VOL. 2, NO. 3, April 2012 ISSN 2225-7217 ARPN Journal of Science and Technology ©2011-2012. All rights reserved. http://www.ejournalofscience.org Understanding Color Models: A Review 1 Noor A. Ibraheem, 2 Mokhtar M. Hasan, 3 Rafiqul Z. Khan, 4 Pramod K. Mishra 1 Department of Computer Science, Faculty of Science, Aligarh Muslim University, Uttar Pradesh, India 1 E-Mail: [email protected] ABSTRACT Colors are important for human for communicating with the daily encountered objects as well as his species, these colors should be represented formally and numerically within a mathematical formula so it can be projected on device/ computer storage and applications, this mathematical representation is known as color model that can hold the color space, by the means of color’s primary components (Red, Green, and Blue) the computer can visualizes what the human does in hue and lightness. In this paper; a review of most popular color models are given with the explanation of the components, color system, and transformation formula for each other, application areas and usages are also included in this work with the classification of color models according to its dependence and independence on the hardware device used in specific application, a summary of the advantages and disadvantages of the color models are also demonstrated in this work. Keywords: Color model, color model applications, Luminance, Chromaticity. their corresponding advantages and disadvantages and their suitable application area. 1. INTRODUCTION One of the interesting fields that developed This paper is organized as follows; Section 2 instantaneous is digital image processing, which its explains common color model systems. Section 3 applications meet various life requirements such as demonstrates color models applications, classifications, communications, tracking signals, television, space, and their taxonomy. A summary of color model intelligent transportation systems etc. [1]. The main two advantages and disadvantages is given in Section 4. application areas of digital image processing as Finally conclusion is illustrated in Section 5. mentioned in [2]are to improve the picturing information for better human understanding as well as processing 2. COLOR MODELS image information for machine representation [2] which We can define the color model as the digital means to establishing a bridge between human-machine representation of possibly contained colors, a different communication by improving both ends. Using the color definition is found in [7] that defines it as the way that spectrum in image processing provides an interesting we can recognize color, where human can visualize color tool for objects recognition and extraction from the through its attributes such as; hue, and brightness [7]. scene[2] and to enable the extension of the domain space Color models is a system for measuring colors that can compared with gray images. Color model explains how perceived by human, and a process of combining the colors are represented and specifies the components different values as a set of primary colors [8]. Typically of color space accurately to learn how each color color models have three or four color components [5]. spectrum looks like[3].Color models are used for Different color spaces are available for different different applications such as; computer graphics [4], applications [7]. Color models can be divided into three image processing [4], TV broadcasting [4][6], and categories according to image processing applications; computer vision [3][4]. With knowledge of color models and different 1. Device-oriented color models: Also called color formats we can represent the color information of device dependent color models that relates and the input digital image acquired by a camera or scanner affected by the signal of the device [4], and the that can recognize three primary ingredient spectrums; resulted color affected by the tools used for red, green, and blue from the light beam [2] that displaying [7].These models are used widely in considered the primary components. The properties that many applications that demand the color be used to distinguish different colors are brightness, hue, consistent with hardware tools used [4], and saturation; these parameters are classified into two examples are any hardware devices that used components; luminance (the brightness) and for human vision perception such as TV [1] and chrominance (hue and saturation), so each color is video system [6]. represented with two characteristic components 2. User-oriented color models: Considered as a luminance and chrominance [2] that are suitable for path that existed between the observer and the human interaction [4] since they can represent the device handles the color information[4], these human skin pigment regardless the lightning used. Color models enable the user to describe and model is a mathematical model that briefly convert the approximate what he percepts from presenting light color coordinates position into three color the color [4]. components [5] in the three dimensional space using 3. Device-independent color models: The color some mathematical functions [6].We have prepared a model not affected by the given device global review paper for different color models used as properties [4], and the same color will be well as the mathematical representation of each with resulted from the set of parameters without any 265 VOL. 2, NO. 3, April 2012 ISSN 2225-7217 ARPN Journal of Science and Technology ©2011-2012. All rights reserved. http://www.ejournalofscience.org consideration for the devices performance and as CIE color space; XYZ is created by international [7].This type of color models are useful in commission on illumination 1931[5].These models are network transmission information so that the created manually with the help of human judgment visual data has to traverse through different ability of visualization and appearances matching, and hardware devices [4]. the chosen colorimetry is based on this matching procedure[4].Figure 2(A) explains the color matching Table 1: Illustrates the distribution of different function. The shapes of the sensitivity curves parameters color models according to latter mentioned X, Y and Z are measured with some plausible accuracy classification. [5].Mathematically speaking, the model can be described as luminance component Y along with two chromaticity Table 1: Color models classifications. coordinates X and Z[4], however, sometimes the XYZ Color Model Classifications color model is represented by its luminance parameter Y [5], furthermore, a normalized tristimulus (chromaticity Munsell Device independent coordinates) can be used as a representative for such model and calculated as in (1) and (2)(denoted by small RGB, CMY(K) Device dependent case xyz). YIQ,YUV, YCbCr Device dependent User oriented-Device HSI, HSV, HSL dependent CIE XYZ, CIE L*U*V*, Device independent, color CIE L*a*b* metric 3. THE MUNSELL COLOR SPACE The earliest organization of color perception into color space was Munsell color model [4] created by Professor Albert H. Munsell[5], and most familiar device independent color space [9].Munsell color model represented as a cylindrical shape with three dimensions equals to value (lightness), hue, and saturation (color purity)[4][5], and it was the first model that isolates the three color components into disciplinary independent, regular, and three dimensional space[5]. The principal of equality spacing between the model components is the main idea of Munsell color model [4],these components are hue, value and chroma, the hue is represented by a circular shape broken down into ten sectors defined as; red, yellow-red, yellow, green-yellow, green, blue-green, blue, purple-blue, purple and red-purple [4] which means the hue range is [1, 10],the value divided into eleven sections refer to lightness (white) at value ten or darkness (black) at value zero[4] which means the range is [0, 10] and perpendiculars the Munsell color model,and the chromare presents the saturation of the corresponding Figure 1: Munsell color system [4]. Right side shows selected combination of each of hue and value the hues circle at value 5, chroma 6; along the vertical V parameters and its range is [0, 12] as seen in Figure 1. value from 0 to 10 [5]. 4. CIE COLOR MODELS This color models has different combination as listed below: ……………. (1) I) CIE XYZ color Model One of the first color space defined is CIE ………….... (2) XYZ, also referred as X, Y, and Ztristimulus functions, 266 VOL. 2, NO. 3, April 2012 ISSN 2225-7217 ARPN Journal of Science and Technology ©2011-2012. All rights reserved. http://www.ejournalofscience.org Since only two coordinates used for color match description, the model represented with on xy- plane and the implicitly evaluated as . III) CIE L*a*b* Color Space Also called CIELAB color model the second uniform color space derived from CIE XYZ space II) CIE L*u*v* Color Space The first uniform color space derived from CIE in1976, with white reference point [4]. L*a*b* color XYZ space with white reference point as shown in model determines the color depending on its position in Figure 3[4] where the white reference point (Xn, Yn,Zn) a3D color space [11], the L* components is the lightness is the linear RGB (1, 1, 1) values. L* represents the of the color (when L* =0 means black and when L* lightness, and u* and v* represent the correlates of =100 referred to while) and the chroma*(for positive chroma and hue [4]. The details of conversion from CIE values indicate red and for negative values indicate XYZtoCIEL*u*v and the inverse from CIEL*u*v to CIE green) and the hue b* (positive values refer to yellow XYZ color models are available in reference [4]. while negative values refer to blue) [4] as illustrated in Figure 4. CIELAB is device independent and considered very important for desktop color [10].The details of conversion from CIE XYZ to CIEL*a*b*and the inverse from CIEL*a*b*to CIE XYZ color models are available in reference [4].