In this essay I'll explain a little about color gamut, why it's important to
examine print files for out of gamut areas, and how to bring those areas
inside the gamut of your printer, when possible. The goal is to enable you to make better color prints,
not to turn you into a mathematician or physicist, so we'll stick to the fundamental concepts
and forget the subtleties. In all of this it is
assumed that you use photo editing
software that allows soft proofing and has a "gamut warning". Adobe
Photoshop is one such program. Soft proofing makes use of your printer/paper
profile to show how your printed image will look without actually printing
it. Turning the gamut warning on while soft proofing will mark "out of
gamut" areas so you know where they are. In Photoshop these areas
turn gray when the gamut warning is turned on.
There are quite a few people who do
not use soft proofing and many who
use it do not use the gamut warning feature. I'm not sure why because in some cases these
features allow making significantly better prints. Ideally the soft proof view should
always be used during image optimization, and the gamut warning should be
checked in that process to help make image gamut corrections if needed.
All images are edited within some working color space, or working gamut. Every color
can be defined by three parameters, so each
working color space or gamut can be represented by a three dimensional
volume. Examples from largest (biggest range of possible colors) to
smallest are Pro Photo RGB, Adobe 1998 RGB, and sRGB. It's best to work in
larger color spaces because data in your images that extends beyond the
working color space is lost.
Like the working gamut, the set of all colors that any given device can produce is
referred to as the device's color gamut. The monitor you are using to read this has a distinct color gamut, for
instance. Printers are a little more complicated because their color gamut
depends on the inks they use, the specific paper being used, and the
for that specific ink and paper combination. As with the
working color space, a device color gamut can be represented by a three
dimensional volume. When a color in the image is beyond the outer surface of that volume
it is "out of gamut" and beyond the ability of the device to produce. The
most serious problems happen when making prints because the working color
gamut, and often the gamut of the image inside of it, will not fit entirely
within the printer's color gamut.
There are a number of
ways to describe three dimensional color coordinates, and there are other ways
to describe colors in four dimensions, which can be hard to visualize. For
the purpose of the remaining discussion we'll use hue (H), saturation (S),
and brightness (B) coordinates to describe colors since they relate
directly to image properties that we can adjust. Every color in the
universe can be specified by a hue, saturation, and brightness value, and every image is produced by variations in one, two, or all three of those
parameters. Objects and/or details in any image are made visible when
adjacent or nearby pixels can be distinguished from one another by the eye.
Let's look at two practical situations where colors being beyond the gamut
of a printer can destroy details, or even entire objects, that are present
in an image file.
Case 1: Suppose we have a gradient within an image that is all of the
same hue and all beyond the gamut of our printer. In this case the print
will show only a solid area of that hue and no gradient will be visible. In
other words, part of the image is lost. Exactly this situation can occur in
photographs of sunset scenes that contain clouds. Differing brightness and
saturation, typically define the structure and details in clouds, which
often have essentially the same hue throughout. Part of the cloud may be
right on the edge of the available gamut, and as the brightness increases
over a distance it gets further and further out of gamut. That entire area
of the cloud will show no detail at all. Instead it's a just an area of that
single hue at the maximum brightness available for that hue in the gamut. In
this case decreasing the brightness so the part that's furthest out of gamut
is back within the gamut will recover the whole gradient. Because hue,
saturation, and brightness all interact, it may also be possible to recover
the gradient by decreasing saturation. Typically recovering an area like
this requires adjustment of both brightness and saturation. What works best
depends on which of those 3 coordinates (really 2 assuming we don't want to
change the hue) is most out of kilter. A little experimentation is your
friend in these situations.
Case 2: Suppose you have a smooth
gradient in hue from red to blue that is all at the same brightness. Further
suppose the whole gradient is out of gamut. This will produce banding of the
hues instead of a nice smooth gradient. If we want to recover the smooth
gradient the only parameters we can possibly change are Hue, Saturation and
Brightness. Again assuming we don't want to change the hues, saturation and
brightness are the only possible adjustments. Completely de-saturating the
gradient will produce a single shade of gray because everything has the same
brightness, but de-saturating it just enough to bring the colors within the
gamut of our printer could work. Decreasing brightness is another option to
bring the gradient back within the available gamut. In reality a little of
both is usually the best solution.
In every case there are only
three things you can change to get an out of gamut area back within the
printer's gamut. In the cases discussed here we have assumed that changing
the hue is not desirable. Hue is changed when the white balance of the image
is changed, and that does change the areas of an image that will be in or
out of the printer's gamut. Since white balance is usually set very early,
while converting a camera's RAW file to a different format, it's important
to set white balance very carefully. After it is set to achieve the desired
look it is not typically a parameter photographers will want to change. In
most cases you're left with adjusting saturation and brightness to bring the
gamut of an area within the available gamut of the printer.
far we have only talked about bringing the colors in an image back within
the gamut we have. The other option is expanding the gamut we have to better
accommodate the image. That can be accomplished by using a different paper
and profile. For instance, a color that is out of gamut on a mat paper may
well be within the much wider gamut of different papers like Canson Baryta
Photographique or Ilford Gallery Gold Fibre Silk, assuming a good profile.
An optimal profile is quite important. A lousy one can produce out of gamut
areas on papers that would not have any if a good profile was used.
Sometimes small out of gamut areas are not a big deal in a print and there
is little to be gained by bringing them within the available gamut. More
often bringing an area within the available gamut will restore missing
details in the image. The adjustments we've discussed can be done globally,
but that sometimes produces undesirable effects in different areas of the
print. Because of that subtle localized corrections are sometimes in order.
The moral of this story is to always soft proof your images and
always check the gamut warning while doing so. Otherwise you may unknowingly
obliterate details in your prints.
So soft proof those images, gamut!