Three-Dimensional Control of Color Management Systems - Control of Color Imaging Systems: Analysis and Design - page 381

3. Filter the CMYK values generated in step 2 using multidimensional filtering

algorithms [108], if required.
4. Rerun steps 2 and 3, if required, in a computer program. After convergence,
store new CMYK values corresponding to each node color in the starting
LUTs for use in the run-time algorithm.

7.5.6 K-S UPPRESSION M ETHODS
Now, during the updating process in the

field where the press would operate, due to
print engine drifts, this 4-to-3 control approach can inject black toner in regions
where it is not needed (e.g., scum dots in neutral flesh tones or excessive black in the

flesh tones) even after designing good GCRs with no black in the initial CMYK LUT.
Figure 7.53 illustrates a quantitative assessment of the problem where scum dots for
neutral

flesh tones or excessive black in flesh tones, sky tones and other important

=

tone scales can appear dirty grainy or nonuniform. Such an appearance would be

unacceptable for high quality color rendition. This is particularly enhanced at the
neutrals because of the high degree of degeneracy, that is, the number of CMYK
combinations that can produce the same color is high near neutrals but low near the
boundary of the gamut. There is only one unique CMYK solution on the surface of
the gamut.
The gain matrix shown in Figure 7.41 (for each node color) is designed with
LQR to minimize a selected quadratic objective function over the iteration length.
This will provide suppression of the scum dots.

250

Cyan
Magenta
Yellow
Black

200

150

Black can cause
graininess. Most visible
for neutrals.

100

50

0

0 50 100 150 200 250

<----Black R = G = B 8 bit input ( a *= b * = 0) White---->

FIGURE 7.53 CMYK separation without the use of a K-suppression algorithm for neutrals.