Trapping

Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between colors on the final output. To compensate for potential gaps between colors in artwork, print shops use a technique called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate, dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.

There are two types of trap: a spread, in which a lighter object overlaps a darker background and seems to expand into the background; and a choke, in which a lighter background overlaps a darker object that falls within the background and seems to squeeze or reduce the object.

When overlapping painted objects share a common color, trapping may be unnecessary if the color that is common to both objects creates an automatic trap. For example, if two overlapping objects contain cyan as part of their CMYK values, any gap between them is covered by the cyan content of the object underneath.

Trapping type can present special problems. Avoid applying mixed process colors or tints of process colors to type at small point sizes, because any misregistration can make the text difficult to read. Likewise, trapping type at small point sizes can result in hard-to-read type. As with tint reduction, check with your print shop before trapping such type. For example, if you are printing black type on a colored background, simply overprinting the type onto the background may be enough.

The Trap command creates traps for simple objects by identifying the lighter-colored artwork—whether it’s the object or the background—and overprinting (trapping) it into the darker artwork. You can apply the Trap command from the Pathfinder panel or as an effect. The advantage of using the Trap effect is that you can alter the trap settings at any time.

In some cases, the top and bottom objects may have similar color densities so that one color is not obviously darker than the other. In this case, the Trap command determines the trap based on slight differences in color; if the trap specified by the Trap dialog box is not satisfactory, you can use the Reverse Trap option to switch the way in which the Trap command traps the two objects.

For more precise control of trapping and for trapping complex objects, you can create the effect of a trap by stroking an object and setting the stroke to overprint.