a reader posted a question on the Chroma-Tech Web site (www.chroma-tech.com)
asking for an explanation of rendering intents - one of the more difficult to
understand aspects of color management.
common desktop publishing software applications such as Adobe Photoshop,
Illustrator, InDesign and Quark Inc.s QuarkXPress, along with many RIPs,
allow you to select rendering intents to be used when conversions using ICC
profiles are performed.
intents are even used when we view images on our monitors. And wherever a
rendering intent can be used, a rendering intent has to be used.
not surprising, then, that questions are posed about rendering intents. What are
rendering intents and which one do you use? The answer to that question is the
same as the answer for just about every question we can think of: that depends.
use of rendering intents goes hand-in-hand with the use of ICC profiles. ICC
profiles are used to convert images and graphics from one color space to
another, such as from RGB to CMYK or from one CMYK space to another CMYK space.
of the objects an ICC profile defines is the color gamut of the device. Gamut
means all of the color that a device can capture, display or output. RGB devices
almost always have a larger gamut than CMYK devices (see Figure 1, which
illustrates the difference between a theoretical RGB and a CMYK color space.)
you convert an image from one space to another, there may be some colors that
one device is capable of reproducing that the other device is not.
Figure 1, the three dots labeled 1, 2 and 3 represent three pixels whose colors
are in-gamut while the three dots labeled 4, 5 and 6 represent three pixels
whose colors are out-of-gamut.
color that exists in the source color space but that does not exist in the
destination color space will need to have its value changed into that of a color
that is available in the destination space.
example, lets assume that I have a photograph of a flower that is vibrant red
in color. Lets also assume that my press cannot reproduce a red that
red color of the flower (pixels 4, 5 and 6 in figure 1) will need to be adjusted
or changed to a red that I will be able to print. The rules that govern how this
adjustment takes place when using ICC profiles for conversion are known as
intents come in four versions: perceptual, saturation and two flavors of
colorimetric, relative and absolute.
is the least-used intent (see Figure 2). Notice that pixels 4, 5 and 6 happen to
be in a straight line out from the center of the color space.
the saturation rendering intent, because each out-of-gamut pixel is treated
individually without regard to any surrounding pixel, the closest color to each
of these three pixels that are out-of-gamut happens to be the same color.
also that the three pixels that are in-gamut are also changed. These pixels are
remapped toward the outside edges of the CMYK color space, effectively
increasing the saturation of the image. Using the saturation rendering intent
will bring all of the colors into gamut but will increase the saturation of all
in-gamut colors during the conversion. Although this is useful when working with
cartoons, comics, business graphics or when trying to improve a weak image, it
will seldom be used in your day-to-day workflow.
lets examine perceptual rendering intents, one of the most commonly used
versions (see Figure 3).
using this intent the entire gamut of the image is compressed to fit within the
gamut of the destination device.
means that all of the colors that were actually in-gamut will need to be
adjusted to, in effect, make room for the out-of-gamut colors. Moreover, unlike
what happened in the saturation rendering intents, with perceptual rendering
intents all pixels are treated with respect for each other.
a result, out-of-gamut pixels may not be moved to the closest reproducible
color. For example, pixel 4 is moved well past the nearest reproducible color at
the edge of the CMYK color space to allow room for pixels 5 and 6.
may think that this adjustment, combined with the fact that pixels 1, 2 and 3 -
pixels that could be accurately be reproduced - are changed, makes every color
assumption, in fact, is correct. Therefore, dont perceptual rendering intents
make things even worse? The answer is no. Thats because all of the colors
have been adjusted proportionally, even those that the destination device could
accurately reproduce. The chance that a viewer will notice that all of the
colors have been modified is minimized. This is the most common rendering intent
that is used when converting from RGB to CMYK color space.
leaves us with the colorimetric intents, both relative and absolute (see Figure
using the colorimetric intent, all of the colors that are in-gamut (1, 2 and 3)
are not compressed but are simply adjusted for accuracy.
the colors that are out-of-gamut are compressed, and each color is modified
individually without regard for any other colors - much like the out-of-gamut
colors were adjusted in saturation rendering intents.
to the fact that this disproportionate color adjustment can be noticeable,
especially in blends and gradients, its use is normally limited to conversions
between color spaces of similar sizes, such as a CMYK-to-CMYK conversion.
you use the colorimetric rendering intent when converting to a smaller color
space from a much larger colors pace, the resulting conversion could introduce
banding, posterization and other artifacts into the image.
you opt to use the colorimetric rendering intent, you still must determine
whether to use either the relative or absolute versions. These two rendering
intents treat all colors the same way and only differ in their treatment of the
using relative colorimetric, the white point of the source color space is
mapped, or changed, to the paper white of the destination. The result: White in
the original image effectively remains white in the final output.
using absolute colorimetric, the white point of the destination becomes the
white point represented by the source profile.
clarify further, imagine that your source profile was made from your press
printing on newsprint stock. The white point in this profile will be the color
of the newsprint stock itself, which is certainly not a pure white.
the absolute colorimetric rendering intent is used to convert an image from this
color space to, say, your proofing color space, the subsequent printout will
have the newsprint paper color simulated over the entire image on the output.
Although this would give us the most accurate proof, it is important that this
image is never actually printed on the newsprint stock itself, because the
simulated color of the newsprint stock would be printed on top of the newsprint
stock, significantly muddying the image.
the absolute colorimetric rendering intent is only used to simulate an output on
your monitor or on your proofing system and should never be used for final
wraps up the lengthy answer to the simple question of What is a rendering
intent and which one do I use?
comfort in the knowledge that once you have your head wrapped around the above
concepts, you will understand what is possibly one of the most complicated
aspects of color management.s
note: In the next article, Nate will describe how to use Photoshop to ensure
you are selecting the best rendering intent for each image youre
Nate is a color specialist for Chromaticity Inc., an integrator of color
technologies for the newspaper and other industries. Nate can be reached via
phone at 616.361.7773 or via e-mail at firstname.lastname@example.org.