Color Models

by Patrick Kraft

Hochschule Ravensburg-WeingartenTechnik | Wirtschaft | Sozialwesen

Doggenriedstraße88250 Weingarten

Contents

• Introduction• Physical Aspects of Colored Light• Physiological Characteristics of the Human

Visual System• Tristimulus Color Spaces• Polar Coordinate Color Models• Conclusion

Introduction

Vision sensor / RGB camera is designed for the reproduction of Color for the human eye.

We first need to understand the human vision to use that kind of vision sensor.

Than we are able to convert RGB camera colors into an appropriate model for image processing.

Physical Aspects of Colored Light

Visible Light

Light and surfaces

Radiance = reflectance*radiance

Physiological Characteristics of the Human Visual System

Color vision

Normalized spectral sensivity curves of the three different types of cones

Colored light stimulates the cones and the rods

cones: s, m, l-cones

Relative spectral sensivity curves of the three different types of cones

Rods are most sensitive at 498 nm and used for night vision.

short-wavelength (s) cones 420 nm middle-wavelength (m) cones 534 nmlong-wavelength (l) cones 564 nm

There are three types of cones.

Tristimulus Color Spaces

Monochromatic Color Response

Every base color causes a certain activity on all three cones

Tristimulus space / values

We then define a given colour as

or just

CIE 1931 Standard RGB Color Space

Because of the linear algebraic properties stated by Grassmann’s laws, it is possible to represent colour stimuli by vectors in a three-dimensional space, called the tristimulus space.

CIE defined the CIE 1931 Standard RGB Colorimetric System with the monochromatic primaries:

Metamerism

Metamerism implies that two objects which appear to have exactly the same color, may have very different colors under different lighting conditions.

There are several different spectra that can appear as the same color to the observer.

Without metamerism there would be no color image reproduction on paper or screen

Bayer Array

A pattern of red, green, and blue filters on the camera image sensor.

Interpolation of surrounding colors to get the right color at each pixel

Brightness

There are twice as many green filter elements in the array as red and blue elements because: the human eye is more sensitive to green light than both red and blue light.

Therefore green colour accuracy is more important. The human eye is also more sensitive to changes in brightness than colour.

Luminance = 0.35*R + 0.55*G + 0.1*B

Polar Coordinate Color Models

Polar Coordinate Color Spaces allows movements in color space which correspond more closely to human perception

1. HueWavelength of the pure colour observed in the signal.Normalized -> Rainbow colorMore the 400 hues can be seen by the human eye.

2. SaturationPurity, a pure colour has 100% saturation

, the white and grey have 0% saturation. About 20 saturation levels are visible per hue.

3. BrightnessAmount of light emitted.Distinguishes the greylevels.The human eye perceives about 100 levels.

These spaces use a cylindrical (3D-polar) coordinate system to encode the following three psycho-visual coordinates:

HSB / HSV Color ModelTransformation from RGB <> HSV by•Travis •Foley and Van Dam•Gonzalez und Woods

Cone: Mathematically incorrect Cylinder: Mathematically correct

HSL / HLS Color ModelHue , Saturation, Lightness (also Luminance or Luminosity)

Cone: Mathematically incorrect Cylinder: Mathematically correct

IHLS Color ModelImproved Hue, Luminance and Saturation (IHLS) colour model

Saturation weighted histogram

Saturation weighted HSV histogram Saturation weighted IHLShistogram

Advantages and Disadvantages of the IHLS Color Model

Advantages

Use of real luminance: Luminance = 0.35*R + 0.55*G + 0.1*B

Saturation can be used as a continous increasing/decreasing factor on hue histograms to eleminate spikes (i.e. no jumps at black/white)

Disadvantages

expansive calculation (trigonometry) but with shader technology faster to computate

Conclusion

It is application dependent, which color model to use

There are more interesting color models used in colorimetry which are worth to study