IPCV ‘06August 21 – September 1

Budapest

Jussi ParkkinenMarkku Hauta-Kasari

Department of Computer ScienceUniversity of Joensuu, Finland

http://spectral.joensuu.fi/

Introduction to spectral color

COLOR ANALYSIS

Eriväristen lehtien spektrejä

Spectral images from real and artificial indoor plants Kanae Miyazawa

Display characterictics

Principle of color detection

Light source

Colored object

Detection system•biological•artificial

Color image formation in human eye

CIE Illum inations D65, A and C

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100

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200

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300

300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

D65

A

C

RGB vs. Spectrum

• Is the spectrum needed?

• RGB is just a 3D projection of a spectrum

• RGB can produce nice colors on display, but not correct colors

Spectral approach to color

• In spectral approach, color is represented by color signal. This causes the color sensation

• The signal is part of electromagnetic spectrum

- in human color vision the range is 380-780 nm

• In spectral approach, we are not limited into this human visual range

What should be the spectralresolution, i.e.

the sampling rate in wavelenths?

Spectral dependence on sampling

Color dependence on sampling

Change of RGB-values due to sampling

Can we reproduce a spectrum from the RGB-values?

Data• 1494 color samples in 200 color images

– Munsell colors, Color checker, natural colors, wall paint

– cameras: Fuji FinePix and Canon Powershot

– illuminants: A and D65

Methods

1. Polynomial model (R, G, B, R2, G2, B2, ..)

2. Kernel models

Evaluation

• delta E and RMSE (for spectra)– average, std, maximum

Spectra with largest delta E polynomial model

Some preliminary tests with mobile phones cameras

Some preliminary tests with mobile phones cameras

Spektrikuvan kanavakuvia

Spectral Face Image

Spectral Image

Image Types

TYPE SPECTRAL CHANNELS---------------------------------------• Gray-scale• Trichromatic• Spectral

– Hyperspectral• Real-time spectral

• Single• Three• >3• Numerous• Numerous

MEMORY REQUIREMENTS OF IMAGES

Image size 256x256 512x512

gray-level image 65 kB 262 kB

color (RGB-) image 196 kB 786 kB

spectral, 20 nm resol. 1 MB 4 MB

spectral, 5 nm resol. 3 MB 15 MB

Pixels in color image are vectors

• What is the order of color?

• What means the average color?

• How to compute distance in spectral space?

• What is the structure of spectral color space?

Statistical Analysis of Natural Images

Statistical Analysis of Natural Images

Munsell system for color representation

Spectrum and Hue, Saturation, and Value

(a) (b) (c)

400 500 600 7000

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Wavelength(nm)

Reflec

tance

(%)

400 500 600 7000

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Wavelength(nm)

Reflec

tance

(%)

400 500 600 7000

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Wavelength(nm)

Reflec

tance

(%)

Motivation for spectral color

• Not to loose important color information

• To define optimal color sensors

• To develop better color vision models

• To develop novel instruments

• To develop spectral color classifiers and optical implementations for them

What is color?

”Color is more than light” ”Computer cannot describe color correctly”

”Color is a perception (of human beings)”• => Color cannot been measured!• => Is this fair to animals?

• In spectral approach, color is represented by color signal. This causes the color sensation

• In spectral approach, we are not limited into this human visual range