With the other approach -- overprinting -- the colors of the top object mix with the colors of the bottom object. So a blue oval set to overprint a yellow object would create a green area where the objects overlap, as shown in Figure 5.

Figure 5: When set to overprint, 100-percent cyan combines with 100-percent yellow to create a green.

These days, setting an overprint is relatively easy in vector programs such as Illustrator. You simply check the overprint option for either fill or stroke in the Attributes panel as shown in Figure 6.

Figure 6: In Illustrator, you set overprint in the Attributes panel.

Although setting an overprint is easy, until recently there were only two ways to see the results you would get with an overprint setting. One way was to create film separations and print the file -- hardly a quick and easy proof.

The other way was to open the Illustrator file in Photoshop as a CMYK image. The Photoshop file would show the image as it would print with separations. (FreeHand users needed to save the file as an EPS in order for Photoshop to open it.)

With Illustrator 9, though, Adobe has added an Overprint Preview. This allows you to see the effects of overprinting quickly and easily, in the Illustrator document. So when the blue oval is set to overprint, the results are visible right on the screen, as in Figure 7.

Figure 7: Illustrator's Overprint Preview makes it easy to see the effects of setting an object to overprint.

Sadly, Macromedia FreeHand does not have a similar feature. The best FreeHand users can do is turn on the Redraw Preference setting for "Display overprinting objects." This fills objects set to overprint with a series of small O's, as seen in Figure 8. However, this only lets you know that overprinting has been applied; it doesn't show you the effect of the overprinting. Another workaround is the time-honored tradition of saving the file as an EPS and opening it in Photoshop.

Figure 8: The small O's in the fill indicates the FreeHand object has been set to overprint.

Confusions in Overprinting
Of course the real world isn't quite so black and white (or yellow and blue). As soon as you deal with multiple colors, the rules regarding overprint get more complicated. Let's keep the bottom object as 100-percent yellow. Now suppose the top object isn't a plain 100-percent cyan, but also contains 20-percent yellow. What happens when overprinting is turned on? The answer is nothing. There is no overprinting effect at all. Figure 9 shows the results.

Figure 9: Adding 20-percent yellow to the top object negates the effect of overprinting.

Why? Well, the first thing you have to realize is that the top object cannot (absolutely NOT) add more yellow to the bottom object. (There is no such thing as 120-percent yellow.)

OK, most people can easily accept that. But then someone inevitably wonders why the 20-percent yellow in the top object doesn't combine with the yellow in the bottom object to create an area filled with 100-percent yellow. After all, shouldn't the overprint command indicate some sort of mixture?

I agree that this seems like it should work, and perhaps there were print shops in medieval days that would have interpreted an overprint command that way. But PostScript doesn't! The way the PostScript language interprets overprinting is to turn it off if the two objects share a common color plate. Even microscopic amounts of yellow in the top object will turn off the overprinting command.

Overprinting and Process White
So what was the problem with my friend's logo? With Overprint Preview turned on, the logo looked the same as it did in the Transparency panel -- not a pleasant sight. See for yourself in Figure 10.

Figure 10: The logo shown with Overprint Preview turned on.

On closer examination, I saw that the white elements weren't actually solid white. If they had been, Illustrator would not have allowed overprinting to be turned on. That's because process white is not really a color. It's actually an instruction to Illustrator to knockout 100 percent of whatever color is behind the object. So turning on overprinting is a set of conflicting instructions -- knockout and overprint combined. If you don't want the white knockout, you shouldn't turn on overprinting. Just delete the knockout object.

However, the white elements in the logo were actually a very subtle gradient that changed from white to a light gray. And Illustrator does allow you to set an overprint to a gradient that contains process white.

To fix the problem, we simply selected the white elements and turned off the Overprint setting. However, one question still lingered.

Why did someone turn overprinting on to begin with? They couldn't have missed the huge warning box that appears when you set an object with process white to overprint.

Figure 11: A warning box tries to stop you from setting objects with process white to overprint.

My friend really didn't have a clue. His client had sent him the logo from its ad agency and he knew no one who could answer my question. My best guess lay in the shadows that had been applied behind the white elements. Perhaps someone mistakenly thought that turning on overprinting would help those shadows blend with the blue behind them. Bad choice: Those shadows are controlled by the Multiple blending mode setting, and overprinting has nothing to do with it.

Another successful case stamped "Solved."

Read more by Sandee Cohen.