How to Apply Trapping in Graphic Processing Software

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 Trapping is a specialized prepress technique used to correct misregistration between solid CMYK color areas in printing. Even minor misalignments can create visible white gaps along color edges, which is particularly critical in professional book printing.

In general, trapping should not be applied to continuous-tone images like photographs, as it may reduce detail, create color artifacts, and degrade overall image quality. Over-trapping solid colors is unnecessary and may produce visible keylines or crosshair patterns on CMYK plates, often appearing only during film output or plate making.

The principle of trapping is straightforward: slightly enlarge adjacent color areas to create a very narrow overlap. This overlap compensates for minor registration errors while remaining visually undetectable in the final print. Most vector-based software, such as Adobe Illustrator and FreeHand, implements trapping using this method.

Trapping in Adobe Illustrator

In Illustrator, trapping can be applied through Pathfinder panel → Merge / Trap → Trap.

Main Parameters:

  • Thickness: defines trap width. Typical range: 0.5–1 point depending on press tolerance (default 0.25 points).

  • Height / Width: controls horizontal vs. vertical trap ratio.

  • Tint Reduction: reduces ink in lighter colors while keeping darker colors at 100%, softening edges (default 40%).

Options:

  • Traps with Process Color: traps darker areas.

  • Reverse Traps: traps lighter areas.

Trap color priority follows: Magenta → Cyan → Yellow.

Trapping in Adobe Photoshop

Photoshop performs trapping through color channel spreading, with a single trap width parameter under Image → Trap.

Trapping rules:

  • All colors spread toward black.

  • Lighter colors spread toward darker colors.

  • Yellow spreads into Cyan, Magenta, and Black.

  • Pure Cyan and Magenta spread equally.

This method can produce visible dark edges. To control trap density (similar to Illustrator’s Tint Reduction), use this workflow:

  1. Open an image (e.g., “TP”) and duplicate it twice (“TP Copy” & “TP Copy 2”).

  2. Apply Image → Trap to TP Copy 2 (1 pt), then copy and paste it into TP Copy.

  3. In TP Copy, a new Layer 1 appears. Set its blending mode to Difference (F7). The nearly black result allows precise control of trap visibility and ink density.

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