A "trap" is a method of overlapping abutting colored objects to compensate for the imperfect registration of printing presses. Because registration, even on good presses with good operators, can be off by a quarter point or more,abutting elements in your publication may not end up abutting perfectly when the publication is printed by your commercial printer. What happens then? The paper shows through where you don't want it to (see Color Example 2 and Example 3).
Do I need to tell you what happens when you take your work to a press that's badly out of register or run by turkeys? Disaster. Before this happens to you, talk with your commercial printer regarding the tolerances of their presses and/or operators. Don't ask them if they're turkeys- it's considered rude.
If you can't (or don't want to) use InDesign's automatic trapping methods (In-RIP or built-in),you can still trap your publication - you'll just have to do it yourself. I'll describe the process, because I believe that you should know how to add and subtract, multiply and divide before you ever use a calculator.
Before I start describing manual trapping techniques, however, I need to state that InDesign's automatic trapping methods can trap your publications better than you can (assuming that you have both deadlines and a finite amount of patience), and if you use them, you usually won't even have to think about trapping.
Object-Level Overprinting. The key to trapping,in InDesign and elsewhere,is in control ling which objects - or which parts of objects - print on top of other objects as the printing press prints your publication. The only way to make manual trapping work is to control the overprinting characteristics of individual objects (see Color Example 1 and Example 4).
Luckily, you can. Any InDesign path can be specified as an over-printing object (that is, it won't knock a hole in any objects behind it when you print), regardless of the object's color. The controls for object-level overprinting are the Overprint Fill and Overprint Stroke options found in the Attributes palette (see Figure 1). These controls, used in combination with InDesign's Paste Into command, can be used to create virtually any trap.
I have to stress the importance of the Weight Changes Bounding Box option on the Stroke palette menu. You cannot create traps when this option is turned off,so you 'l l have to make sure it's turned on as you follow any of the procedures in this section.
When you're working with trapping, you'll be creating spreads (outlines of objects, in the same color as the object,that are slightly larger than the object itself) and chokes (outlines of the object that are the same color as the underlying object's color). Spreads make the object larger so that the edges of the object print over the underlying object; chokes make the area knocked out of the underlying object smaller than the outline of the foreground object.
Use chokes when the foreground object is a darker color than the background object; use spreads when the foreground object is lighter. In other words, trap from light colors into darker colors. Sound subjective? It is. I use chokes when I'm trapping type-text characters of ten look distorted when you use spreads (the eye is very critical when it comes to text).