Adobe InDesign CS5 - Manual (Page 495)

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Last updated 11/16/2011
Chapter 16: Trapping color
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.
Avoid the need for trapping by eliminating the possibility of misregistration in your use of color. Prevent
misregistration by making sure that abutting process colors have common inks. For example, if you specify a dark
purple stroke with a vivid red fill, they will both contain a significant percentage of magenta. The stroke’s and fill’s
common magenta will print as a single area, so that if misregistration occurs in the other process inks, the magenta
printing plate will make any resulting gap hard to see.