Methods of Trapping (Overprinting Compensation) in Prepress Using Image and Graphics Software

In the prepress stage, it is common to encounter images with distinctly different color blocks, where one color overlaps another. During printing, each color is printed separately, and due to mechanical inaccuracies, misregistration between color blocks often occurs. To address this issue, prepress technicians use a process known as trapping (also called overprinting compensation or leak white correction).

Trapping is a complex task used only to correct registration misalignment between solid CMYK colors. It should not be applied to continuous-tone images such as photographs, as it can degrade image quality. Excessive trapping between color blocks is unnecessary and can cause "key lines" or even "cross lines" in the CMYK plates. These issues may not appear in composite channels but will become visible when films are output. Therefore, trapping does not enhance image beauty nor solve mechanical misregistration problems; it only helps visually mask minor misregistration defects in print.

The main principle of trapping is to slightly expand color areas so that adjacent color regions overlap minimally. This overlap should be very narrow—just enough to be invisible to the naked eye on the printed piece. Most vector-based design software (such as Adobe Illustrator and FreeHand) uses this method to achieve trapping.

Trapping in Adobe Illustrator

In Adobe Illustrator, the Pathfinder panel contains a "Trap" button under "Blend/Trap" used for creating traps.

1. Three Key Parameters

1. Thickness – Determines the width of the trap, from 0.01 to 500 points (default: 0.25 points). Typically, a value between 0.5–1 point is used, depending on the measured mechanical tolerance of the printing press.
2. Height/Width – Specifies the ratio of horizontal to vertical trap widths.
3. Tint Reduction – Reduces the tint value of the lighter color in the trapping area while keeping the darker color at 100%. The default is 40%. This helps soften the dark edges of the trap by reducing ink density.

2. Option Settings

1. Traps with Process Color – Trapping occurs in the darker color region. Reverse Traps – Trapping occurs in the lighter color region.
2. The trapping color priority in Illustrator is in the order of Magenta → Cyan → Yellow (from dark to light).
3. When working with multiple spot colors, decide whether to trap in the darker or lighter area based on the actual ink colors used.

Trapping in Adobe Photoshop

In Adobe Photoshop, trapping is achieved by allowing channel colors to spread into one another. It is accessed via Image → Trap, which offers only a single Width option. The rules are:

• All colors spread toward black.
• Lighter colors spread into darker ones.
• Yellow spreads into Cyan, Magenta, and Black.
• Pure Cyan and Magenta spread equally toward each other.

This type of trapping inevitably creates darker boundaries between colors. To control the trap's tonal intensity—similar to the Tint Reduction option in Illustrator—you can manually adjust ink coverage by following these steps:

1. Open a file named "TP".
2. Duplicate it twice using Image → Duplicate, naming the copies "TP Copy" and "TP Copy 2".
3. Apply Image → Trap (Width = 1 point) to "TP Copy 2". Then Select All, Cut, and Paste it into "TP Copy".
4. In "TP Copy", a new Layer 1 will appear. Open the Layers Panel (F7) and change the blend mode of Layer 1 to Difference. Since the Difference mode calculates the absolute difference in brightness between corresponding pixels in each channel, the overlapping regions will appear black.
5. Merge the layers, then Invert (Ctrl+I) the image to create a white background with trapped areas in tone. Select All, Cut, and Paste this into the original "TP" file.
6. In the "TP" file, set the new Layer 1's blend mode to Multiply.

By carefully adjusting the trap width and the tint reduction percentage based on the actual press tolerance, prepress technicians can effectively minimize white gaps and misregistration issues during printing.