Three-Dimensional Control of Color Management Systems - Control of Color Imaging Systems: Analysis and Design - page 384

lime green) that appear on the display monitor cannot be printed using subtractive
process colors.
The out-of-gamut colors are converted to printable colors through a transform-
ation called gamut mapping or gamut compression. In other words, gamut mapping
is a means of tone scale modi cation that attempts to preserve the original appear-

ance of a color image captured with a wider gamut device on a smaller gamut device.
Otherwise, these colors cannot be reproduced on the printer and would simply be
clipped. It is a key feature used in every color reproduction device.
There are at least two gamuts involved in mapping colors: (a) source

image
gamut (i.e., gamut of the image whose colors need mapping) and (b) desti-
nation gamut (i.e., the gamut of the device in which the image will be reproduced).
Particularly when displays are involved, as is normally the case when soft proo

=

ng

images, there will be three gamuts involved: (1) display =

image gamut, and (3) printer gamut. For instance, producing a mostly saturated blue
color as displayed on a monitor as a printed output on paper (using a typical CMYK
printer) will likely fail. The blue color printed on paper with CMYK toners

= monitor gamut, (2) source

=

inks may
not be as saturated as the one seen on a monitor. Conversely, the bright cyan of an
inkjet printer may not be easily presented on an average computer monitor. In
addition to this problem, a gamut-mapping algorithm that produces pleasing results
for one image may not work well for another. This is true for pictorial images as well
as business graphics and computer-generated images. The color management system
can utilize various gamut-mapping methods to achieve the desired results and give
experienced users control of the gamut-mapped outcome.
There are several techniques used for gamut mapping [111 - 133]. There is no

unique gamut-mapping method that satis es all requirements for image reproduc-

tion, such as pleasing color, contrast, lightness, chroma, hue, and the like across all
imaging devices. Some gamut-mapping algorithms offer feature enhancements in
one region of the gamut and others are more favorable elsewhere in the gamut. As
such, device designers generally compromise regarding the gamut-mapping func-
tions they wish to employ in their respective color management systems.
Techniques used for dealing with out-of-gamut colors include gamut clipping
and gamut compression. In gamut clipping, all out-of-gamut colors are mapped to a
color on the gamut in some way that minimizes the degradation of the

resultant output, while in-gamut colors are left unaltered. A common form of
clipping involves a ray-based approach, wherein a ray is drawn from a desired out-
of-gamut color to a point on a neutral axis. The location or point where the ray
penetrates the gamut surface is the gamut-mapped color. Such a strategy is imple-
mented to preserve hue through the gamut-mapping operation. In gamut compres-
sion, both in-gamut and the out-of-gamut colors are altered in order to map the entire
range of image colors to the printer gamut. For ef

surface

'' ''

cient computational processing of
images in practical systems, the gamut-mapping operation is incorporated into the
nodes of a 3-D LUT.

7.6.1 G AMUT M APPING WITH R AY -B ASED C ONTROL M ODEL
All gamut-mapping methods need to determine accurately whether the node color is
inside or outside the destination gamut. A color that is wrongly considered as outside