One-Dimensional, Two-Dimensional, and Spot-Color Management and Control Methods - Control of Color Imaging Systems: Analysis and Design - page 463

automatically. The spot-color control may run while another job is printing, or
the print job may be held until the current print job completes. The advantages of
running spot-color control automatically include reduction of long-term engine drift
effects, spot-color stability, time savings, reduced operator error, and reduced
operator intervention. In essence, this kind of automation would decrease the time
needed to improve the accuracy and repeatability of spot colors in print jobs.
Currently, such a system is still not commercially available, but is conceivable for
digital printing systems of the future.
A spot-color control algorithm is comprised of the of following key steps:

(a) Determining whether or not the spot-color targets are inside the printer
gamut
(b) Mapping out-of-gamut colors to printable colors using appropriate gamut
mapping algorithms and determining the new target L*a*b* values
(c) Selecting appropriate GCR (or gamut classes) for the target L*a*b* values
(d) Applying closed-loop control algorithm for the GCR constraints set in Step c
(e) Selecting best CMYK recipe out of multiple iteration steps

These steps are described in detail next.

8.7.1 G AMUT M APPING FOR S POT -C OLOR C ONTROL
One of the key components in finding a CMYK recipe for a given spot color is

determining whether the target L*a*b* values are either inside

= on-boundary or

outside of a printer s boundary

could be mistakenly reported as outside the gamut by algorithms that are not
suf

s gamut. Colors located very near or on the gamut

' '

ed as outside the gamut will
be handled by the gamut mapping algorithm. Thus, this mapping algorithm will, in
turn, map the colors in reference to a point in the surface of the printer

ciently accurate. A color that is wrongly classi

s gamut.
The consequence of this action may lead to the reproduction of a color that is
slightly different from the original one; obviously, this could have been avoided
since there was no need to map the original color. The ray-based control model
described in Section 7.6.1 can be used to determine whether the spot-color targets are
inside or outside the gamut surface. Any of the gamut clipping or gamut compression
methods can be used to map out-of-gamut colors.

'

D E 2000 is one of the preferred [11]
methods since it can map the out-of-gamut colors perceptually close to the gamut
boundary.

8.7.2 G AMUT C LASSES
The basic algorithm requires the use of GCR-constrained inversion for the target
L*a*b* values of the given spot color. A tricolor GCR is preferred for spot colors
due to the advantages described in Section 7.5.4.3. The overall color printer gamut is
represented as a composite of the gamut subclasses wherein each gamut subclass is
comprised of a subset of CMYK. Selected spot-color targets are assigned to one of
the gamut subclasses for ef ciently calculating the CMYK recipe for a given spot-

color target. An example of a tricolor gamut class is comprised of four gamut classes