Chromaticity coordinates in CIE-XYZ system.(continued)

In chromaticity diagram, the spectral locus extends outside the triangle formed by three primaries (R₀, G₀, B₀) which are located at (1,0), (0,1) and (0,0). The region within this triangle is referred to as a “color gamut” of primary sources i.e. contains all colors reproducible by R₀, G₀ and B₀.However as observed, there is a large range of spectra colors that cannot be synthesised by non negative amounts of R₀, G₀, B₀.

            In (x, y) chromaticity diagram, on the other hand, all spectral colors including the line of purples lie within the triangle with vertices  (0,0), (1,0) and (1,1). Therefore, all real colors can be synthesized by positive amounts of X, Y, Z primary colors. But , all  three vertices are outside the spectral locus, thus making them non real. It is sometimes useful to indicate the qualitative color appearance in terms of the specification (x, y).

            For this, we plot the chromaticity coordinates of the spectral colors individually for each wavelength. This is shown in Fig (2.14). These chromaticity plots are different from color matching functions of Fig (2.9),  and should not be confused with them.

Figure (2.14a) Chromaticity coordinates (r₀,g₀,b₀) of spectral colors for the 2⁰ Standard Observer Primaries R₀,G₀,B₀ at wavelengths 700.0nm, 5461.1 nm and 435.8nm respectively

Figure (2.14b) Chromaticity coordinates (x,y,z) of spectral colors.

            From the (x, y, z) plots of the spectral colors, we observe that a large value of x indicates a substantial amount of red light that can be matched by colors that appear orange, red or reddish purple.

            If y is large, the color appears green, bluish-green or yellowish-green. A small value both for x and y indicates chromaticity z is large and such a color is matched by colors appearing blue, violet or purple. Note from Fig (2.14) that unlike the chromaticity coordinates (r₀,g₀,b₀) the chromaticity coordinates (x,y,z) shown in the same figure are always positive.Summarising we can say that color images and its representation for human vision plays an important role in image and video compression and coding. Present day image and video coding standards such as JPEG 2000, MPEG 2, 4 etc. employ various color spaces to achieve high compression ratios.