One-Dimensional, Two-Dimensional, and Spot-Color Management and Control Methods - Control of Color Imaging Systems: Analysis and Design - page 453

present in the image pixel and a print engine can try to print it, but the printed result is
far from certain. C in ¼ 0 is another passive control patch as in this case (ideally) no

toner is developed. Using the active control patches at C in ¼ 30,

extrapolation can be used to produce a target C out highlight value corresponding to
the input at C in ¼

13 and C in ¼

1. In Figure 8.19, we used C out ¼ 8. We also used C out ¼ 0 corre-

sponding to the C in ¼ 0 patch. After smooth extrapolation was done with these two

passive patches (C in ¼ 0 and C in ¼ 1), C in ¼ 0 was mapped to 0, C in ¼ 1 was mapped

to 4, C in ¼ 13 was mapped to 20, and C in ¼ 30 was mapped to 46. So, although we

chose a C out ¼ 8 corresponding to C in ¼ 1, after extrapolation and smoothing, the

actual value at C in ¼ 1 was mapped to a lower value. Extrapolation, such as linear or

polynomial extrapolation, of three or more active control patches can also be used to
produce adequate results. The algorithms produce better results when more passive
and active control patches in the neighborhood of the highlight are used to de

ne the
TRCs in the extreme highlight region. If the stability of marking engines is not good
for highlights, then this strategy has to be reconsidered to produce good highlights on a
dynamically varying process.

8.6.2.2 Shadow Corrections

As discussed above, calibration TRCs in the shadow region can also be problematic.
Control points cannot be joined with linear interpolation in the shadow region
without good shadow region smoothing. When there are no active control patches
in that region, then algorithms producing TRCs must extrapolate.
In Figure 8.21, we show the approach to solve shadow region problems for the
cyan separation. The maximum desired saturation is 255 because that is the extreme
value along the input cyan axis. We use another passive control patch at C in ¼ 255,

250

200

SmoothTRC
Control patches

150

100

100 150 200 250

C in

FIGURE 8.20 Cyan gray-balanced TRC with shadow region.