Digital Processing Of Color Images

ABSTRACT Digital processing of black and white images has received most attention during the last 25 years, and has led to various algorithms for the enhancement, smoothing, and zooming of images. Due to the decreasing cost and increasing availability of color image acquisition and display equipment along with the computing systems of higher performance, it is expected that the digital processing of color images will rapidly grow in demand. In this paper we review our experiences with the processing of color images. In particular, algorithms for the contrast enhancement, smoothing and zooming of color images are investigated. The effects of applying these methods in color coordinates other than RGB are examined and considerations with respect to performance, time efficiency and compatibility with some popular algorithms developed for monochrome images are discussed. 1. INTRODUCTION Although color perception of the human visual system is not yet fully undrestood, experimental and theoretical results support formal bases for expressing the physical nature of color. In 1666, it was discovered that the sunlight consists of a continuous spectrum of colors. Six broad regions: violet, blue, green, yellow, orange, and red can be distinguished with no abrupt changes in the spectrum where each color blends smoothly into the next. We perceive the color of an object by the nature of the light reflected from it. An object that reflects light relatively balanced in all visible wavelengths appears white. On the other hand, if the reflected light is in a limited range of the visible spectrum the object appears to have some shades of color. Due to the structure of the human eye, wesee all colors as variable combinations of three primary colors red (R), green (G) and blue (B^. In a trichromatic color system, tristimulus values of a color or some function of the tristimulus values are used for color measurements. A particular color can be described as a vector in a three dimensional color space with the tristimulus values as its coordinates. In such a system, the coordinates of the vectors, rather than the vector length, specify the color. It is evident that a linear function of the tristimulus values is simply a conversion to a new set of primaries. In 1931, the CIE introduced a standard primary reference system with three monochromatic primaries at wavelengths: red = 700.0 nm; green = 546.1 nm; and blue = 435.8 nm. The units of the tristimulus values Rc> Gc, and Bc are chosen to be equal when matching an equal-energy white throughoutthe visible spectrum. In NTSC primary system, these units are normalized to Rjsj, Gjsj, and B^ so that thetristimulus values are equal when matching the flluminant C white reference. To compensate for the problem of having negative tristimulus values, the CIE developed X-Y-Z primary system. The X-Y-Z primaries have been chosen so that the Y tristimulus value is equivalent to the luminance of the color. In the development of the United States color television system, the NTSC formulated the Y-I-Q coordinate system. The Y component is the same as the Y of the X-Y-Z coordinate system and is a measure of the luminance of a color. The I and Q jointly describe the/u/e and saturation of the imagedHue associates a color with some position in the color spectrum and saturation describes the "vividness" of a color, pure spectral colors being "fully saturated colors." Y-I-Q space has been in use as a television broadcast standard since 1953. The primary objective of this transformation was to use the fixed available amount of bandwidth to maximize the perceptual information of the encoded color information. By arranging the Y axis to be an approximation of the luminance, Y-I-Q space becomes compatible with existing black and white transmission standards. Alloting as little bandwidth as possible to color information that the human eye cannot resolve, I axis is chosen for encoding information along a blue-green to orange vector, and the Q axis is arranged