Color

Page
5

Usually projected to display:

(x,y) = (X/(X+Y+Z), Y/(X+Y+Z))

CIE XYZ Color matching functions

Slide 42

HSV color space

Hue, Saturation, Value

Nonlinear – reflects topology of colors by coding hue as an angle

Matlab: hsv2rgb, rgb2hsv.

Image from mathworks.com

Slide 43

Distances in color space

Are distances between points in a color space perceptually meaningful?

Slide 44

Distances in color space

Not necessarily: CIE XYZ is not a uniform color space, so magnitude of differences in coordinates are poor indicator of color “distance”.

McAdam ellipses:

Just noticeable differences in color

Slide 45

Attempt to correct this limitation by remapping color space so that just-noticeable differences are contained by circles  distances more perceptually meaningful.

Examples:

CIE u’v’

CIE Lab

Uniform color spaces

CIE XYZ

CIE u’v’

Slide 46

Today: Color

Measuring color

Spectral power distributions

Color mixing

Color matching experiments

Color spaces

Uniform color spaces

Perception of color

Human photoreceptors

Environmental effects, adaptation

Using color in machine vision systems

Slide 47

Color and light

Color of light arriving at camera depends on

Spectral reflectance of the surface light is leaving

Spectral radiance of light falling on that patch

Color perceived depends on

Physics of light

Visual system receptors

Brain processing, environment

Slide 48

The Eye

The human eye is a camera!

Iris - colored annulus with radial muscles

Pupil - the hole (aperture) whose size is controlled by the iris

Lens - changes shape by using ciliary muscles (to focus on objects at different distances)

Retina - photoreceptor cells

Slide by Steve Seitz

Slide 49

© Stephen E. Palmer, 2002

Cones

cone-shaped

less sensitive

operate in high light

color vision

Types of light-sensitive receptors

Rods

rod-shaped

highly sensitive

operate at night

gray-scale vision

Slide credit: Alyosha Efros

Slide 50