Page 183 - DCAP504_Computer Graphics

Computer Graphics

The figure 12.4 depicts CIE Chromaticity graph.

Figure 12.4: CIE Chromaticity Diagram

Source: Xiang. Z., Plstock. R. (2006). Computer Graphics. 2nd Edition. Tata McGraw Hill. Pg no. 292
When chromaticity of two colors varies then they are represented by two different points in the diagram
to indicate that the two colors have different chromaticity co-ordinates.
The chromaticity co-ordinates (x, y) can be converted into a specific color in the XYZ color space and to
do this an additional piece of information is required, typically Y:
x 1 − x − y
X = , Y Y = , Y Z = Y
y y
We can find all spectral colors along the upper border of the figure. Point C is referred as Illuminant C.
The point C corresponds to a reference white that is obtained from a spectral distribution close to
daylight.
The CIE chromaticity diagram can be used as a universal reference for measuring and comparing visible
colors. A typical color monitor will have the following chromaticity co-ordinates: R (0.62, 0.34), G (0.29,
0.59), and B (0. l5, 0.06). As a result of these co-ordinates, a triangular region is defined within the
diagram. The region is known as the color gamut of the monitor which represents all the colors which a
color monitor is able to display.
Color Gamut Mapping
Let us consider that the phosphors of an RGB color monitor have the following co-ordinates: (x r, y r),
(x g, y g), and (x b, y b). We will now introduce auxiliary variables as shown below.
Cr=X r+ Y r+ Z r
Cg =X g+ Y g+ Z g
Cb=X b + Y b +Z b
where, (X r, Yr, Zr), (Xg, Y g, Z b), and (X b, Y b, Z b) represent the respective XYZ co-ordinates of the red,
green, and blue colors the monitor can display. We have
X r = xrCr, Y = yrCr, Zr = zrCr where zr = l – xr-yr
X g= x gCg, Y g = y gCg, Z g = z gCg where z g = l – x g – y g

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