As tempting as a no-nonsense dismissal of rendering intents might seem -- "My intent is simply to get some good-looking output!" -- choosing the right rendering intent in Photoshop and other applications is important if you want accurate, predictable color.

Many would-be color-management heroes get caught up in the widespread confusion about rendering intents, and not without good reason: Applications often refer to rendering intents differently, controls aren't always where you'd expect to find them, and keeping the various intents straight can be a challenge. But making the best choice for any given image isn't so difficult if you understand what rendering intents are and where they come from.

What Are Rendering Intents?
Rendering intents are simply methods -- sets of rules, say -- for converting colors from one color space to another. They are a standard part of the ICC (International Color Consortium) profile format used by virtually all current color-management systems, including ColorSync on the Macintosh and ICM on Windows. While they are primarily designed to address the problem of handling out-of-gamut colors, they have an effect on all colors.

Out-of-gamut colors are simply the colors present in the source color space that the destination color space cannot reproduce. For example, the color displayed on a monitor from RGB values of 0,0,255 is not readily reproducible in print, and the color produced in print using 100 percent cyan ink cannot be displayed by most monitors. When we try to create a match between monitor and print, we have to do something with out-of-gamut colors, and that's precisely where the rendering intent comes into play. The rendering intent being used defines how out-of-gamut colors are mapped (or not) to colors that exist in the destination color space.

The ICC profile specification defines four different rendering intents: absolute colorimetric rendering, relative colorimetric rendering, perceptual rendering, and saturation rendering. To give us a good starting point for discussing each, in Figure 1 we show a full spectrum in the Adobe RGB 1998 color space. Figures 2 through 5 show that same spectrum converted to an inkjet space using each of the four rendering intents.

Figure 1: A full spectrum in Adobe RGB 1998.

Absolute Colorimetric Rendering
Absolute colorimetric rendering reproduces in-gamut colors exactly, and clips out-of-gamut colors to the nearest reproducible hue, sacrificing saturation and possibly lightness. From this description, it might sound like you'd always want to use absolute colorimetric rendering, but there are two big problems with this approach.

First, our eyes are highly adaptable to different lighting conditions, and one of the many tricks they play is adapting to different "colors" of white, a phenomenon known as chromatic adaptation. When we judge color, our eyes seek out white, then judge other colors in relation to that white. Our eyes accept a wide range of different colors as white, so the white in the scene we're evaluating biases our perception of all the other colors (this is one reason why we use standard lighting for proofing, since the color white is greatly influenced by the light source that illuminates it).

Absolute colorimetric rendering tries to reproduce the source white exactly in the target space. If your target space represents a print and there's a visible paper-white border, an absolute colorimetric print will usually look strange: The white areas in the image will almost always have some color added, but our eye adapts to the paper-white surround, so the image appears to have a color cast.

Second, our eyes are much better at evaluating color relationships than they are at evaluating absolute colors. When you clip the out-of-gamut colors in an image to their nearest reproducible hue, you change the relationship between the in-gamut and out-of-gamut colors, which often destroys the image.

Figure 2: Converted to an inkjet space using absolute colorimetric rendering. Note the discontinuities, particularly in the blue areas, and the apparent color shift on the grayscale.

Absolute colorimetric rendering is mostly useful for proofing, when our proofing device has a larger gamut than the final output we want the proofer to simulate. Since the source space (the final output) has a smaller gamut than the destination space (the proofer), no gamut clipping takes place, and the absolute colorimetric rendering makes the proofer lay down ink in the white areas to simulate the paper color of the final output.