InDesign CS's Auto-Trapping

November 2, 2004 - 3:00am ||| 0 Comments | Add new

A friend of mine, a meticulous designer (and a relative newbie to ID) e-mailed me two simple questions a few days ago: Does InDesign automatically trap, like Quark? And can you see those traps when you preview separations (in Window -> Output Preview -> Separations)?

The answer to the first question is yes. Sort of.

Like QuarkXPress 6, InDesign CS automatically creates traps (overprinting edge colors to compensate for slight misregistration on the press) for page layout elements. You can see the default trap settings if you open the [Default] trap preset from Windows -> Trap Presets. Use the same palette to create and apply other trap settings.

I say "sort of" because the only time InDesign (or Quark 6, for that matter) actually generates those traps is when you print out separations. When you print a composite proof to your laser writer, or export a page layout file to a composite PDF, no trapping is applied. When you send a composite file to your print vendor, their software does the trapping when it's separated. They don't use InDesign's built-in trapping settings. The only traps they may honor are any strokes you manually set to be overprinting via the Attributes palette.

Which brings me to my friend's second question. He was sure he could detect automatic traps with InDesign's Separations Preview palette. I took a look at his file and thought so too, at first.

Try it yourself … put some spot-colored type on top of a process color background, open the Separations preview palette (choose Separations from its dropdown menu too), and look at the type's outlines closely in InDesign. There's a slight edge to the type that is a mix of the spot and process color background.

If you hover your mouse over the relevant page elements, you'll see the ink percentages change in the Separations palette. Move the tip of the cursor ever so slightly from the type interior, to the type edge, to the background. Watch as the palette's ink % readouts change from 100% spot color (when you're over the type) to 50% spot/50% process mix (when you're exactly over the edge of the type) to 100% of the process color (when the cursor is wholly on top of the background color).

If you look at individual plates on screen, you can also detect the edge of the spot color fade to 50%.

What the heck? I double-checked the documentation and called some printers I know. Nope, all said it was impossible. You can't see traps in Separations preview, only in actual separation printouts.

In the meantime, my friend was freaking out, because assuming he was actually seeing the automatic trapping here, he was also seeing that it was incorrect. Some elements had no traps that should, other traps had a bad mix of inks.

After a bit of head-scratching, I figured it out. The cursor was picking up the *screen antialiasing* of the type, not the trapping, which (as I said) didn't exist at this point. When hovering over the antialiased edge, InDesign was converting those RGB numbers to CMYK in the Separations preview palette, just as it does with placed RGB images.

Solution: Turn off anti-aliasing in Preferences -> Display Preferences. Voila, the phantom screen traps are gone. Everything is pure knockout/kiss fit, just as the documentation says.

If you really want to see the effects of InDesign's built-in trapping on-screen, you can. (Just remember that your print vendor will very likely *not* be using these settings, unless this is the final file you turn over. And in that case, better warn them that's what you did!)

What you'll be doing is creating a trapped, composite PDF file.

1. Choose File -> Print, and choose the Adobe PDF printer as your printer. (This means you'll need to have Acrobat 6 properly installed, it's what adds the Adobe PDF virtual printer to every program's Print dialog.)

2. In the Print dialog, in the Output panel, look for the Color: dropdown menu. Change the color output from Composite CMYK to In-RIP Separations.

3. Right below that you'll see the Trapping: dropdown menu. It's probably set to Off. Change it to Adobe Built-In.

4. Make any other changes you'd like in the Print dialog — page range, printer's marks, etc.

5. Click the Printer button (or Setup button if you're on Windows) at the bottom of the Print dialog. Change Adobe PDF options to the Press preset, and click Print to back out of the Printer/Setup dialog.

6. When prompted, name your PDF and tell the infernal machine where to save it. Click Print again in the InDesign Print dialog.

Open the PDF in Acrobat. To view the traps (the overprinting strokes), turn on Overprint Preview in the Advanced menu. You may need to use the Loupe zoom tool if your traps are really small.

And they said it couldn't be done ! ;-) (Thanks to Nick Hodge at for the basics of this technique.)

Doing it in Quark
By the way … as far as I can tell, it *can't* be done in QuarkXPress 6. I experimented for a long time but I couldn't get Quark to show its automatic trapping in a composite PDF.

However, I was able to get halfway there. I pretended I was going to print separations to my laser printer … by choosing those options in Quark's print dialog … but then made an end-run by clicking the Printer button and changing the OS printer to Adobe PDF. That created a color-separated PDF (one page/plate per color) that showed the traps Quark automatically creates.

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