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June 2007

Avoid registration mishaps by setting a perfect trap

by Ren�e Dustman

Application(s): Adobe InDesign 2/CS/CS2

Operating System(s): Macintosh, Microsoft Windows

Download: * Subscribers Only *

At A Glance:
Probably the last thing on most designers’ minds as they begin a new project is “I wonder how the InDesign document will trap?” But with the level of graphic artistry we pump out these days, keeping a close eye on trapping is a must. InDesign allows you to set trapping parameters early on so you can avoid disastrous printing results.

To prevent misregistration from botching up your jobs, we’ll:

• Explain what trapping is and why it’s so important.
• Show you how to create and apply custom trap presets to specific pages.
• Step you through the process of enabling trapping in InDesign.

Before the computer age, image trapping was a manual skill held by press operators alone. It involved vacuum frames, original films, spacer sheets, unexposed films, diffusion sheets, and overexposure. The need for trapping still exists in today’s more advanced workflows, but the tools for trapping and who is responsible for doing it have changed greatly. When is it time to trap, what is trapping, and how does one go about it? We’ll answer those questions (and probably a few more) in this article.

When is it time to trap?

There are ways to eliminate the need for trapping entirely. You can, for example, never overlap colored elements in your InDesign documents or you can stroke every colored object with black. Another way to eliminate the need for trapping (that is far less limiting to your creativity) is to use strictly process colors defined with common elements of at least 20%, and then set them to overprint. Figure A shows how this works.

On the left, the large gift box is filled with 100% cyan and sits atop of a shape filled with 50% yellow. The cyan fill is set to overprint via the Attributes palette and, by choosing View > Overprint Preview, you can see how this combination will print. By simply adding 20% yellow to the 100% cyan, however, you can ensure the colors print correctly without trapping, as shown on the right in Figure A.

Figure A:

Naturally, overprinting isn’t the answer to everything. When a document requires solid process and spot colors, you should use overprinting carefully and with the intention of producing colors other than what’s in your Swatches palette. Otherwise, you might end up with much more unsightly results than what’s shown on the left in Figure A.

What if, for example, we want the background of our graphic to be 100% magenta, the gift box to be 100% cyan, and the bow to be 100% yellow? Overprinting 100% cyan and magenta creates purple—not what we wanted, but not terrible either. Printing yellow on top of the purple, however, gives us a very disturbing gray, as shown in Figure B. That just won’t do! And that’s why InDesign overprints black and knocks out all colors by default.

Figure B:

Rather than overprint the colors, InDesign knocks out (removes) the cyan and magenta inks under the yellow object. This allows the yellow ink to print on bare substrate and not be affected by other colors. This process is also the reason why you need to trap your documents.

What is trapping?

Trapping specifies where the different colors meet and then either spreads (increases) the knockout area or chokes (decreases) it. The purpose of this is the reason for trapping: To prevent the substrate from showing through in the event misregistration occurs on the press and inks don’t line up correctly. An example of this is shown in Figure C. Another indication of misregistration is when colors overlap too much and cause an object to appear blurry.

Figure C:

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Who should set a trap?

The best-case scenario is to consider trapping before and during the design stage. However, trapping specifications vary depending on the substrate and printing processes. Therefore, it’s best for designers to discuss trapping requirements with their printers (and vice versa) early on. Generally, a printer will either ask you to disable trapping—so dedicated software can be used—or provide you with custom trap settings.

Note: If you need to start work on a document and a printer has not yet been chosen, your best bet is to leave trapping disabled and find a printer who can do it.

How do you set a trap?

In the event that you’re given the task to trap an InDesign document, there are a few ways you can go about it. Make sure to choose a method everyone involved feels comfortable with and that suits your workflow.

Trapping methods include:

Automatic trapping. As long as you have a PPD that supports separations, you can use InDesign’s built-in trapping feature to trap InDesign objects, Adobe Illustrator 9 and later native or vector PDF files (not already trapped), and placed images, but not vector EPS files.

Note: EPS files must be trapped in their native application or by a dedicated trapping program. Warning: To avoid serious conflicts, use the same settings to trap your EPS files as you do your InDesign files.

Adobe In-RIP Trapping. If you have a PPD and an Adobe PostScript 2 or later RIP that supports Adobe In-RIP Trapping, you can choose this option to fully trap InDesign objects, placed images, and graphics. Bonus: In-RIP Trapping is faster because it processes all traps at the RIP, instead of at your computer.

Manual. Generally, there isn’t much call for setting traps manually. However, you can read the info box “Set your own traps” if you’re curiosity gets the best of you.

Dedicated trapping software. Often more capable than built-in trapping engines, dedicated trapping software also lets you delay trapping until it’s time to RIP the files for print.

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Set trapping parameters

When you use InDesign’s built-in or In-RIP Trapping engine, adjustments are made automatically using the default Trapping settings. You can, however, create trap presets with custom settings for specific pages with special trapping requirements.

Version note: Trap Presets are referred to as Trap Styles in InDesign 2.

To create a trap preset:

Choose Window > Output > Trap Presets (Window > Trap Presets in earlier versions) to show the Trap Presets palette, as shown in Figure D, and then [option]-click ([Alt]-click in Windows) the Create New Trap Preset button .

Type a name for the preset in the New Trap Preset dialog box. Default settings are shown in Figure E.

Set the options according to specifications determined by the printer. See the info box “Preset the perfect trap” for an explanation of each option.

Click OK.

Figure D:

Figure E:

Preset the perfect trap

When creating a trap preset, set the following options so they correspond to your printing conditions:

Trap Width. Set the Default (all colors but black) and Black width of the trap to determine how far colors spread or choke. The maximum width is 4 points for built-in trapping and 8 points for Adobe In-RIP trapping. InDesign’s default settings are generally suitable for sheetfed presses and gravure.

Trap Appearance. Choose an option from the Join Style pop-up menu to specify the shape of the outside join of two trap segments; and choose an option from the End Style pop-up menu to control the intersection of three-way traps.

Images. Specify the way in which you want to trap vector objects to bitmap images by choosing an option from the Trap Placement pop-up menu. Also, if applicable, select the Trap Objects To Images, Trap Images To Images, Trap Images Internally, and Trap 1-bit Images check boxes. Note: In most cases, deselecting the last two options will improve results.

Trap Thresholds. In the Step field, specify the percentage at which color values must vary before they’re trapped. Adobe recommends a value from 8% to 20%. In the Black Color field, set the minimum amount of black ink required before InDesign applies the Trap Width for Black setting. Adobe recommends a value no lower than 70%.

Tip: To ensure InDesign applies the proper trap in the event of extreme dot gain, decrease this value and screen back blacks and rich blacks. In the Black Density field, specify the percentage at or above which InDesign considers an ink to be black. In the Sliding Trap field, specify the percentage at which the trap is moved from the darker side of a color edge toward the centerline in order to create a smoother transition along a gradient edge. In the Trap Color Reduction field, enter the percentage at which InDesign uses components from overlapping colors to lessen the trap color.

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To apply a trap preset:

Choose Assign Trap Preset from the Trap Presets palette pop-up menu.

Choose the preset you want to apply from the Trap Preset pop-up menu.

Specify the pages or range of pages to which you want to apply the preset, as shown in Figure F.

Click Assign, and then click Done. (Clicking Done without clicking Assign saves the preset without applying it.)

Tip: The process of trapping can take a while, but there’s a way you can speed things up. Simply apply the No Trap Preset to pages that don’t require trapping.

To disable a trap preset:

Choose Assign Trap Preset from the Trap Presets pop-up menu.

Choose [No Trap Preset] from the Trap Preset pop-up menu.

Select the Range option button and enter the pages or page range you don’t want trapped.

Click Assign, and then Done.

Figure F:

Print a perfect trap

With your trap presets saved and assigned to pages accordingly, you’re ready to print your InDesign document. At this stage, you may need to set a few additional parameters via the Ink Manager.

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To enable trapping:

Choose File > Print.

Select an appropriate PostScript printer and PPD that supports separations.

Set the remaining options in the Print dialog box as usual. When you get to the Output panel, however, choose Separations or In-RIP Separations from the Color pop-up menu.

Choose Application Built-In or Adobe In-Rip from the Trapping pop-up menu, as shown in Figure G.

Click the Ink Manager button if printing with any specialty inks, such as metallic inks.

Select the ink in the Ink Manager dialog box, shown in Figure H, and then set the option as needed. For Type, choose the type of ink you’re printing with. For Neutral Density, specify the correct value for the type of ink you’re printing with. For Trapping Sequence, specify the order in which you want the ink to print. For example, you may want an opaque ink to print first and allowed to dry before printing another ink over it.

Click OK, and then click Print.

Figure G:

Figure H:

Set your own traps

When manually trapping process colors, it’s important to remember that overprinting may produce unwanted color. Therefore, you need to consider ink values and their resulting color combinations when setting a trap.

To spread an object using process colors:

Create three new color swatches with the following values:

C=20 M=100 Y=0 K=10
C=0 M=100 Y=50 K=0
C=0 M=0 Y=50 K=0

Create a rectangle and fill it with the first color. Set the stroke to None.

Draw an ellipse directly on top of the rectangle and fill it with the second color.

Apply a stroke weight of .5 pt (or twice the width of the desired trap) to the ellipse.

Apply the third color swatch to the stroke of the ellipse.

Select the Overprint Stroke check box in the Attributes palette.

The final object is shown in Figure I. If the ellipse were to contain the darker shade, you’d instead choke it using a stroke color that contains only the largest color component used in the fill color—in our example, that’s 100% magenta.

Figure I:

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