How to Apply Trapping in Graphic Processing Software
Trapping is a time-consuming and highly specialized prepress technique used to correct misregistration between solid CMYK color areas.In general, trapping should not be applied to continuous-tone images such as photographs, as improper trapping may degrade image quality.
Excessive trapping of color blocks is also unnecessary and may result in visible keylines or even crosshair patterns on CMYK plates. These problems are often invisible in composite previews but become apparent during film output or plate making.
Therefore, trapping does not enhance image quality nor does it solve mechanical registration issues; it merely helps mask minor misregistration defects in printing.
The basic principle of trapping is to slightly enlarge adjacent color areas, creating a very narrow overlap between them. This overlap should be subtle enough to remain visually undetectable in the final printed piece.
Most vector-based software, such as Adobe Illustrator and FreeHand, implement trapping using this method.
Trapping in Adobe Illustrator
In Adobe Illustrator, trapping can be applied via the Pathfinder panel → Merge / Trap, using the Trap button.1. Main Parameters
(1) ThicknessDefines the width of the trap, with a range of 0.01–500 points. The default value is 0.25 points.
In practice, values between 0.5 and 1 point are commonly used, depending on the press registration tolerance.
(2) Height / Width
Controls the ratio between horizontal and vertical trap widths.
(3) Tint Reduction
Reduces the ink amount of the lighter color in the trap while keeping the darker color at 100%. The default value is 40%.
This helps soften dark edges in the trapping area, making them less noticeable in print.
2. Options
Traps with Process Color: traps are created in darker color areasReverse Traps: traps are created in lighter color areas
In Adobe Illustrator, trap color priority from dark to light is:
Magenta → Cyan → Yellow
Trapping in Adobe Photoshop
Adobe Photoshop performs trapping through color channel spreading.Under Image → Trap, only a single parameter for trap width is available.
The trapping rules are as follows:
- All colors spread toward black
- Lighter colors spread toward darker colors
- Yellow spreads into cyan, magenta, and black
- Pure cyan and pure magenta spread equally into each other
This method inevitably produces visible dark edges between colors. To control the ink density of the trapping area—similar to the Tint Reduction option in Illustrator—the following workflow can be used:
- Open an image named "TP".
- Duplicate it twice to create "TP Copy” and "TP Copy 2".
Apply Image → Trap to "TP Copy 2" with a width of 1 point, then select all, cut, and paste it into "TP Copy". - In "TP Copy", a new Layer 1 will appear. Open the Layers panel (F7) and set the blending mode of Layer 1 to Difference.
Since Difference mode calculates the absolute difference of pixel values between layers, the result appears nearly black, providing a basis for controlling trap ink density.
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