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Chapter 19:
Issues of Digital Color Imaging
Excerpt A digital color image is a multidimensional entity. It is sampled in a 2D plane with width and length, having quantized values in the third dimension to indicate the intensities of three or more channels (trichromatic or multispectral) for describing color attributes. The smallest unit in the 2D image plane is called the picture element (pixel or pel), constituted by the pixel size (width and length) and depth (the number of tone levels). However, the appearance of a color image is much more intriguing than a few physical measurements; there are psychological and psychophysical attributes that cannot be measured by any existing instruments—only by human vision. Therefore, the most important factor in color-image analysis is human vision because a human being is the ultimate judge of image quality. Human vision provides the fundamental guidance to digital-imaging design, analysis, and interpretation. Because human vision is subjective and instrumentation is objective, there is a need for establishing the correlation between them. This makes digital color imaging a very interesting and complex process, involving human vision, color appearance phenomena, imaging technology, device characteristics, and media properties. With rapid advances in color science and technology that resulted in a better understanding of color images in the forms of human visual system (HVS) and color appearance model (CAM) on the one hand, and the developments of various color-imaging technologies and computation approaches on the other, it is now possible to address the color-image reproduction at the system level in a quantitative manner and to produce good image quality across all media involved. To achieve these goals, cross-media color-image reproduction consists of four core elements: device characteristics and calibration, color appearance modeling, image processing, and gamut mapping. This chapter describes the scope and complexity of the digital color imaging at the system level and provides the sRGB⇆CIELAB conversions as examples to illustrate the modular architecture and technical requirements for a successful implementation.
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