The jarring ring of the telephone rattled through my office like a tin-lizzy bouncing down an ancient cobblestone street. Pixel, the cat, stirred on the windowsill.

The voice on the other end was one of my best friends, a top-flight artist whose clients are some of the biggest companies in the world. I knew instantly that this would be a hush-hush case.

I could hear the concern in his voice. "Pick up your e-mail," he whispered, "and call me back when you open my file. I'll explain then what the problem is."

Wading through a stack of junk-mail messages, I quickly downloaded his file. Launching Adobe Illustrator, I saw the logo for a well-known company. (Since I don't want to embarrass my friend, I've substituted the following imaginary logo in Figure 1.)

Figure 1: While the names have been changed to protect the innocent, this is an example of the type of logo that caused the problem.

I called him back. He picked up the phone on barely half a ring. If a voice could sweat, his was dripping. "Did you get it? Is it open?" I answered in my most soothing tones, "Calm yerrself, buddy. There's nothing wrong with the logo."

"But there is, there is! Select all the objects and look at the preview in the Transparency panel. The white elements are gone! Missing! And even worse, the file prints that way!"

Sure enough, when I selected all the elements in the logo, the Transparency panel gave me quite a start. Instead of the white elements, there was... nothing!

Figure 2: The Transparency panel displays a preview of the selected objects. Notice that the logo is missing the white elements.

I felt as if I'd been hit in the gut with my ancient G3 "Wallstreet" Powerbook. How could this happen? Elements don't show in one place and disappear in another. Could it be another one of the infamous bugs in Illustrator Nine-point-Oh? Somehow I doubted it. I was about to tell him to trash the Illustrator preferences and rebuild his desktop when a light bulb clicked on. Suddenly, it all became clear to me -- like the screen on an Apple Cinema Display.

"Hey, buddy, take a gander at what happens when you choose View > Overprint Preview. See, your white elements are gone from the screen." The silence on the other end of the phone was deafening. Then my friend stammered out his questions. "Overprint Preview? What's that? And why would someone turn on overprinting?"

That was just the beginning of the mystery.

Understanding Overprinting
Before I explain what effect overprinting had on my friend's file, a little background is in order. Start with two overlapping objects, in this case a yellow oval and a cyan oval like those in Figure 3.

Figure 3: To understand knockouts and overprints, begin with two overlapping objects as they appear on screen.

In the ancient days, before computer graphics, instructions would be sent to the separator to set the cyan oval to either "knockout" or "overprint" the yellow oval. A knockout would mean that the cyan oval would act like a cookie cutter to knockout, or cut away, any objects underneath, as shown in Figure 4.

Figure 4: When set to knockout, the top oval causes any portion of the underlying objects to be discarded. This is the usual default for computer graphics programs.

When I first started teaching computer graphics, many of my students wondered if they could knockout images in computer programs. They thought they would have to do it themselves. Fortunately, computer graphics programs are set to automatically knockout objects from each other.