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Avoid registration mishaps by setting a perfect tr

Added on Monday 15th of June 2009 11:25 pm EST

Before the computer age, image trapping was a manual skill held by press operators alone. It involved vacuum frames, original fi lms, spacer sheets, unexposed fi lms, diff usion sheets, and overexposure. The need for trapping still exists in today’s more advanced workfl ows, but the tools for trapping and who is responsible for doing it have changed greatly. When is it time to trap, what is trapping, and how does one go about it? We’ll answer those questions (and probably a few more) in this article.

When is it time to trap?
There are ways to eliminate the need for trapping entirely. You can, for example, never overlap colored elements in your InDesign documents or you can stroke every colored object with black. Another way to eliminate the need for trapping (that is far less limiting to your creativity) is to use strictly process colors defi ned with common elements of at least 20%, and then set them to overprint. Figure A shows how this works.

On the left, the large gift box is fi lled with 100% cyan and sits atop of a shape fi lled with 50% yellow. The cyan fi ll is set to overprint via the Attributes palette and, by choosing View > Overprint Preview, you can see how this combination will print. By simply adding 20% yellow to the 100% cyan, however, you can ensure the colors print correctly without trapping, as shown on the right in Figure A.
Naturally, overprinting isn’t the answer to everything. When a document requires solid process and spot colors, you should use overprinting carefully and with the intention of producing colors other than what’s in your Swatches palette. Otherwise, you might end up with much more unsightly results than what’s shown on the left in Figure A.
What if, for example, we want the background of our graphic to be 100% magenta, the gift box to be 100% cyan, and the bow to be 100% yellow?
Overprinting 100% cyan and magenta creates purple—not what we wanted, but not terrible either. Printing yellow on top of the purple, however, gives us a very disturbing gray, as shown in Figure B. That just won’t do! And that’s why InDesign overprints black and knocks out all colors by default.