RGB primaries play an important role in building colorimetry. CIE color-matching functions are derived by CIE standard RGB primaries. Many imaging devices are additive systems, such as television and digital cameras that are based on RGB primaries, where image signals are encoded in a colorimetrical RGB standard. Moreover, colorimetrical RGB encoding standards are a frequently encountered representation for color reproduction and color information exchanges.
This chapter presents an extensive collection of the existing RGB primaries. Their gamut sizes are compared. Methods of transforming between CIEXYZ and colorimetric RGB are discussed. A conversion formula between RGB standards under the same white point is also provided. The general structure and representation of the RGB color-encoding standard is discussed. Several important RGB encoding standards are given in Appendix 4. Finally, conversion accuracies from RGB via CIEXYZ to CIELAB of these primaries are examined.
6.1 RGB Primaries
Many RGB primary sets have been proposed for use in different industries such as motion picture, television, camera, film, printing, color measurement, and computer. We have collected 21 sets of RGB primaries; their chromaticity coordinates are given in Table 6.1.
Usually, the gamut size of RGB primaries is plotted in a chromaticity diagram. The mixing of these colorimetric RGB primaries obeys Grassmann's additivity law; therefore, the color gamut boundary is indicated by the triangle that connects three primaries in the chromaticity diagram. From the size of the gamut, RGB primaries can be classified into three groups. The first group has a relatively small gamut size in which RGB primaries are located inside of the spectral color locus as shown in Fig. 6.1. AdobeâRGB98, BruseâRGB, EBUâRGB, GuildâRGB, InkjetâRGB, ITU.BT709âRGB, NTSCâRGB, SMPTE-CâRGB, SMPTE-240MâRGB, and Sony-P22âRGB fall into this group. SMPTE-CâRGB, the early standard of the Society of Motion Picture and Television Engineers, gives the smallest color gamut. Both Sony and Apple Computer use P22âRGB; it has the same blue primary as SMPTE-CâRGB but different red and green primaries. ITU709âRGB is slightly larger than SMPTE-CâRGB; it has the primaries used in several important RGB encoding standards such as sRGB and PhotoYCC.