InDesign’s Trapping in a Composite PDF

Trapping is the correction that is often necessary to cover up for slight misregistration on press. Humans should not have to do manual trapping with overprinting strokes any longer. (If you don’t know what I’m talking about, just smile and be happy that you never had to manually trap a file.)

But every now and again, we hear someone say, “Yes, but we really need to do trapping in InDesign.” Well, InDesign’s Attributes palette lets you control overprinting, and its Strokes palette lets you control stroke thickness and position. Or, even better, you can use InDesign’s built-in trapping feature by printing separations to disk.

But what if you want to create a composite-color PDF file with trapping… that is, you want to be able to open the PDF in Acrobat and actually see the color and print separations there (instead of just seeing black and white pre-separated plates)? For example, let’s say we want to trap this page:

Here’s what you do:

Instead of using File > Export, choose File > Print, and then choose PDF from the Printer popup menu. (You’ll need Acrobat Professional installed to see this, I believe.)
Set up the various panes in the Print dialog box. For example, if you want crop marks, you’ll need to add them in the Marks and Bleed pane.
In the Output pane, choose In-RIP Separations from the Color popup menu. In this case, the “RIP” is Acrobat Distiller.
Now you can chose Application Built-In from the Trapping pop-up menu. That tells InDesign to handle the trapping when it writes PostScript to disk (before it gets to the Distiller).

Note that you can control the Acrobat Distiller settings by clicking the Printer button (on the Mac OS) or Setup button (in Windows) at the bottom of the Print dialog box. Then, when you’re ready, click Print. InDesign writes trapped PostScript to disk and then launches the Distiller to convert it to PDF.

When you open the file in Acrobat, be sure that Overprint Preview is turned on for best effect.

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