Adobe Illustrator and CorelDRAW
9.1 The result of misregistration between color elements when no trap has been applied.
I have chosen these rather lurid colors as an example of misregistration (fig. 9.1) because they just happen to be two components of the CMYK system and are printed by different ink units on the press. If I were actually picking colors for a two-color print job, I would probably select something more tasteful from the Pantone book.
Modern printing presses are fantastic pieces of engineering and, in the hands of a good operator, it is possible to put a cyan dot right on top of a magenta dot that is right on top of a yellow dot, and so on. But that is on a good day, with a brand-new press. It is much more likely that the cyan will be ever so slightly out of register with the magenta, which in turn will be ever so slightly out of register with the yellow. The problem is that if you use a drawing program to create an image like the one shown in fig. 9.1, then the hole knocked out of the cyan square is exactly the same size and shape as the magenta circle that is doing the knocking. Given even a small amount of misregistration, the magenta element is almost certain to print with a slight overlap onto the cyan along one side, thus leaving a thin, crescent-shaped white space along the other.
The way to avoid this problem is to produce a very slight overlap between the magenta and cyan shapes. The area both colors then share is known as a trap.
It is not always possible to apply a trap in the way you might want. For example, in this case, if you were using Illustrator or CorelDRAW, it would be difficult to trap (i.e. expand) the cyan into the magenta. It could be done, but it would involve much more work. It would be easy, on the other hand, to trap the magenta into the cyan, simply by adding an outline and setting it to overprint. The thing to remember with traps is that it is always best to trap the lighter color into the darker, and not the other way round. If the darker color expanded into the lighter to create the trap, the result would compromise the shape of the lighter-colored object. A good way to remember which way round to do this is to imagine the effect, both ways, for yellow type on a black background. Then you will remember that the only thing to do is to trap the yellow into the black, because that option leaves the shape of the type unchanged.
If the overlapping area is small enough, you will not see it. The juxtaposition of two bright colors, especially if they are close to being opposites, slightly dazzles the eye. Thus you would not notice even quite a large overlap (much more than you would want to apply as a trap) because the interaction between the colors, together with the after-image they produce on your retina, would cause you to see a slight optical illusion along their common edge that would make it difficult to see—even if no trap was applied. Alternatively, if the colors are not as bright as my example, or as contrasting, you are not going to see the trap between them simply because they are not bright or contrasting. When you look at an image, you are probably not really looking at detail. It is much more likely that at first you are just getting an overall impression. Maybe then you look a bit closer at the individual elements, but you tend not to investigate the edges where different elements touch, so long as the trap is small enough. Why? Because it is convincing. However, that only works if the trap is small enough to do the job it is intended to do, and not if it is so big that you can easily see it. So, obviously, the amount by which two colors are set to overlap is important.
Adobe Illustrator
In Illustrator there are two ways to deal with trapping. One is automatic, one is manual. To create a trap manually, add an outline of 0.25pt to an object and set it to overprint. You can do this by opening the Attributes palette and checking Overprint Stroke while the object is selected. To automate trapping between selected objects, use the Pathfinder palette. Select Trap and enter a width. Again, you will usually want 0.25pt. You can also specify a tint value to reduce the percentage of the lighter color being trapped, which will make it even less visible.
CorelDRAW
In CorelDRAW, you can also create manual traps between objects by adding a thin outline and setting it to overprint. Select the object, add the outline, then right-click on it and choose Overprint Outline. While saving or exporting the file, make sure you have trapping set to “Preserve the document overprint settings” (you will find this option if you click on the Advanced button in the Export window), otherwise the trapping information will get left behind. This will also be the case if you try to export it as a TIFF. To create automatic traps, you will need to export the drawing as an EPS file. During this process you can click on the Advanced tab and choose Auto-spreading. The Maximum command lets you specify the width of the resulting trap.
Incidentally, when using Illustrator or CorelDRAW it is also useful to apply Overprint Fill to anything black that runs over an object of another color. As black is the only color that can overlay other colors without creating a visible mix, there is no need to knock out the image beneath it. Therefore, simply allowing it to overprint avoids the need for a trap. The page-layout programs generally allow black to overprint as a default.