Definition of Rendering Intent
Rendering intent describes the four modes by which color management systems adapt some or all colors in a picture to the limitations of a given display or printer.
"For most images, Relative Colorimetric rendering produces superior results. For others, Perceptual will be far better. These cases include images with significant shadow details where a slight lightening of the print is acceptable to open up the shadows. Also images with areas of highly saturated color can benefit from Perceptual rendering. If you see color banding in the soft proof with Rel. Color. selected, try Perceptual. With experience you will get a feel for which images best pair with each rendering intent."
Intent Definitions - Per the Photoshop Print Dialog
Perceptual - Aims to preserve the visual relationship between color so it's perceived as natural to the human eye, even though the color values themselves may change. This intent is suitable for photographic images with out-of-gamut colors.
Relative Colorimetric - Compares the white of the source color space to that of the destination color space and shifts all color accordingly. Out-of-gamut colors are shifted to the closest reproducible color in the destination color space. Relative colorimetric preserves more of the original colors in an image than Perceptual.
Rendering Intents by Analogy
Another challenge to the neophyte in color management is understanding rendering intents. While there are four rendering intents when using a profile to print, the two that are most often used are Perceptual, and Relative Colorimetric. To someone who hands you an image and says "I just want it to print correctly," you don't really want to launch into a long explanation describing transforms and lookup tables and Profile Connection Spaces. But you can explain that they have options about what should be done with colors that are outside of a printer's ability to print.
Here are a few analogies I have found useful:
There are different ways to take an object and make it smaller when necessary.For my final project in wood shop in Junior High, I chose to make a bowling ball out of wood. I glued together several different exotic blocks of wood and then turned this big, blocky mess on a lathe to trim off the excess portions until it was round. I basically had a specific gamut in mind and trimmed off everything that was outside of that gamut. This is somewhat analogous to the Relative Colorimetric rendering intent. ( I realize it's an imperfect analogy. To be a true Rel Col RI, all the wood outside would have to be squished and pressed until it were moved to just within the circumference of the bowling ball. But let's not get too technical; come on - work with me here...)
The Perceptual rendering intent is analogous to a balloon. Think of a balloon with a pattern of stars on it. When fully inflated, the stars are in a certain position. If some air leaks out, the balloon gets smaller and each of the stars move closer to the center of the balloon. The relationship between each point on the balloon remains the same, but the gamut of the balloon gets compressed.
If there were a picture of Mickey Mouse on the balloon, the picture would continue to be recognizable as the balloon got smaller, because the relationship between each point on the surface would be retained as each point evenly moved closer together. Not so for the bowling ball. If my original glued blocks bore a resemblance to Mickey Mouse when I started, then much of the original would be cut off as I trimmed it down on the lathe. There go the ears! That's what we'd call "clipping of the gamut."
Naturally, the choice of these rendering intents depends on one's intent for the image. If it is more important that the relationship between all the colors be retained, then Perceptual is the way to go, while understanding that many of the colors will be desaturated. If the greater importance is to maintain the most saturation in the colors, then choose Relative Colorimetric and accept some clipping of the out of gamut colors.
The above was written by Pat Herold of Chromix, Red River Paper's official profile maker.