Introduction to Perception and Color David S. Ebert Computer Science & Electrical Engineering Department University of Maryland Baltimore County ebert@umbc.edu http://umbc.edu/~ebert

Perception Basics

Perception Basics Light enters eye and is focused on the retina Retina contains photoreceptors They send electrical impulses through the neurons to the brain Rods -used under low illumination (night) achromatic very sensitive higher concentration at periphery

Retinal Cones 3 Types: L-receptors: sensitive to long wavelengths of light (red) M-receptors: sensitive to medium wavelengths of light (green) S-receptors: sensitive to short wavelengths of light (blue)

Receptive Fields Millions of receptors in the retina Only ~1 million optic nerve fibers Receptors are grouped into fields and the output of the field is sent to the brain So vision reacts at the level of receptive areas (fields), not individual receptors

Mach Bands and Receptor Fields

Another Intensity Illusion

Cones & Rods vs. Luminance Eye can detect light variations on a range of 1:100,000,000,000,000 cd/cm2 Rods most sensitive at low illumination levels Become saturated between moonlight and low room light illumination Cones start functioning in the upper range of rod sensitivity

Color Vision Model

Trichromatic Color Theory vs Opponent Color Theory Three primary color receptors in the eye Opponent Color Theory: Certain phenomena can not be explained by trichromatic color Why don’t we see reddish greens or yellowish blues ?

Opponent Color Theory (cont.) Neural processing of trichromatic values yields 2 major opponent color classes Spectrally opponent processes (channels) Red/Green Channel: difference of the red and green cone signals Yellow/Blue Channel: difference of the blue cones and the sum of red+green

Opponent Color Theory (cont.) Luminance : Spectrally non-opponent process (black vs. white) based on input from all cones Greater sensitivity to luminance than chrominance Greatest bandwidth for conveying information Chrominance sensitivity decreases with low luminance

Color Spaces RGB YIQ HSV CIE Perceptually Linear Color Spaces

RGB Color Space

YIQ – TV broadcasting Recording of RGB for transmission efficiency and downward compatibility with B&W TV. Recoded signal is transmitted using NTSC standard Y = Luminance (only one shown in B&W TV) I = Chromaticity

HSV Color Space User Oriented H = hue S = saturation V = Value measured by angle around verticle axis with red at 8º, green at 120º, etc. . . S = saturation ranging from 0 to 1 V = Value

Perceptually Linear Color Spaces CIE created 2 perceptually linear (uniform) color spaces: CIE lab and CIE L*u*v* CIE L*u*v* L = lightness u* = red-green v* =blue-yellow Not commonly used- Why? Ignorance Complexity of conversion Need cube root - expensive Need tristimulus coordinates for monitor

CIE Chromaticity Diagram

What’s Wrong with RGB? Interpolation in RGB will not give correct perceptual information RGB linear color maps will give incorrect information What about vector interpolation of RGB? Is there something better and quick? Approximations based on table look-ups

Possible Alternative for RGB Interpolation Use Opponent Color Model R-G B-Y Luminance Use Hue, Saturation, and Value Can you understandably display three variables using these techniques?

More Color Issues Viewing on a CRT: Non-linear device Luminance = voltagegamma Small range of luminance values compared to visual sensitivity

Other Visual Perception Factors Depth Curvature Orientation Transparency Texture Motion

Pre-attentive Processing Hardwired processing without conscious processing At-a-glance processing Processed in parallel Less than 200 milliseconds Accurate Independent of the display size

Which Attributes are Pre-attentive? Chromaticity (2 dimensions) Luminance (1) Curvature (1?) Orientation (1?) Texture (3+) Motion (4) Opacity?

Pre-attentive Bandwidth Very little discrimination pre- attentively 2 -3 bits per channel Color: studies show seven color pre- attentively Can we successfully combine several pre-attentive channels? Sometimes Interference can occur

Good References Pages Colin Ware: The Joy of Visual Perception: http://www.omg.unb.ca/~cware/cs5405 The Joy of Visual Perception: http://www.yorku.ca/eye Chris Healey http://www.cs.berkeley.edu/~healey/PP

Conclusion Illumination, and Shading Affect Your Resulting Images Can improve perception of information or decrease it Visual Perception must be taken into account in choosing colors, illumination, and shading techniques Visual system is very sensitive to certain characteristics, not very sensitive to others Take advantage of this!