Three-Dimensional Control of Color Management Systems - Control of Color Imaging Systems: Analysis and Design - page 380

Corn e rs = 0 0 0 255

250

K
C
M
Y

200

150

100

50

0

0 10 20 30 40 50 60 70 80 90 100

From 0 to gamut corner

FIGURE 7.52 CMYK response to colors on neutral axis for four gamut class GCR after two
iterations of preconditioning steps ( ' 0 ' on the x-axis corresponds to white (C ¼ M ¼ Y ¼ K ¼ 0)).

K GCR is preferred over either a medium K GCR, or a low K GCR, etc. These
adjustments are stored as standard pro les). When a print job has

already been produced, and a future reprint may be expected, then the color in the
future reprint has to be retrieved from the original electronic documents with as
much original GCR design as possible while making sure that the embedded or the
associated graphics are intact.
Below we show a method to reproduce images by retrieving GCRs from
different sources. Direct use of a GCR-embedded pro

les (e.g., ICC pro

le as an initial CMYK LUT
may produce contours and other undesirable image artifacts since the pro le

LUT may not represent the inverse of the printer in which the GCRs have to be
retrieved. Hence, in this section we show a preconditioning

= processing method that

is applied to a pro le. Using this approach, the GCRs can be reproduced with

reasonable precision as compared with the desired GCRs, resulting in expected
image quality appearance in documents without contours or other undesirable
image artifacts, which otherwise would have appeared.
The preconditioning method includes the following steps:

1. Extract the CMYK values for all the node colors (L*a*b* values) from the
GCR contained input pro le.

2. Run the 4-to-3 control iterations for each node color using the appro-
priate printer model or printer using the GCR contained in the input
profile. This step involves using an appropriate controller in the loop (as in
Figure 7.41) based on the Jacobian matrix extracted from the printer model at
the CMYK values of interest. After convergence (i.e., when the errors are near
zero), record new CMYK values corresponding to each node colors.