Three-Dimensional Control of Color Management Systems - Control of Color Imaging Systems: Analysis and Design - page 305

errors, and (iv) pro ling algorithm errors. In spite of improvements with respect to

(i), (ii), and (iii), the pro ling algorithm errors can make it impossible to achieve

required rendition accuracy since the pro le algorithms use a many-to-one inversion.

To build a multidimensional LUT, one must find the right CMYK formulation for

each color in the table, and this requires rules to choose from the many possible
CMYK formulations (called gray component replacement [GCR]) and inversion
methods.

7.3.2 G AMUT L IMITATION
A device must be able to produce good color from input image files that contain

colors outside the nominal device gamut (Figure 7.3). Since many devices do not
have full gamut coverage, out of gamut colors are mapped to the destination device
gamut that they are able to produce. For example, some colors outside a destination
device gamut need to be mapped to the inside of the gamut or on the boundary as
they otherwise cannot be reproduced on the output device and would simply be
clipped. For instance, printing a mostly saturated blue color, as displayed on a
monitor, on paper using a typical CMYK printer will likely fail. The paper blue
may not be as saturated as the one displayed on the monitor. Conversely, the bright
cyan of an inkjet printer may not be easily presented on an average computer
monitor. Each multidimensional transform must utilize various methods to achieve
the desired color and also give experienced users the opportunity to control the
gamut-mapping behavior.

7.3.3 S MOOTHNESS
Reproduction of color sweeps that cross the gamut boundary must be smooth.
Otherwise, output images can have contours. Generally, these image quality defects
are related to gamut mapping in particular regions of the color space (e.g., dark
brown region). Contours can occur due to a many-to-one mapping at the intersection
between mapped colors and in-gamut colors near the gamut boundary.

FIGURE 7.3 Vector plot of out-of-gamut colors shown with respect to reproducible gamut

of a printer.