HISTORY OF COLOUR IMAGE
PROCESSING

History of Color Image Processing
Over the last three decades, we have seen several important contributions in the field of color image processing. While there have been many early papers that address various aspects of color images, it is only recently that a more complete understanding of color vision, colorimetry, and color appearance has been applied to the design of imaging systems and image processing methodologies. The first contributions in this area were those that changed the formulation of color signals from simple algebraic equations to matrix representation . More powerful use of the matrix algebraic representation was presented where set theoretic methods were introduced to color processing. The first overview extending signal processing concepts to color was presented in IEEE Signal Processing Magazine in 1993. This was followed by a special issue on color image processing in IEEE Transactions on Image Processing in July 1997, where a complete review of the state of the art at that time was found in. More recently, we have seen the introduction of several texts that address color image processing . The articles selected for this special section concentrate, in general, on the processing and application of color for input and output devices. They have been chosen to provide the reader with a broad overview of color techniques and usage for hardware devices. However, innovative color applications extend far beyond the topics covered within this issue. To this effect, color has been widely utilized and exploited for its properties in many applications, including multimedia and video, database indexing and retrieval, and exchange of information, to name a few. Furthermore, the extension of color concepts to multispectral imaging has been shown to provide significant increases in accuracy by recording more than the usual three spectral bands. Applications of these special devices have been primarily concentrated in the fine arts.
 COLOR FUNDAMENTALS
Color
Color is the brains reaction to a specific visual stimulus. Although we can precisely describe color by measuring its spectral power distribution (the intensity of the visible electro-magnetic radiation at many discrete wavelengths) this leads to a large degree of redundancy. The reason for this redundancy is that the eye’s retina samples color using only three broad bands, roughly corresponding to red, green and blue light. The signals from these color sensitive cells (cones), together with those from the rods (sensitive to intensity only), are combined in the brain to give several different “sensations” of the color.
Characterization of light is central to the science of color. If the light is achromatic (void of color), its only attribute is its intensity, or amount. Achromatic light is what viewers see on a black and white TV set, and it has been an implicit component of discussion of image processing thus far. The term gray level refers to a scalar measure of intensity that ranges from black, to gray, and finally to white.
For humans and other animals cones are the sensors in the eye responsible for color vision. It has proved that 6 to 7 million cones in the human eye can be divided into three principal sensing categories, corresponding roughly to red, green, and blue. Approximately 65% of all cones are sensitive to red light, 33% are sensitive to green lights and only 2% are sensitive to blue light(but the blue cones are most sensitive). Due to this absorption characteristics of the human eye, colors are seen as variable combinations of the so-called primary colors red®, green(G), blue(B).
The primary colors can be added to produce the secondary colors of light--- magenta(red + blue), cyan(green + blue), and yellow(red + green). Mixing the three primaries or a secondary with its opposite primary color, in the right intensities produces white color, this result shown in Fig.4 which also illustrates the three primary colors and their combinations to produce the secondary colors.
Difference between the primary colors of light and the primary colors of pigments or colorants is important. In the later, the primary color is defined as one that subtract or absorbs a primary color of light and reflects or transmits the other two. Therefore the primary colors of pigments are magenta, cyan, and yellow, and the secondary colors are red, green, and blue. Those colors are shown in Fig.4. A proper combination of the three pigment primaries, or a secondary with its opposite primary, produces black.
• Brightness: the human sensation by which an area exhibits more or less light.
• Hue: the human sensation according to which an area appears to be similar to one, or to proportions of two, of the perceived colors red, yellow, green and blue.
Colorfulness: the human sensation according to which an area appears to exhibit more or less of its hue.
• Lightness: the sensation of an area’s brightness relative to a reference white in the scene.
• Chroma: the colorfulness of an area relative to the brightness of a reference white.
• Saturation: the colorfulness of an area relative to its brightness.
The tri-chromatic theory describes the way three separate lights, red, green and blue, can match any visible color – based on the eye’s use of three color sensitive sensors. This is the basis on which photography and printing operate, using three different colored dyes to reproduce color in a scene. It is also the way that most computer color spaces operate, using three parameters to define a color.
CIE chromaticity diagram
◆ Hue and saturation together are called ‘chromaticity’.
◆ Let the intensity of R, G, B be [0, 255], and X, Y, Z denotes their intensity, respectively
◆ In chromaticity diagram x-axis is X/(X+Y+Z) and y-axis is Y/(X+Y+Z)
The chromaticity diagram is useful for color mixing because a straight-line segment joining any two points in the diagram defines all the different color variations that can be obtained by combining this two colors additively. Consider, for example, a straight-line drawn from the red to green points shown in Fig.5. If there is more red light than green light, the exact point representing the new color will be on the line segment, but it will be closer to the red point than to the green point. Similarly, a line drawn from the point of equal energy to any point on the boundary of the chart will define all the shades of that particular spectrum color.
The triangle in Fig.6 shows a typical range of colors(called the color gamut) produced by RGB monitors. The irregular region inside the triangle is representative of the color gamut of today’s high-quality color printing devices.
The boundary of color printing gamut is irregular because color printing is a combination of additive and subtractive color mixing, a process that is much more difficult to control than that