Photoshop, InDesign, and Quark

First, I should mention that you do not need to apply a trap to photographic images in Photoshop. Trap settings should only be used if you are creating a non-photographic type of image in which there are areas of adjacent flat or graduated colors that will be printed by different ink units on a press.

If you do want to apply a trap in Photoshop, read on.

The default setting is a 1-pixel trap, but of course the size of the pixels is up to you. A 1-pixel trap on a 300dpi image is ideal for most work. Choose Image > Trap, enter a pixel count of “1,” and click OK. A 1-pixel trap on a 300dpi image will not really be noticeable, even when using it between two colors like cyan and magenta, whereas the alternative—a thin white crescent down one side of the magenta circle—would be very noticeable indeed.

Adobe InDesign’s default trap setting is 0.25pt. InDesign also allows trapping for rich blacks, which is a very sophisticated extra (see “Creating and Using a Rich Black,” opposite). In Quark, however, the default trap amount is 0.144pt. Now, if a point is 1⁄72 in, then 0.144 of a point is roughly 1⁄500 in. If you ask your printers if they can hold registration throughout a four-color run to 1⁄500 in (or just over 0.05mm), they will probably laugh. To change the default trap settings in Quark, first ensure that no project is currently open (or the new settings will only apply to it) and choose (PC) Edit > (or Mac) QuarkXPress > Preferences > Default Print Layout > Trapping (fig. 9.2). The “Absolute” option assigns the given trap amount everywhere it is needed, whereas “Proportional” will alter the amount of trapping depending on the lightness of the color. I suggest setting process trapping to “on” and increasing the default amount to 0.25pt. That increases the trap to 1⁄288 in—the same as InDesign, and roughly the same as a 1-pixel trap on a 300dpi image.

In fig. 9.3 I have created three images with a background of solid cyan and a magenta circle in the middle. On the left, it has been generated by Adobe Illustrator and exported as an EPS file with a 1-pt trap. The center image shows the same EPS file but with a 0.25-pt trap. On the right, the same image elements were created in Photoshop as a 300dpi TIFF file with a 1-pixel trap. (Incidentally, the file size for these images was only 220kb for the vector EPS files, but nearly 1.35MB for the TIFF, so that is clearly one area where vector EPS has an advantage. Of course, given current computer storage capacities it is not such a big deal as it used to be.)

9.2 The Preferences window, in which the Quark default trapping settings can be changed.

9.3 Left: Adobe Illustrator EPS, 1-pt trap. Center: Adobe Illustrator EPS, 0.25-pt trap. Right: Photoshop 300dpi image, 1-pixel trap.