Basic Trapping Concepts

What is Trap?
Trapping is the intentional overlap of abutting colors in a printed project that prevents inevitable errors in printing from showing. Figure 1 illustrates how an image may appear with trapping and without trapping. Notice there are no unsightly gaps between the cyan and magenta colors in the example with trapping—the colors are in register.

 

Figure 1

Knockout vs. Overprint
Two abutting elements require trapping when one or more of the following conditions occur:

1. The elements are 2 different spot colors.(Example: PMS 123 and PMS 456)
2. The elements consist of a process build and a spot color.(Example: C, M, Y, K, and PMS 123)
3. The elements consist of two different process builds or process colors. (Example: C & Y and K)
4. One of the elements is a scan.(Example: text over a picture image)

If two abutting elements contain the same colors, trapping is not needed. Examples include:

1. Abutting elements have C, M, Y, and K
2. Abutting elements are different percentages of the same PMS color (Figure 2)
3. Abutting elements are process builds containing the same colors at different percentages (Figure 2)

For elements requiring trapping, Ink & Ideas  standard trap is .25 pt (maximum).

 

Figure 2

How to Trap: Spread vs. Choke
Spread and choke are two words used to describe trapping. As their names suggest, spread increases the size of an image for trapping purposes, whereas choke reduces the size of an image.

The rule of thumb for trapping is to spread lighter areas into darker areas. In doing so, the integrity of the darker image is maintained while the lighter color underprints. Figure 3 illustrates this key point: with a 100% magenta circle on top of a 100% blue square, the magenta moves or spreads onto the blue. In an opposite scenario, where the circle is 100% blue and the large square is 100% magenta, the magenta moves under or chokes to the blue. Notice, in both examples the lighter color (magenta) moved and the darker color (blue) did not.

 

Figure 3

Knockout vs. Overprint
The difference between knockout and overprint are illustrated below in Figure 4. Knockout involves eliminating a shape from a background object to create an opening for a superimposed object. Overprint combines colors by printing over an already printed area. Notice the change in the overprinted object's color. The resulting green circle is produced by combining the cyan circle with the yellow background. Many postscript separation programs today will automatically knockout underlying colors and overprint black.

 

Figure 4

As a general rule, small black type on any background overprints well and requires no trapping.

 

Figure 5

However, when using large black type (type over 1'' in height)—or graphics—you should knockout the underlying color to prevent a two-tone appearance in the black object. The two-tone appearance results when black is set to overprint, rather than having the background knockout. Figure 6 compares the effects of overprinting black with knocking out the background color.

 

Figure 6

While this technical tip summarizes a few basic concepts of trapping, there are many other aspects which factor into how and when to trap. The most important rule in trapping is DO NOT GUESS. If you are uncertain about how to trap or have questions regarding trapping, please ask.

If you are currently working on a project for print with Ikn & Ideas, Inc. please contact your customer service or sales representative with trapping questions.