Three-Dimensional Control of Color Management Systems - Control of Color Imaging Systems: Analysis and Design - page 391

Thus, this control-based iteration of the ray model not only gives an accurate
determination of whether or not the colors are located inside or outside the gamut
surface, but also performs accurate gamut mapping along the ray axis. Additional
constraints can be included during the iterations while performing error minimization
(i.e., D E 2000 etc.) using ray-based control model to provide variations to the com-

ponents such as lightness, L*, chroma, C, and hue, H.
Next we show how to apply the ray-based control model to perform few well-
known gamut-mapping strategies.

7.6.2 C ENTROID C LIPPING
In centroid gamut mapping, the node colors with a constant hue are mapped toward a
focal point inside the gamut [131]. In our illustration, we use the method that maps to
the gamut surface along the ray toward the gamut centroid (x 0 )

the focal point.
Though this type of gamut mapping reduces chroma, it is straightforward to use with
ray-based control model to map colors accurately.
Figure 7.57 shows rays pointing from the sample node colors to the centroid
(L* ¼ 60.59, a* ¼ 6.62, b* ¼ 13.11), which are overlaid on top of the actual
printer gamut. Figure 7.58 shows the

—

D E a * convergence values at the end of 10
iterations for six out-of-gamut node colors shown in Table 7.13. Each ray was
divided into 101 points. Clearly,

D E a * convergence value during iteration
deviates to greater than zero at the gamut intersection as shown by arrows. Mapped
CMYK values, either for minimum

the

D E a * mapping (also called nearest point clipping)
or for centroid clipping to the gamut surface, can be easily obtained using the
procedure outlined in Section 7.6.1.

FIGURE 7.57 Rays pointing to the focal point in centroid gamut mapping.