Dean M. Chriss

Color Gamut Ins and Outs
Out of gamut areas can obliterate image details
November 24, 2013

HSB Graph
HSB Color Model, adapted under Creative Commons License
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 color profile 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.
So 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!


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Copyright 2013 Dean M. Chriss