Color Models

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Artificial color systems, which include computer scanners, monitors, printers, and other peripherals, attempt to reproduce, or model, the colors that we see, using various sets of components of color. If the model is a good one, all the colors we are capable of detecting are defined by the parameters of the model. The colors within the definition of each model are termed its color space. Because nearly all color spaces use three different parameters such as colors (red, green, and blue, for example) or qualities (such as hue, saturation, and brightness), we can plot them as x, y, and z coordinates to produce a three-dimensional shape that represents the color gamut of the model.

The international standard for specifying color was defined in 1931 by the Commission Internationale L'Eclairage (CIE); it is a scientific color model that can be used to define all the colors that humans can see. However, computer color systems are based on one of three or four other color models, which are more practical because

Thisdocumentis crejat^wfch trial version ofCHM2PDF Riot2.16.100., those colors.

None of these systems can generate all the colors in the full range of human perception, but they are the models with which we must work. There have been some efforts to define new color working spaces, such as sRGB, but so far nothing has appeared that will completely take over the photography and computer graphics industries. Indeed, learning to work with Photoshop's sRGB setting was one of the most popular topics among puzzled users when Photoshop 6 was introduced. The topic has been renewed with the popularity of digital cameras, because some of them, like my own Nikon D70, can use several slight variations of color models, including sRGB and what is called Adobe RGB. Your best bet is to learn something about all color working spaces, since image editors like Photoshop support several different color models.

Of the three most common models, the ones based on the hue-lightness-saturation (HLS) and hue-saturation-value (HSV) of colors are the most natural for us to visualize, because they deal with a continuous range of colors that may vary in brightness or richness. You use this type of model when you adjust colors with the Hue/Saturation dialog boxes.

Two other models, called additive color and subtractive color, are easier for computers to handle, because the individual components are nothing more than three basic colors of light. Throughout this book, I've referred to these models as RGB and CMY (or CMYK, for cyan, magenta, yellow, and black).

Additive color is commonly used for image capture by cameras and scanners, and in computer display monitors, while subtractive color is used for output devices such as printers. Since you need to understand how color works with these peripherals, I'll explain the additive and subtractive models first.

Additive Color

Computer monitors produce color by aiming three electronic guns at sets of red, green, and blue phosphors (compounds which give off photons when struck by beams of electrons), coated on the screen of your display. LCD and LED monitors use sets of red, green, and blue pixels to represent each picture element of an image that are switched on or off, as required. If none of the colors are displayed, we see a black pixel. If all three glow in equal proportions, we see a neutral colorgray or white, depending on the intensity.

Such a color system uses the additive color modelso called because the colors are added together, as you can see in Figure 6.2. A huge selection of colors can be produced by varying the combinations of light. In addition to pure red, green, and blue, we can also produce cyan (green and blue together), magenta (red and blue), yellow (red and green), and all the colors in between. As with grayscale data, the number of bits used to store color information determines the number of different tones that can be reproduced.

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