COMPUTER GRAPHICS
CS 482 – FALL 2015
SEPTEMBER 1, 2015
HUMAN VISUAL PERCEPTION
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EYE PHYSIOLOGY
COLOR BLINDNESS
CONSTANCY
SHADOWS
PARALLAX
STEREOSCOPY
EYE PHYSIOLOGY
RODS & CONES
THE THIN LAYER OF NERVE CELLS AT THE
BACK OF THE EYE IS CALLED THE RETINA.
THE SECTION OF THE
RETINA THAT REACTS TO
COLOR IS CALLED THE
FOVEA.
HIGH RESOLUTION COLOR
IMAGING IS PROVIDED BY
LIGHT SENSOR CELLS
CALLED CONES, LOCATED
IN THE FOVEA.
CS 482 – FALL 2015
THE LIGHT SENSOR CELLS CAPABLE OF
WORKING OVER A WIDE RANGE OF
ILLUMINATION LEVELS AND OF PROVIDING
QUICK RESPONSE TO CHANGES ARE CALLED
RODS.
THE “TRISTIMULUS THEORY
OF COLOR” HYPOTHESIZES
THAT EACH CONE IS
SENSITIVE TO EITHER RED,
GREEN, OR BLUE.
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EYE PHYSIOLOGY
PHOTORECEPTORS
PHOTORECEPTOR CELLS ARE ARRANGED IN
CIRCULAR REGIONS KNOWN AS RECEPTIVE FIELDS.
ON-CENTER FIELDS HAVE A CORE OF LIGHTSENSITIVE RECEPTORS SURROUNDED BY DARKSENSITIVE RECEPTORS.
OFF-CENTER FIELDS HAVE A CORE OF DARKSENSITIVE RECEPTORS SURROUNDED BY LIGHTSENSITIVE RECEPTORS.
RECEPTIVE FIELDS ARE
FULLY ACTIVE WHEN LIGHT
IS RECEIVED BY THE LIGHTSENSITIVE RECEPTORS BUT
NOT BY THE DARKSENSITIVE RECEPTORS.
CS 482 – FALL 2015
RECEPTIVE FIELDS ARE
FULLY INACTIVE WHEN
LIGHT IS RECEIVED BY THE
DARK-SENSITIVE
RECEPTORS BUT NOT BY
THE LIGHT-SENSITIVE
RECEPTORS.
CENTER/SURROUND
ANTAGONISM OCCURS
WHEN LIGHT IS RECEIVED
BY BOTH THE LIGHTSENSITIVE AND THE
DARK-SENSITIVE
RECEPTORS.
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EYE PHYSIOLOGY
LATERAL INHIBITION
THE HERMANN GRID PICTURED AT LEFT BELOW GIVES THE IMPRESSION OF GRAY
SPOTS AT EACH INTERSECTION, UNLESS THOSE INTERSECTIONS ARE VIEWED
DIRECTLY.
THIS IS DUE TO LATERAL INHIBITION, IN
WHICH ON-CENTER RECEPTIVE FIELDS
HAVE THEIR LIGHT-SENSITIVE RECEPTORS
RECEIVING LIGHT WHILE THEIR DARKSENSITIVE RECEPTORS RECEIVE A MIX OF
LIGHT AND DARK.
MACH BANDING, THE CURVED APPEARANCE OF THE SOLID-COLORED
BANDS ABOVE, IS CAUSED BY RECEPTIVE FIELDS LINED UP AT THE
BORDER BETWEEN BANDS, WITH A CENTER RECEIVING MAINLY LIGHT
OR DARK AND A SURROUND RECEIVING A HEAVY MIX OF BOTH LIGHT
AND DARK.
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COLOR BLINDNESS
COLOR VISION DEFICIENCIES
MALFUNCTIONING OR MISSING CONES OF A PARTICULAR COLOR SENSITIVITY (RED,
GREEN, OR BLUE) RESULTS IN ABNORMAL COLOR PERCEPTION.
NORMAL VISION
(NO DEFICIENCIES)
92% OF MALES
99.5% OF FEMALES
PROTAN
(RED DEFICIENCY)
2% OF MALES
0.11% OF FEMALES
DEUTERAN
(GREEN DEFICIENCY)
6% OF MALES
0.33% OF FEMALES
TRITAN
(BLUE DEFICIENCY)
0.05% OF MALES
0.05% OF FEMALES
AN ISHIHARA COLOR
BLINDNESS TEST AND
HOW IT IS PERCEIVED
BY NORMAL AND
DEFICIENT VIEWERS
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CONSTANCY
COLOR CONSTANCY
ALTHOUGH THE SQUARES ON THE RIGHT APPEAR DARKER THAN THOSE ON THE
LEFT...
…THEY ARE ACTUALLY IDENTICAL.
IF LIGHT IS DETECTED
SOLELY BY THE CENTER
PHOTORECEPTORS IN
AN ON-CENTER
RECEPTIVE FIELD, THEN
THE FIELD’S RESPONSE
IS PROPORTIONAL TO
THE LIGHT INTENSITY.
CS 482 – FALL 2015
HOWEVER, IF LIGHT IS
ALSO DETECTED BY THE
DARK SURROUND
PHOTORECEPTORS,
THEN THE CENTER AND
SURROUND REGIONS
CONFLICT WITH EACH
OTHER AND IT TAKES A
GREATER AMOUNT OF
LIGHT TO ACHIEVE A
FIELD RESPONSE.
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CONSTANCY
GAMMA CORRECTION
BECAUSE OF THE HUMAN VISUAL SYSTEM’S SENSITIVITY TO LIGHT CONTRAST, RGB
VALUES ARE STORED AS GAMMA-CODED VALUES (R, G, B), USUALLY WITH   0.45.
WITHOUT GAMMA CODING
(0.25,0.25,0.2
5)
(0,0,0)
(0.5,0.5,0.5
)
(0.75,0.75,0.7
5)
(1,1,1)
WITH GAMMA CODING
(0,0,0)
(0.25,0.25,0.2
5)
(0.5,0.5,0.5
)
(0.75,0.75,0.7
5)
(1,1,1)
THIS RESULTS IN A WIDER VARIETY OF BRIGHTER SHADES, PROVIDING GREATER
ABILITY TO ACCOMMODATE THE HUMAN’S EYES SENSITIVITY TO LIGHT.
WHEN TRANSFERRING THE STORED IMAGE TO THE DISPLAY, IT MUST BE GAMMADECODED, USING A POWER OF   2.2.
GAMMA CODING
CS 482 – FALL 2015
GAMMA
DECODING
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CONSTANCY
SHAPE CONSTANCY
HUMANS PERCEIVE THAT AN OBJECT’S SHAPE AND SIZE HAVEN’T CHANGED, EVEN
IF THE PERSPECTIVE FROM WHICH THE OBJECT IS VIEWED DOES CHANGE.
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SHADOWS
VISUAL CUES
SHADOWS PROVIDE VISUAL CUES TO NOT ONLY AN OBJECT’S ELEVATION, BUT
ALSO ITS DISTANCE FROM THE VIEWER.
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PARALLAX
ILLUSION OF DEPTH
BY MODELING BACKGROUND IMAGES TO MOVE PAST THE CAMERA AT A SLOWER
RATE THAN FOREGROUND IMAGES, GRAPHICS DEVELOPERS CREATE THE ILLUSION
OF DEPTH.
BEFORE THE DEVELOPMENT OF 3D COMPUTER
MODELING, PARALLAX SCROLLING WAS
IMPLEMENTED VIA MULTI-PLANE CAMERA SYSTEMS
IN TRADITIONAL ANIMATION, VIA DEPTH BUFFERING
IN RASTER-GRAPHIC GAMES, AND VIA HTML5 AND
CSS ON WEB SITES.
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STEREOSCOPY
VERGENCE VS. ACCOMMODATION
PROJECTING A 3D IMAGE ONTO A 2D SCREEN OR TELEVISION DISPLAY TENDS TO
PLAY TRICKS WITH ONE’S VISUAL PERCEPTION.
IN THE REAL WORLD, WHEN A PERSON
TRIES TO FOCUS ON A DISTANT OBJECT,
THE EYES EXPERIENCE VERGENCE, WHERE
THEY BOTH ROTATE ON THEIR OWN
VERTICAL AXES UNTIL THEY BOTH POINT
AT THE OBJECT.
MEANWHILE, THE EYES EXPERIENCE
ACCOMMODATION, WHERE THEY
REFLEXIVELY FOCUS ON THE OBJECT IN ITS
DISTANT POSITION, CAUSING NEARBY
OBJECTS TO GO OUT OF FOCUS.
WITH 3D IMAGES, HOWEVER, THE EYES
ROTATE SO THEY ARE POINTED TO WHERE
THE OBJECT SEEMS TO BE, BUT THE
REFLEXIVE FOCUS MUST ADJUST TO THE
SCREEN’S ACTUAL LOCATION,
ELIMINATING THE NATURAL BLURRING OF
NEARBY OBJECTS.
CS 482 – FALL 2015
THIS RESULTS IN SERIOUS
EYESTRAIN AND
HEADACHES FOR MANY
3D AUDIENCE MEMBERS.
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STEREOSCOPY
DIGITAL 3D
ADVANCES IN DIGITAL PHOTOGRAPHY AND STEREOSCOPY HAVE ENABLED FILMMAKERS TO
AFFORDABLY PRODUCE 3D FILMS AND THEATRE OWNERS TO AFFORDABLY SHOW THEM.
A 3D DIGITAL CAMERA RECORDS VIDEO,
CIRCULARLY POLARIZING THE RIGHT
VIEWPOINT USING RIGHT-HANDED
POLARIZATION...
BOTH IMAGES ARE PROJECTED
SIMULTANEOUSLY ONTO THE THEATRE
SCREEN, IN A MANNER THAT
GUARANTEES THAT THE VIEWPOINTS ARE
SYNCHRONIZED.
CS 482 – FALL 2015
...WHILE RECORDING THE
LEFT VIEWPOINT USING
LEFT-HANDED CIRCULAR
POLARIZATION.
THE MOVIEGOER WEARS POLARIZED
GLASSES THAT ONLY ALLOW THE RIGHTHANDED POLARIZED IMAGES TO PASS
THROUGH THE RIGHT LENS AND THE
LEFT-HANDED POLARIZED IMAGES TO
PASS THROUGH THE LEFT LENS.
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