Trapping

Trapping is useful whenever multiple colors are used, and areas of different color touch each other. The following problems often occur:

Problem 1: All of these can create gaps of white between colors when printing either spot or process colors (Do you remember the Sunday comics when the colors did not seem to quite line up?).

The registration can be off because the plates can shift between colors.

Paper can shift on the press.

Paper can stretch on the press.

Ink can bleed outside of it's correct area (depends on the type of paper & ink).

Ink can shrink inside it's correct area (depends on paper & ink).

Problem 2: Printing one color of ink over another often creates new unwanted colors.

Trapping is One Solution

Trapping refers to expanding neighboring areas of color so that they overlap slightly, as .5 pt. "Traps Zap Gaps!"

The two basic trapping options are 1) Choking, where the larger object moves in on the smaller one, or 2) Spreading, where the smaller object expands into the larger one.

Traditional trapping guidelines include "Cyan spreads. Magenta chokes." It is seldom this simple. Decisions must be made on the percentage of common process colors in each object, whether or not the overlap might produce another unacceptable color, the type of printing process to be used, and so on.

Trapping is built in to Quark Xpress & PageMaker, and a few other programs. Their options are limited. Additionally, EPS files must have the trapping done in the original program (such as Illustrator) before they are placed into publishing programs.

Trapping in object-based programs such as Illustrator usually involves adding a line to overprint both edges (50% + 50%). The fill of the background object knocks out.

Trapping is tricky. Trapping specialty software exists (Aldus TrapWise is $4995 ! ) but needs an expert to run it.

Problems Commonly Occur when Trapping!

Dark lines appear where colors overlap. One solution is to reduce the "density" of ink in the trap.

Trapping gradient fills is tough. It is difficult to judge where, how much, and at what density to trap.

Almost never useful for continuous tone images anyway.

Other Solutions that Avoid the Need for Trapping!

Don't have colors meet. Design in white lines between colors, and make large enough to be obvious (2 pt +)

Adjacent colors that share process colors (even green & red) usually don't need trapping because the color will be printed continuously across from one area to the next. The amount of shared process color must vary no more than 50% across the line.

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