Adobe InDesign CS4 - Manual (Page 492)

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Chapter 17: Trapping color
The quality of a printer’s work depends in large part on getting different inks to print exactly in register. To minimize
the effects of misregistration, commercial printers developed a technique called trapping.
Trapping documents and books
About ink trapping
When an offset printed document uses more than one ink on the same page, each ink must be printed in register
(perfectly aligned) with any other inks that it abuts, so that there is no gap where the different inks meet. However, it’s
impossible to ensure exact registration for every object on every sheet of paper running through a printing press, so
misregistration of inks can occur. Misregistration causes an unintended gap between inks.
You can compensate for misregistration by slightly expanding one object so that it overlaps an object of a different
color—a process known as trapping. By default, placing one ink over another knocks out, or removes, any inks
underneath to prevent unwanted color mixing; but trapping requires that inks overprint, or print on top of each other,
so that at least a partial overlap is achieved.
Misregistration with no trap (left) and with trap (right)
Most traps employ spreading—expanding a light object into a dark object. Because the darker of two adjacent colors
defines the visible edge of the object or text, expanding the lighter color slightly into the darker color maintains the
visual edge.
Trapping methods
Trap a document using any combination of methods, including the following:
• Use process colors that don’t need trapping.
• Overprint black.
• Manually overprint strokes or fills.
• Use Adobe InDesign CS4 built-in trapping or Adobe In-RIP Trapping.
• Trap imported graphics using the trapping features in the illustration programs in which they were created. Refer
to the documentation for these applications.
Choose a trapping solution that complements the color output workflow you’re using, such as Adobe PostScript or PDF.
Updated 18 June 2009