Three-Dimensional Control of Color Management Systems - Control of Color Imaging Systems: Analysis and Design - page 303

series of base transforms are included in the ICC pro le that perform the conversion

between different color spaces. A source profile, a three-to-three (RGB-to-L*a*b*)
or a four-to-three transform (CMYK-to- L*a*b*), is assigned to each image le that

de file (i.e., the

amount of red, green, and blue light seen by the source camera) and a reference color
space. This reference color space, speci

nes a mathematical transform between the numbers in the input

ed in XYZ or L*a*b*, describes the color as
perceived by the human eye. These L*a*b*orXYZ values are passed to the
destination profile, which de nes yet another mathematical transformation (e.g., a

three-to-four transform used for a CMYK printer) between the amounts of the
colorants from reference color space and the destination color space that the device
would need to use to render the color. In general, these mathematical transformations
are embodied as multidimensional LUTs. Using this kind of source, reference, and
destination architecture, the source and destination devices from different vendors
can be used in a system to produce the color with good results that is most suitable
for a particular application.
The destination pro

les have an additional complexity because they must also
handle requests for colors the destination device cannot make. This situation is
called the gamut-mapping problem and it is normally handled inside the destination
pro le. In the ICC pro le architecture a reference color space in L*a*b* or XYZ

called the pro le connection space (PCS) [1], is used for standard observers,

measurement geometry, and illuminants. Also, there can be many different rendering
intents (colorimetric, perceptual, saturation, etc.) included in the ICC.1:2004-10
speci cations because of the need for tuning the pro les to speci c applications.

Device Link pro les that provide a dedicated

transformation from one input device color space (e.g., CMYK) to another output
device color space (CMYK) without going through the PCS. Thus the ICC pro

les are another special kind of ICC pro

le
format is made generic to the extent that each application vendor can foster their
own proprietary technology to differentiate their art from their competitors. How-
ever, the basis for creating a device link pro

le or pro les with different rendering

intents has always been an ordinary ICC pro le.

Hence, in this chapter we focus our algorithms on the generation of an accurate
colorimetric pro

le such as the colorimetric pro

le.
Figure 7.1 shows a footprint of the destination ICC pro

le, in particular, a destination pro

le used with various
components to translate color information from the PCS to the device-speci

c
CMYK space. A color lookup table (CLUT) or a 3DLUT is a major component
of the destination ICC pro le that is a GCR-constrained, gamut-mapped LUT

from RGB triplets to device CMYK space (Figure 7.2). Although the destination
pro le is simply an XYZ or L*a*b* (input of Figure 7.1) to CMYK transform

(output of Figure 7.1), in order to obtain the biggest possible color gamut with
minimum amount of toner and improved color rendition accuracy, the pro

le is
divided into several components as shown in Figure 7.1: the XYZ or L*a*b*to
RGB matrix, a tone reproduction curve (TRC), a 3DLUT, and a second TRC. This
type of division will help the process of creating a destination pro

le. See Section
7.8 for detailed steps involved in the creation of a colorimetrically accurate
destination pro le.