Graphics Programs Reference
In-Depth Information
tion and hue because our eyes will notice differences in luminance far
more than differences in saturation or hue. Luminance information pro-
vides shape and detail; this is the most important factor to perception. In
the early days of color management on the desktop, perceptual was often
referred to as “photographic,” since the prevailing consensus at the time
was that this rendering intent was best used on photographic images.
As we will see in Tutorial #3, this isn't always the case, therefore we
shouldn't assume that perceptual is the right answer simply because
we are working with images. The perceptual intent is a good option if
there is a lot of out-of-gamut colors in the original image compared to
the final destination color space.
Note that there are no specifications in how a perceptual rendering
intent should be conducted, at least with profiles built to version 2 (v2
ICC profile format), which at this time are most prevalent. Each manu-
facturer of a product that builds ICC profiles that can conduct a percep-
tual rendering intent can use whatever techniques they feel will produce
the most pleasing colors. Two profiles built from the same printer made
by two different profile packages will almost always produce two differ-
ent prints when a user selects the perceptual rendering intent. Some of
the ambiguities have been addressed with the newer v4 ICC profile
format, which at the time of this writing, is just starting to come onto the
scene in some profile packages.
The saturation rendering intent was at one time recommended for
solid graphics like logos or pie charts (sometimes referred to as business
graphics) and the gamut mapping is weighted to produce the most vivid
saturated colors (hence the name). For this reason, using this intent on
images can produce less than desirable results. However, depending on
the profile and how it was built, the saturation intent might be fine for
some images so don't dismiss it outright. In most cases, this intent really
is going to be best used for files that don't contain images and for use on
business graphics and similar types of imagery. Like the perceptual
rendering intent, there are no specifications for how this intent should
be applied, so various profiles from different manufacturers will produce
differing results.
The relative colorimetric rendering intent uses the gamut clipping
technique and takes into account the white of both the source and des-
tination color spaces when converting colors. The ICC profile knows all
about the device it describes, including the paper white of the device. A
color in the source is perceived relative to the white of its paper. This
method takes into account this relative perception and calculates how the
same color will appear relative to the destination paper color. The colors
that fall within gamut are not affected at all. I find that the relative col-
orimetric intent works quite well when the source and destination gamuts
are similar. Picking an intent is both image- and profile-dependent.
The absolute colorimetric rendering intent reproduces the exact color
that existed in the source—absolutely. If the source was light color on
