) Y/(X+Y- + Z) = green/(red + green + blue) Z = Z/(X + Y + Z) = e/(red + green + blue) BLACK The coordinates X, y, and Z are called chromaticity coordinates, and they always add up to 1. As a result, Figure 7.2-13 Double Hexcone HSI Color Model. For consistency with the HSV model, we have changed from the Tektronix Z can always be expressed in terms of X and y, which convention of blue at 0° and depict the model as a double hexcone means that only X and y are required to specify any color, rather than as a double cone. and the diagram can be two-dimensional. Colour spaces CHAPTER 7.2 Table 7.2-5 75% HSI color bars. For consistency with the HSV model, we have changed from the Tektronix convention of blue at 0° Nominal range White Yellow Green Magenta Black 0° to 360° 0 to 1 0 to 1 0.375 0.375 0.375 0.375 0.375 0.375 0.375 Typically, a source or display specifies three (x, y) information is not included in the standard CIE 1931 coordinates to define the three primary colors it uses. chromaticity diagram, but is an axis that is orthogonal to The triangle formed by the three (x, y) coordinates en- the (x, y) plane. The lighter a color is, the more restricted closes the gamut of colors that the source or display can the chromaticity range is. reproduce. This is shown in Figure 7.2-15, which com- The RGB chromaticities and reference white (CIE pares the color gamuts of NTSC, PAL and HDTV. Note illuminate C) for the 1953 NTSC standard are: that no set of three colors can generate all possible colors, which is why television pictures are never completely R : X yr=0.33 accurate. G : yg = 0.71 In addition, a source or display usually specifies the B : 0.08 (x, y) coordinate of the white color used, since pure white : Xw = 0.3101 Yw = 0.3162 white is not usually captured or reproduced. White is defined as the color captured or produced when all Modern NTSC, 480i and 480p video systems use three primary signals are equal, and it has a subtle a different set of RGB chromaticities (SMPTE "C") and shade of color to it. Note tha