Last month, we explained rendering intents and the role they play in converting images in a color-managed workflow (see “Understanding rendering intents: Which one and why? Newspapers & Technology, March 2004).

John Nate

To recap: When you convert from one color space to another, you are often faced with color gamuts of differing sizes. This means colors that exist in one color space, such as a larger RGB space, might not exist in a smaller CMYK space.

As a result, these colors need to be remapped or redefined so they can be reproduced.

The rules that govern how these colors are redefined are called rendering intents. Of the four rendering intents available, we usually use only two: perceptual and relative colorimetric.

Nice and neat

Here come the two general rules. When converting from an RGB color space to a CMYK color space, use the perceptual rendering intent. When converting from RGB to RGB or CMYK to CMYK color spaces use the relative colorimetric rendering intent.

So there you have it, nice and neat. But now the questions: Do we always use these rendering intents in these situations and if we do or don’t, how do we figure it all out?

The answer to the first question is no, and we have at our disposal a very good tool to use in order to answer the second question: Adobe Photoshop.

Let’s begin our exploration by assuming that we have an RGB image opened in Photoshop that needs to be converted into the CMYK space of our press. If Photoshop were properly set up, we could simply perform an Image>Mode>CMYK color conversion.

Unfortunately, this method won’t show us whether or not we have the proper rendering intent selected. Instead, let’s examine our options by going to Image>Mode>Convert to Profile.



Fig.1
Graphic: Chromaticity

Right or wrong?

As shown above in Figure 1, our document is currently in the Adobe RGB (1998) color space and we are converting it to CMYK using our custom press profile and the perceptual rendering intent.

Although the theory suggests this is the correct rendering intent, it may not be the case.

Why? While the gamut of Adobe RGB (1998) is almost certainly larger than the profile of our press, the gamut of the image itself could actually be the same size as the gamut of the press.

In fact, it may even be smaller. And if it isn’t, using the perceptual rendering intent will needlessly throw away color by over-compressing the gamut.

To determine the ideal rendering intent to use, select the one that you think is most appropriate for the conversion that you are performing.

Then, while observing the image, toggle the Preview button in the “Convert to Profile” dialog box on and off. What we are looking for are two main visual alterations - limited image change and banding. Sadly, we can’t always have both.

The perceptual rendering intent will probably result in the largest visual change to the image while the relative colorimetric rendering intent may cause banding. Look for this banding in image areas containing vivid or saturated colors.

Remember, the relative colorimetric rendering intent may result in ranges of colors being mapped to a single color. The visual effect of this is banding, harsh color breaks, and/or posterization. Try different rendering intents and toggle the Preview button on and off while closely examining the image.

Use whichever combination gives you the least amount of image change or the combination that does not introduce banding.

Other options

What about the other options in the “Convert to Profile” dialog box?

Engine - The engine is the part of the software that actually performs the conversion. None of the options will make or break an image conversion, but for the sake of consistency make sure that your entire staff uses the same engine. When in doubt, use the default Adobe (ACE). Using Dither allows Photoshop to do a better job of blending colors to create colors that it needs to make an accurate conversion. Leave it on.

Black Point Compensation - This is a little more difficult to explain. I’ll cover that in the next article. For now, leave it on.

So there you have the rules to select rendering intent and how to break them.

And don’t be afraid to experiment. Although you’ll more than likely stick to the general rule of using perceptual rendering when converting from RGB to CMYK and relative colorimetric when performing an RGB to RGB or a CMYK to CMYK conversion, take some chances.

Doing so will either ensure that the traditional approach is correct or allow you to break the rules. The result? Improved image reproduction.

John Nate is a color specialist for Chromaticity Inc., an integrator of color technologies for the newspaper and other industries. He can be reached at 616.361.7773 or via e-mail at jnate@chromaticity.com.