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Visual Perception and Color CS/BIOEN 4640: Image Processing Basics April 3, 2012 The Human Eye Source: National Eye Institute, NIH Photoreceptors in the Retina Cones Color vision - three wavelengths (S, M, L) Mostly in fovea Less

SLIDE 1

Visual Perception and Color

CS/BIOEN 4640: Image Processing Basics April 3, 2012

SLIDE 2

The Human Eye

Source: National Eye Institute, NIH

SLIDE 3

Photoreceptors in the Retina

Source sciencephoto.com

Cones

◮ Color vision - three

wavelengths (S, M, L)

◮ Mostly in fovea ◮ Less sensitive to light ◮ ∼6-7 million

Rods

◮ Mostly see blue

wavelengths

◮ Not in fovea ◮ Sensitive to light ◮ ∼120 million

SLIDE 4

Density of Rods and Cones

Source: webvision.med.utah.edu

SLIDE 5

Blindspot Due to Optical Disk

SLIDE 6

The Visual Pathway

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Source: www.aph.org/cvi/brain.html

Nice presentation here:

http://www.sumanasinc.com/webcontent/animations/content/visualpathways.html

SLIDE 7

The Visual Pathway

Source: Wikipedia

Dorsal Stream (green) processing “where” Ventral Stream (purple) processing “what”

SLIDE 8

Receptive Fields in the Retina

◮ Groupings of photoreceptors ◮ Center receptor acts in

pposition to those in ring

◮ Input to a ganglion ◮ Act as little edge detectors ◮ Single receptor can be

involved in multiple fields

Nice presentation here:

http://www.sumanasinc.com/webcontent/animations/content/receptivefields.html

SLIDE 9

Contrast Sensitivity

Source: Wikipedia

SLIDE 10

Cones and Color

Basis for Trichromatic Theory

400 450 500 550 600 650 700 Wavelength (nm) Normalized cone response S M L

Source: Wikipedia Source: webvision.med.utah.edu

SLIDE 11

Purkinje Shift

Source: Wikipedia

As light level falls, your rods (night vision) take over from your cones (day vision). Bright reds that looked lighter will appear darker.

SLIDE 12

Luminance and Color Constancy

Source: Wikipedia

We perceive objects to have the same color under different lighting conditions. Similar to white balance

n a camera

SLIDE 13

Color Opponency Theory

◮ Extends trichromatic color

theory

◮ Color perception is

relative to adjacent colors

◮ Differences in color are

more important

◮ Receptive fields involved

SLIDE 14

Cornsweet Illusion

SLIDE 15

Cornsweet Illusion

SLIDE 16

Links to Further Illusions

Wikipedia is a good starting place: en.wikipedia.org/wiki/Optical_illusion See “Luminance” and “Color” sections here: www.michaelbach.de/ot/ Inventor of the checkerboard illusion: persci.mit.edu/gallery See “Colour perception” section here (checkered box): www.echalk.co.uk/amusements/ OpticalIllusions/illusions.aspx