Visual Perception and Color CS/BIOEN 4640: Image Processing Basics - PowerPoint PPT Presentation

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 sensitive to light ◮ ∼ 6-7 million Rods ◮ Mostly see blue wavelengths ◮ Not in fovea ◮ Sensitive to light Source sciencephoto.com ◮ ∼ 120 million

Density of Rods and Cones Source: webvision.med.utah.edu

Blindspot Due to Optical Disk •

The Visual Pathway 1+/$%)2% 32&&)*+&+- ./#)* ,$%0$ ./#)* 34)"-5 !"#$%"& '$()*+&"#$ ,+*&$+- ./#)* 6"7)"#)2(- 8%)5"%9 :)-+"&;32%#$< Source: www.aph.org/cvi/brain.html Nice presentation here: http://www.sumanasinc.com/webcontent/animations/content/visualpathways.html

The Visual Pathway Dorsal Stream (green) processing “where” Ventral Stream (purple) processing “what” Source: Wikipedia

Receptive Fields in the Retina ◮ Groupings of photoreceptors ◮ Center receptor acts in opposition 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

Contrast Sensitivity Source: Wikipedia

Cones and Color Basis for Trichromatic Theory S M L Normalized cone response 400 450 500 550 600 650 700 Wavelength (nm) Source: webvision.med.utah.edu Source: Wikipedia

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

Luminance and Color Constancy We perceive objects to have the same color under different lighting conditions. Similar to white balance on a camera Source: Wikipedia

Color Opponency Theory ◮ Extends trichromatic color theory ◮ Color perception is relative to adjacent colors ◮ Differences in color are more important ◮ Receptive fields involved

Cornsweet Illusion

Cornsweet Illusion

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