IPCV ‘06 August 21 – September 1 Budapest Jussi Parkkinen Markku Hauta-Kasari Department of Computer Science University of Joensuu, Finland

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

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

What should be the spectral resolution, 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, R 2, G 2, B 2,..) 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

Spektrikuvan kanavakuvia

Spectral Face Image

Spectral Image

Image Types TYPE SPECTRAL CHANNELS Gray-scale Trichromatic Spectral –Hyperspectral Real-time spectral Single Three >3 Numerous

MEMORY REQUIREMENTS OF IMAGES Image size256x x512 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

Munsell system for color representation

Spectrum and Hue, Saturation, and Value (a) (b) (c)

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