About ink trapping

When you design documents using multiple ink colors, each ink must print in perfect alignment—called printing in register—with adjacent inks. However, paper movement and mechanical tolerances make exact registration impossible throughout an entire press run. This misregistration creates unwanted gaps where different colored inks meet, revealing the white paper underneath.

Trapping solves this problem by intentionally overlapping adjacent colors, ensuring that minor press misregistration won't create visible gaps in your final printed piece. This technique represents a crucial quality control step for professional commercial printing, particularly when working with spot colors or process color builds that don't share common ink components.

How trapping works

Trapping compensates for misregistration by slightly expanding one ink area so it overlaps an adjacent ink of a different color. This expansion—typically measured in fractions of a point—creates a safety margin that hides registration errors that occur during printing.

The process requires overprinting rather than knockout behavior. When inks knock out, the underlying ink is completely removed to prevent unwanted color mixing. Trapping intentionally allows inks to overprint in narrow overlap zones, creating a buffer against misregistration while maintaining clean color boundaries.

Misregistration without trapping (left) reveals white gaps; with trapping (right), the overlap prevents visible gaps

The spreading principle

Most trapping employs spreading—expanding a lighter colored object into an adjacent darker object. This approach maintains the visual edge definition because the darker color defines the perceived boundary between objects. When you expand the lighter color slightly into the darker color, the darker boundary remains visually intact even when the lighter color spreads beyond its design edge.

This technique preserves the integrity of your design intent while accommodating the mechanical realities of offset printing. The human eye perceives the darker color as the definitive edge, making the slight spread of the lighter color imperceptible in the final printed piece.

Trapping considerations for your workflow

Different elements in your InDesign documents require different trapping approaches:

Process color builds: Adjacent colors containing common process inks require minimal or no trapping. For example, a purple stroke (cyan + magenta) and red fill (magenta + yellow) share magenta. The shared magenta prints as a continuous area, so misregistration in cyan or yellow creates minimal visual impact. Structure your color builds to maximize shared ink components whenever design requirements allow.

Black ink: Pure black typically overprints underlying colors by default, eliminating trapping concerns. This behavior works because black is dark enough to conceal any underlying colors completely.

Spot colors: Adjacent spot colors with no shared ink components require careful trapping to prevent gaps. InDesign's trapping engines calculate appropriate spreads based on the neutral density (relative lightness or darkness) of each spot color.

Imported graphics: Vector graphics from Illustrator or placed PDF files trap according to InDesign's trapping rules when you use automatic trapping. Bitmap images like photographs also trap to adjacent text and graphics, with trap boundaries calculated at the edge of the image frame.

When trapping matters most

Not every document requires trapping. Consider your specific printing scenario:

• Single-color printing: Documents using only black or any other spot color ink need no trapping.
• Digital printing: Many digital presses print all colors simultaneously, eliminating registration variation between inks.
• Shared process inks: Adjacent colors built from process inks that share components (such as a warm red and orange both containing magenta and yellow) require minimal trapping.
• Large solid areas: Trapping becomes more critical when adjacent areas are large and differ in solid color, particularly when spot colors are used.
• Fine details: Small text and thin strokes require more precise trapping calculations than large objects.

Work with your commercial printer to determine appropriate trapping strategies for your specific press conditions, substrates, and quality requirements.