Graphics Programs Reference
In-Depth Information
xyY chromaticity diagram. The larger triangular-shaped device is Adobe
RGB (1998) and the smaller gamut plot is from a printing press. The
entire horseshoe-shaped color plot represents all of human vision. The
center of the plot has lower saturation and as colors move out from
the center, they become increasingly more saturated until they reach the
edge where pure spectral colors lie. As you can see in Fig. 1-9, there are
a huge number of colors outside the gamut of the devices plotted here.
Color Translation
The ultimate goal is to translate the numbers that make up our source
file to the numbers that our printer needs to correctly reproduce the
color. For this to happen, a process known as color space conversion (or
transformation) takes place (see the sidebar, “Source and Destination”).
We want to conduct this conversion using those well-made and accurate
ICC device profiles. The gamut of the original file (source) has to be fit
to the color space of the output device or printer (destination). This
process is known as gamut mapping . ICC-based color management has
tools that allow the best possible gamut mapping with several options
available. When gamut mapping is done poorly, color shifts and other
problems can result. In the past (prior to Photoshop 5.0), users who had
to convert RGB files to CMYK would complain that Photoshop did a poor
job of converting the colors. Part of this was improper use of the Photo-
shop conversion tools available at the time. Part of this was due to the
fact that the CMYK output gamut was much smaller than the original
RGB gamut of the file, and users were, and continue to be, disappointed
when they saw their nice saturated colors muted due to the severe
change in color gamut. Some of the issues also were due to the lack of
precision conversion tools and robust gamut mapping as we have today
using ICC profiles. The last thing we want to see is our bright-saturated
blue skies shifting to purple after a conversion. This was a common
problem. Even with good gamut mapping, we can't overcome the
limitations of the output devices. Saturated colors often will become less
saturated. Nevertheless, we certainly do want options in how the
mapping is done.
Sidebar
Source and Destination: ICC profiles have a very important role. We use ICC profiles so we
can take one set of existing numbers in a certain color space and create a new set of numbers
in a new color space optimized for some other device. Taking an image and converting it from
RGB to CMYK is one simple example of a color space conversion. We need a CMYK file because
the printer we want to send the file to requires a file in a specific CMYK color space. In such a
case, two ICC profiles are required. In fact, for any color space conversion, two ICC profiles are
always required. The typical use of a profile in a pipeline goes something like this: You need to
specify a source profile (where the file came from) and destination profile (where the file is
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