From time to time you might have a document printed and find that two objects of different colors that were set to print next to each other have a thin line separating them. This misregistration isn't uncommon, nor does it mean your printer isn't doing a good job. In any given printing situation, paper can slip or stretch on the press, as can the plates being used to print. When you see a slight misregistration, it most likely happened because you didn't set up trapping in your document.
Trapping is a process by which objects of different colors are created with a slight overlap to avoid any issues with misregistration on the press (see Figure 21.34). To create trapping, you must be working with a document in CMYK mode.
Figure 21.34. A simulation of how trapping works by overlapping page elements of different colors.
To create a trap:
The trap settings dialog opens. The options in this dialog determine how much overlap is created by the trap. Talk to your printer before specifying trap options; in most cases, the printer takes care of trapping at the RIP stage of output, and you don't have to concern yourself with it. Trapping settings are determined by ink colors, press type, and paper type among other things. Also know that the colors you are using might mean you don't have to trap at all. If the two elements share similar percentages of cyan, magenta, yellow, or black, you might not see or notice if items misregister. Your printer is best qualified to give you an idea of the most appropriate choices here, if you have to make these choices at all.