular Cue #3

9. 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. Monocular Cue #4

10. 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). Monocular Cue #5

11. Example 2 Close one eye. Hold your left thumb upward at arm’s length. Hold your right thumb upward at half arm’s length. Positionthe 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. Monocular Cue #5 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. Shading Uses light falling on an object from a certain angle to give form and depth to an object. Cast shadows aid in locating an object. Monocular Cue #6

13. Patterns Use contour lines to infer depth. Monocular Cue #7

14. Accommodation The change of shape performed by the eye lens to focus on an object aids the brain in determining the object’s distance. Monocular Cue #8 Thick lens - object is near Thin lens - object is far

15. 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) Binocular Cue #1 45° 20°

16. 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. Binocular Cue #2 Threading a needle utilizes retinal disparity.

17. S I L Relative response 400 460 490 500 530 600 650 700 Wavelength (nm) Blue Cyan Green Red Color Perception • Trichromats • Humans have three cones that correspond to three ranges of the visible light in the areas of red, green, and blue light.

18. Normal Vision Viewer requires 3 basic colors to match another color. 91% Males ~ 99% Females Anomalous Trichromacy Also requires 3 basic colors to match another color, but the ratios of those three basic colors differ from a person with normal vision. ~ 6% Males Human Color Vision Deficiencies

19. Monochromacy Requires one color to match any other color Two types: Cone monochromats: Only one type of cone (very small % of population) Rod monochromats: Only the rods respond (.003% males) Dichromacy Requires two colors to match any other color; 8-10% Caucasion males Four types: Protanopes: No L cones (1% males) Deuteranopes: No I cones (1% males) Tritanopes: No S cones (very small % pop.) Tetartanopes: Lack chromatic channel (RG or YB, small %) Human Color Vision Deficiencies

20. Human Color Vision Deficiencies

21. 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 Perception

22. 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. Temporal Perception

23. Real Movement An object physically moves. Induced Movement The background moves behind an object causing the perception that the object moved. Motion Perception

24. 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 Motion Perception

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

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

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