Graphics Programs Reference
In-Depth Information
the dingy yellow-white of newsprint, the resulting color on your brilliant
coated ink jet paper will be dingy yellow. This intent is really designed
for making one device simulate the appearance of another device for use
in proofing. Perhaps you wish to proof on an Epson printer what your
image would look like on a printing press. The two papers (Epson and
press) would have different paper whites. The paper used on press likely
would have a slightly yellowier and darker appearance. The white of any
paper greatly affects our perception of all the other colors. The paper
white of the Epson would now appear slightly darker and yellow to
match the paper white of the press. That is the reason this rendering
intent is utilized for proofing. Nevertheless, it is useful when you want
to make one device mimic another!
There are several issues we encounter when we conduct gamut
mapping. First, the gamut of a device is fixed and we just have to work
within its boundaries. Ideally, we want to have a file that has a color
gamut that is at least as large as the output device color gamut. If the
color gamut of the file is smaller than the color gamut of the printer,
there are colors the printer might be able to create that we don't have in
the file.
Color conversions in the ICC architecture are implemented by a soft-
ware engine called the
CMM
(Color Matching Method). The CMM
accesses a table within the profile that describes how this conversion
should occur. Each profile contains multiple tables. These tables are
referred to as the
AtoB
and
BtoA
tags. AtoB tags translate from the device
space to the PCS (Profile Connection Space; this is usually LAB). BtoA
tags translate from the PCS to the device space. There is an AtoB tag and
a BtoA tag for each rendering intent. Not all profiles contain all the tables
and thus all rendering intents.
We will investigate color gamut and gamut mapping a lot more when
discussing Photoshop working spaces and building ICC profiles. At this
point however, you might want to examine the tutorial on rendering
intents. If you are somewhat unfamiliar with working with Photoshop to
do color space conversions, this tutorial will walk you through the effects
of the rendering intents on a series of test images so you can see how
they affect an image. (See Chapter 9, Tutorial #3: “Rendering Intents.”)
If you are not comfortable using Photoshop's
Convert to Profile
command,
feel free to wait until we cover this important command in Chapter 2.
Converting/Transforming and the PCS
ICC profiles allow colors in a source image to be mapped to colors in a
destination where that destination has a different color gamut. To take a
file from one color space and convert it into another we need two pro-
files. Imagine we need to translate a topic from German to Russian. We
