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What is Trapping?

Print Trapping is an acknowledgment that no printing process is perfect. It compensates for mechanical shifts or stretching of paper or plates in the printing process and provides an overlap of colors to prevent unprinted paper from showing in the final printed product.

Trapping described in this documentation is not to be confused with the terms wet trapping or dry trapping which describes behavior between inks or between ink and paper and are used most frequently at the printing press.

Why is Trapping needed?

Generally speaking, the registration precision when printing several colors one on top the other has a certain degree of tolerance. Depending on the printing process used, registration shifts can be large enough to cause visible defects in the final printed result. One of the most conspicuous irregularities are areas of unprinted paper, called "flashes" or "halos".

A flash is a white gap that appears between two relatively dark areas of color which are supposed to meet. This happens when the colors are made up of different separations and the separations do not register exactly on the border between the colors. A small area of paper remains unprinted, which, in contrast to the two darker colors, appears as a bright, very noticeable stripe.

Trapping describes the measures and methods that can be used to eliminate or conceal the occurrence of flashes.

The unprinted area is sometimes called a "flash".

Another problem caused by registration errors is an unwanted overlapping of inks. This can result in a color that is conspicuously different from the colors which are supposed to be present.

The result of the Cyan and Magenta inks mixing can be an unwanted purple color.

Causes of Registration Errors

Inaccuracy of the recording devices—Not all recording devices can produce separations that are consistent in all dimensions.

When the separations belonging to one page are exposed in different positions of the film, e.g. Cyan and Magenta exposed at the beginning of a film roll and Black and Yellow in the middle of the film roll, the separations produced may not match perfectly.

Distortion of the film

Exposing and developing can stretch and distort the film.

Changes in the temperature or humidity during the processing of a film can lead to registration errors. Separations should be exposed on film from the same batch whenever possible.

Inaccuracies in film assembly

Errors in film assembly and platemaking can cause mis-registration, even when greatest care is taken.

Limited accuracy of the printing press.

Film or stripping material instability.

Paper issues

Distortion of the paper caused by pressure, temperature and moisture from atmospheric humidity or dampening solution.

Paper bounce in sheet-fed presses.

Paper stretch in sheet-fed or web presses.

Inconsistent paper quality.

Poorly maintained or badly worn printing presses.

Improperly trained press operators.

Preventing Flashes from Mis-registration

Preventing mis-registration and removing all sources of error in each stage of production can be expensive. There are also photographic and electronic methods of trapping which can remedy more problems. These two methods offer the following modifications:

Overlapping colored objects to prevent unwanted areas of unprinted paper from appearing.

Deliberately creating gaps to prevent inks from being mixed and avoiding unacceptable new colors.

Adding objects such as black frames along the borders of colored areas to hide any possible gaps or overlapping.

When to Trap

If two colors touch or butt, trapping may be required. If the design includes fine linework or small serif type, the decision on how to trap is important. If two or more adjoining colors share a common color, trapping is probably not necessary.

In simple trapping, where two colors are involved, the lighter color is spread into the darker color. The darker, dominant color defines the size and shape of the image.

Designing with the Printing Process in Mind

If two adjoining colors both share an amount of the same process color, trapping is not usually required. Transitions between pairs of colors that have most of their components on different separations are a key target of trapping.

The following example is how the two colors are intended to appear:

If a purely cyan area borders on a purely magenta area, then a mis-registration of the separations would lead to a mis-registration of the objects. Either an overlapping or a gap would appear between the objects. The gap between the objects would appear as a visible flash of unprinted paper.

To avoid this, the following measures are possible:

Avoid adjacent areas of strongly differing colors.

Add black frames to print over the transitions between colors which would otherwise have been trapped. This is often used with images.

Insert gaps or “white frames” between colors that cannot overlap. These gaps should be less visible than mixed colors that will result when a light color such as yellow meets a dark color such as blue.

Overprint when using black text.

Choose suitable object colors which have sufficient common shares of inks at critical transitions.

In trapping, the lighter color migrates or traps into the darker color. The darker color is the dominant color. While the lighter color traps, the darker color retains the integrity of the image.

Light areas are trapped to dark areas.

Overprint

Objects printed over other objects are overprinted. This mix can result in unexpected colors.

Knockout

In PostScript, every object that is placed on another object will normally leave an unprinted image of itself on the lower object. Trapping will be required for all cases because the “hole” is white unprinted paper. The image below represents a knockout in the yellow plate:

Choke

The background color is lighter than the foreground color. The foreground or “island” color is darker. The lighter background color is extended into the island color to provide a trap. No intermediate color is used.

Spread

The island color is lighter than the background color. The lighter island color is expanded into the background to provide a trap that does not use an intermediate color.

Trapping Types

Photographic or Conventional Trapping

Photographic trapping inserts layers of film between the original and the copy during the copying process to allow the light to be scattered around the objects for spread or around the background for choke. Occasionally, this copy process is deliberately overexposed.

This method has several disadvantages:

Photographic trapping requires a great deal of experience and craftsmanship and is time consuming.

It can only be used for fully filled areas, but not when half­tone screening is involved.

Only certain objects or colors can be trapped selectively.

By using diffused light or overexposure, the copy is usually blurred. This applies to the entire image especially linework and fine structures such as serifs and small sized text.

The results can be very difficult to reproduce, especially when overexposure is used.

Photographic trapping cannot be used when printing to plate. There is no means to photographically copy plates.

Electronic Trapping

By comparison, electronic trapping has many advantages:

Electronic trapping can be applied to tinted objects.

Traps can be generated from tinted objects or can have varying widths.

Electronic trapping can always be faithfully repeated.

Electronic trapping can be controlled more easily. It can be restricted to certain objects or colors.

Electronic trapping can be performed automatically especially when standard tasks are involved and manual intervention is not necessary.

Which Color is Darker

There are many ways of finding out which color is darker. The simplest is to print the colors and then photocopy the proof. The colors are transformed into grayscales at that point and can be visually analyzed for relative darkness.

Another method to determine which color is darker is to determine its Neutral Density. Neutral Density values are industry standards for density readings of colors based on their C, M, Y and K components.

A third method to use the R, G and B components to determine the relative darkness by applying the following formula:
Luminosity = .3 (Red Value) + .59(Green Value) + .11 (Blue Value)

The following tables were created in Photoshop. The color table on top was created first and displays the colors at their 100% values:

The grayscale table below provides an illustration as to which colors are darker:

All color perception is subjective. If a light color abuts a darker color, it may appear darker than its numeric value. For that reason, trapping decision tables can be adjusted for specific situations.

When not to Trap

Trapping is not required in the four color process, when overprinting is used or when either black or white margins are placed around the object.

Notes:

Trapping preserves the integrity of the graphic design.

Trapping is used to preserve the integrity or the image shape such as block serifs.

Trapping compensates for the limitations of the printing process.

Designing with the printing process in mind many problems can be avoided.

Black frames are used in the borders of colored areas in comic books.

Opaque inks are almost always printed last.

Metallic inks are considered opaque.

Other opaque inks contain a high content of white pigment so that the color of the underlying paper cannot be seen.

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