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Trapping

Article Description

Imperfect registration of the printing presses at your printer could ruin your documents. Let Real World Adobe Indesign 1.5 author Olav Martin Kvern show you how to trap your publications to avoid disastrous printing results.

[Part 5 of 6]

Process-Color Trapping. Process-color trapping is a bit simpler than spot-color trapping,because it's usually less critical that process-colored elements have traps, but it can be far harder to figure out exactly what color to make the stroke for a process-colored object. And when you're talking about trapping two process-colored graduated fills, watch out!

The main thing to keep in mind, however, is that, for each of the process inks, the ink percentage used in the topmost object in any stack of objects always wins - they knock out all percentages of that ink behind them, regardless of any overprinting settings.

Unless, that is, the ink percentage is zero. If, for example, the percentage of cyan used in the fill color of the topmost object in a stack of objects is zero, turning Overprint off makes the path knock out any other cyan in the area covered by the path. Overprinting the fill, in this case, means that the area taken up by the fill disappears from the cyan plates - the percentage of cyan in the next object in the stack shows through the area where the objects overlap (see Color Example 4).

Another way to think of this is to think of each ink in a process color as behaving like a separate spot ink.

Simple Process-Color Trapping. In process-color trapping, you've got to make your overprinting strokes different colors from either the background or foreground objects. Why? Because process colors have a way of creating new colors when you print them over each other. It's what they do best.

As in the spot-color trapping section above, I'll demonstrate process-color trapping techniques by example. First, create a couple of objects.

  1. Create a rectangle that's filled with "Color 1", which is specified as 20C 100M 0Y 10K.

  2. On top of this rectangle,draw an ellipse and fill it with "Color 2", which is specified as 0C 100M 50Y 0K.

  3. Select both objects and set their stroke to None.

  4. Save the file.

The ellipse needs to be trapped, or you run the risk of having cyan-colored lines appearing around the ellipse when the publication is printed - which could happen if the cyan and yel low plates aren 't in good register, or if your paper stretches.Whether you spread or choke the ellipse depends on its color. If the ellipse is darker than the background rectangle, choke the ellipse. If the ellipse is a lighter color than the background rectangle, spread the ellipse. In this case, the ellipse is a lighter color, so you'll use a spread. To spread the ellipse,follow these steps.

  1. Create a new process color containing only those colors in "Color 2" having higher values than "Color 1". Quick quiz: what component colors in "Color 2" have higher values than their counterparts in "Color 1"? If you said 50Y, you're the lucky winner. Specify a new color:0C 0M 50Y 0K.

  2. Select the ellipse.

  3. Press F10 to display the Stroke palette, if it's not already visible. Enter the stroke weight you want for your stroke in the Weight field.It should be twice the width of your desired trap.

  4. Apply the color swatch "Color 3 " to the stroke of the ellipse and set it to overprint.

When you print, all the areas around the ellipse have some dot value inside them, and the new colors created where the objects abut won't be too obvious.Choose Revert from the File menu to get ready for the next example.

What if the ellipse is the darker color? If it were, we'd have to choke it. To choke the ellipse,follow these steps.

  1. Select the ellipse and fill it with "Color 1". Select the rectangle and fill it with "Color 2".

  2. Create a new color ("Color 3") that contains only the largest color component in "Color 1". That's 100M, so "Color 3" should be specified as 0C 100M 0Y 0K.

  3. Use the Weight field in the Stroke palette to specif y the weight of the trap you want.

  4. Set the stroke color to "Color 3."

  5. Turn on the Overprint Stroke option in the Attributes palette.

When you print, the stroke you applied to the ellipse guarantees that there's no gap around the ellipse, even if you run into registration problems when you print the publication.

At this point, you've read your way through the manual trapping techniques, and are seriously considering hiring twenty house elves to take care of your trapping needs. But wait - InDesign includes two powerful automatic trapping methods : In-RIP trapping and InDesign built-in trapping (see Color Example 5).

For more on automatic trapping, see pages 401-408 of Real World Adobe InDesign 1.5.