Of course you’ve seen them: Perceptual; Relative Colorimetric; Saturation; Absolute Colorimetric. But do you know what they are; what they’re for; what they do?
They are rendering intents. They’re for for handling situations in which you’ve got to convert color information from one color space to a different color space with a smaller gamut. What they do is determine how that conversion is made.
That’s also all they do. Sometimes you may get into a situation where you have an image that isn’t working for you and you try anything and everything, and finally you stumble across rendering intents and click through them and one happens to solve your problem on that one image. Don’t assume that it’s the magic bullet and you can use it all the time. All you’ve done is discover that for the particular image you’re working with you’ve solved a particular problem in a particular way. The next problem image you have may have an entirely different underlying issue.
So why are there four of them and how are they different? Because depending on what your image is, or how it’s going to be used, you might want to handle how you get out-of-gamut colors into gamut differently.
Incidentally, there are also only four. In some cases with some “dumbed-down” printer drivers or applications or the like, you might see them described as “photographic” or “graphics” or some such, but that’s just “helpfulness” on the part of whomever it was that wrote that particular application. Behind the scenes, there are just perceptual, relative colorimetric, saturation, and absolute colorimetric.
So for which to use and in what instances, the first thing to keep in mind is that every ICC profile or color space has as its basis point its white point. So that if you’re moving from one color space to another (and just for illustrative purposes here, think of any ICC profile as a color space), the first thing you’ve got to determine is whether or not to move from the originating color space white point to the destination color space white point.
Most times and most cases in this industry, you do. Unless you’re doing some very specific proofing, it’s almost a given that you’d want to. If you’re printing an image for final sale, in just about all cases, you obviously want to print the entire range of colors available to you in a color space rendered accurately on the media on which you’re printing.
For that reason, three of the rendering intents do make the conversion from originating white point to destination white point. Those three are perceptual, relative colorimetric, and saturation.
How are they different?
Well, let’s just say that you’ve got an image with a lot of subtle green shades in it, running from midtone out to deep shadow. And let’s say that your printer runs out of gamut about midway along their scale. If all you did was compensate for the new white point and then move out-of-gamut colors into gamut, you’d lose all your detail in the green areas past the gamut boundary of the destination space (your printer.) For example, maybe where you had a leaf with some nice detail in it, you’d just wind up with a solid green blob.
So what the perceptual intent does is make a determination to move some in-gamut colors—basically by lightening them—in order to keep this clipping, as it’s called in the trade, from happening.
Great, right? Maybe. But what if you’ve got an image that’s got a corporate PMS color right on the gamut boundary and you want to nail that color, and that’s really all your client cares about?
Perceptual will helpfully lighten it up for you each and every time.
So that brings up relative colorimetric. Relative because it’s compensating for the new white point, colorimetric because it’s just making color value moves to the destination space, and leaving in-gamut colors alone.