Trapping is a little like buying a boat. The saying about buying a boat is, “If you have to ask the price, then you can’t afford it.” My feeling about trapping is, “If you have to ask what it is, then you shouldn’t do it.”

In fact, as I am writing this I can hear production managers all over the country yelling ,“No, no! Don’t write about trapping. Your readers shouldn’t get involved with trapping! It’ll only confuse them.”

I wish I could skip talking about trapping, but the fact is that someone, somewhere, is going to mention trapping to you and it will be much better if you have some understanding of what it’s all about.

Keep in mind that this is hardly a detailed lesson on trapping—these are just the basic facts with a couple of suggestions for how to avoid the need for it.

Before you understand how to trap, you need to understand why to trap. Think about some of those rather horrible inserts you get in Sunday newspapers—the ones with all those coupons for products you never buy. Have you ever seen one of those pages where everything is slightly off-kilter, where colors don’t fit neatly inside the spaces where they belong, where objects look like they are out of position? We say the colors are out of registration, often called misregistration.

Misregistration occurs for various reasons: The paper on the printing press shifts, the plates move, a meteor hits the building (just kidding). But the result is that one of the colored inks is printed just a wee bit out of place. This causes a gap between the colors where the white paper shows through.

Misregistration can’t be prevented. There are some presses that are less likely to have registration problems, but you won’t find any printing press that prints in perfect registration all the time. So we need to compensate for the inevitable misregistration. That’s where trapping comes in.

Let’s say you put one object filled with one color over another object filled with a different color. What happens in the area where the two objects overlap? When you make the separations for your file, the top object knocks a hole in the bottom object. The hole is called a knockout. When the two color plates are registered correctly, the top object sits perfectly inside the knockout. When the two color plates are misregistered, the top object is slightly off from the knockout, creating the slight gap. If the paper is white, you see a white gap.

The nasty gap occurs because the top object is exactly the same size as the knockout. If the knockout was slightly smaller or the object slightly bigger, there would be an area where the two colors would overlap. Then if one object moved slightly, there still wouldn’t be a gap. The overlap is the trap, and you can see it in printed pieces where the overlap of the two adjacent colors creates a third color, as shown at right

Worrying about trapping is a little like worrying about being struck by lightning: Yes, some people do have problems with the registration of colors, and some people do get struck by lightning, but most people never have to worry about it.

Most printing presses today have much less trouble with misregistration than they used to. As long as you understand some of the principles of trapping, you may never have to trap at all (and as long as you stay indoors during a lightning storm, you may never get hit by lightning). Following are some of the ways to avoid trapping problems.

Keep Colors Apart

This is so basic that many people forget it. If you want to avoid registration problems, just don’t let your colors touch. For instance, if you have red type over a blue background, you might be afraid of a misregistration. If you add a white outline around the type, you have created a “buffer zone” where the two colors won’t touch. As long as the colors don’t touch, you don’t have to worry about misregistration. And as long as you don’t have to worry about misregistration, you don’t have to create traps to avoid noticeable gaps.

Milk carton and soft drink cup designs are excellent examples of physically separating colors to avoid trapping. These items are printed on presses that have the potential for enormous misregistration. The size of the traps necessary to avoid gaps would be huge and very noticeable. So most designers avoid the need for trapping by adding white around the elements. No matter how badly the colors are registered, there is no need for traps because the colors don’t touch.

Use an Overprint

The easiest way to avoid misregistration is to overprint colors. Overprinting prevents the knockout in the color underneath so the top color just prints directly on top of the other color. Without the knockout there is no way the nasty gaps can appear. All vector illustration and page layout programs let you overprint selected objects and colors.

Of course, when you overprint you have to accept the fact that your colors are going to change. For instance, yellow overprinting blue creates green. That’s not so bad if you want to create green, but you will have a slight problem if only half of the yellow object overprints blue: Half of the yellow object will be green, and the other half will still be yellow.

If you have software that allows you to turn on a preview of overprinting, you can see what the effect is. If not, you need to print your file with the Simulate Overprint setting turned on in the printer dialog box. However, you shouldn’t turn on overprinting willy-nilly. You could wind up with a muddy effect for your colors.

If you are unsure of what you are doing when you apply overprinting, talk to the print provider that will be printing your job; they will give you the best advice.

Use Common Plates

Trapping is less of an issue in four-color process printing than it is in spot color printing. (Milk cartons and drink cups are usually printed with spot colors.) Remember, the goal of trapping is to avoid the gap between colors where the paper shows through.

When two colors share a common plate (or several plates), the misregistration from one color butting up against another color isn’t as noticeable. For instance if there is a little magenta in a blue background, and a little cyan in a red object, there isn’t any need for trapping.

If the misregistration occurs, the art still won’t fit exactly into their knockouts. But instead of a white gap, there will be a gap filled with either a light tint of magenta or cyan. That tint of color is less objectionable than the glaring white.

So let’s say after all this you decide you want to set the traps in your software (alarm bells should be going off in your head). After all, most software has built-in trapping commands. You’ll just open the dialog box, set a few numbers, and be finished, right? Not really! Setting the size of traps is a very precise science. You need to know the type of printing press, the type of paper, the inks, and many other technical issues before you can decide how big the traps should be. Your print provider has knowledge built on long experience.

But let’s say you do have an idea of how much to trap. Should you do it then? No. Especially not if you are combining text in your page layout program with photos or vector illustrations. The page layout program can only set the traps for the elements that were created in that page layout program—it can’t trap graphics that were brought in from another application. So if you build traps for a headline in InDesign or QuarkXPress, the trap won’t do anything when that headline prints on top of an illustration brought in from Illustrator.

The best solution is: Let the print service provider set the traps! They have special software dedicated to trapping all the elements on your page together. Dedicated trapping software is the best choice for trapping and the people at the print provider are the best people to run it. (In case you were wondering, trapping software costs thousands of dollars. It’s not meant for poor creatures like us to use. Thank goodness.)

Project #1
You have a blue circle at the top left of your page and a yellow circle on the bottom right. Do you need to set the traps?

Project #2
You have black text set to overprint onto yellow. Do you need to set the traps?

Project #3
You have green text (c: 100, m: 0, y: 100, k: 0) sitting on a blue (c: 100, m: 20, y: 0, k: 0) background. Do you need to set the traps?

Project #4
You have a 1-color job that has vector art in it. Do you need to set the traps?

Project #5
The final output of your job is the desktop printer. Do you need to set the traps?

Project #6
You want to buy a boat but need to know the price. Should you buy it?

Project #7
A friend suggests that you set the traps for your layout before you send it to the print provider. Should you set the traps?

The quick answer to all the projects is, “No!” The explanations below tell you why.

Project #1
Since the two colors don’t touch there is no need to trap.

Project #2
If one color overprints another, there is no need to trap.

Project #3
The common cyan plate makes it unnecessary to trap.

Project #4
Vector or raster art in a job doesn’t matter. As long as it’s a single color job there is no need to trap.

Project #5
There are no separations when you send the job to a desktop printer. So there is no need to trap.

Project #6
If you have to ask the price, you shouldn’t buy it.

Project #7
No. Trapping is a technique better left to the professionals.