Most device-dependent color spaces are creations for the convenience of practical usage, digital representation, and computation. They do not relate to the objective definition or the way humans see color. They are used to encode device-specific digital data at the device-control level. Device-dependent color spaces fall into two main classes: additive and subtractive spaces. Additive spaces include RGB spaces and any spaces derived from DeviceâRGB space, such as HSV and HLS spaces. Subtractive spaces include CMY, CMYK, and Hi-Fi spaces that have five or more primary colorants. In this chapter, the color gamut of device-dependent color spaces is discussed. The gamut computation of ideal block dyes is given in detail. The process of determining a color gamut boundary of a real imaging device, which includes the test target, device gamut model, interpolation method, and color gamut descriptor, is given. Finally, we discuss color gamut mapping for cross-media rendition because of the gamut mismatch among real imaging devices.
7.1 Red-Green-Blue (RGB) Color Space
RGB color space defines colors within a unit cube by the additive color-mixing model. Additive color mixing means that a color stimulus for which the radiant power in any wavelength interval, small or large, in any part of the spectrum is equal to the sum of the powers in the same interval of the constituents of the mixture, constituents that are assumed to be optically incoherent. As shown in Fig. 7.1, red, green, and blue are additive primaries represented by the three axes of the cube that defines the color gamut boundary of the RGB space; all colors are located within the cube. Each color in the cube can be represented as a triplet (R, G, B) where values for R, G, and B are assigned in the range from 0 to 1 (or from 0 to the bit depth of the device). In an ideal case, the mixture of two additive primaries produces a subtractive primary; thus, the mixture of the red (1, 0, 0) and green (0, 1, 0) is yellow (1, 1, 0), the mixture of the red (1, 0, 0) and blue (0, 0, 1) is magenta (1, 0, 1), and the mixture of the green (0, 1, 0) and blue (0, 0, 1) is cyan (0, 1, 1). A white (1, 1, 1) is obtained when all three primaries are added together and the black (0, 0, 0) is located at the origin of the cube. Shades of gray are located along the diagonal line that connects the black and white points.