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Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between colors on the final output. To compensate for potential gaps between colors in artwork, print shops use a technique called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate, dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.
There are two types of trap: a spread, in which a lighter object overlaps a darker background and seems to expand into the background; and a choke, in which a lighter background overlaps a darker object that falls within the background and seems to squeeze or reduce the object.
When overlapping painted objects share a common color, trapping may be unnecessary if the color that is common to both objects creates an automatic trap. For example, if two overlapping objects contain cyan as part of their CMYK values, any gap between them is covered by the cyan content of the object underneath.
Trapping type can present special problems. Avoid applying mixed process colors or tints of process colors to type at small point sizes, because any misregistration can make the text difficult to read. Likewise, trapping type at small point sizes can result in hard-to-read type. As with tint reduction, check with your print shop before trapping such type. For example, if you are printing black type on a colored background, simply overprinting the type onto the background may be enough.
The Trap command creates traps for simple objects by identifying the lighter-colored artwork—whether it’s the object or the background—and overprinting (trapping) it into the darker artwork. You can apply the Trap command from the Pathfinder panel or as an effect. The advantage of using the Trap effect is that you can alter the trap settings at any time.
A. Area of overprinting B. Area of knockout C. Background color D. Foreground color
In some cases, the top and bottom objects may have similar color densities so that one color is not obviously darker than the other. In this case, the Trap command determines the trap based on slight differences in color; if the trap specified by the Trap dialog box is not satisfactory, you can use the Reverse Trap option to switch the way in which the Trap command traps the two objects.
Thickness
Specifies a stroke width between 0.01 and 5000 points. Check with your print shop to determine what value to use.
Height/Width
Specifies the trap on horizontal lines as a percentage of the trap on vertical lines. Specifying different horizontal and vertical trap values lets you compensate for on‑press irregularities, such as paper stretch. Contact your print shop for help in determining this value. The default value of 100% results in the same trap width on horizontal lines and on vertical lines. To increase the trap thickness on horizontal lines without changing the vertical trap, set the Height/Width value to greater than 100%. To decrease the trap thickness on horizontal lines without changing the vertical trap, set the Height/Width value to less than 100%.
Tint Reduction
Reduces the tint of the lighter color being trapped; the darker color remains at 100%. This option is useful when trapping two light-colored objects, where the trap line may show through the darker of the two colors, resulting in an unsightly dark border. For example, if you trap a light yellow object into a light blue object, a bright green border is visible where the trap is created. Check with your print shop to find out what percentage of tint is most appropriate given the type of press, inks, paper stock, and so on being used.
Traps With Process Color
Converts spot-color traps to equivalent process colors. This option creates an object of the lighter of the spot colors and overprints it.
Reverse Traps
Traps darker colors into lighter colors. This option does not work with rich black—that is, black that contains additional CMY inks.
Precision (as effect only)
Affects how precisely an object's path is calculated. The more precise the calculation, the more accurate the drawing and the more time is required to generate the resulting path.
For more precise control of trapping and for trapping complex objects, you can create the effect of a trap by stroking an object and setting the stroke to overprint.
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Create a spread by entering the same color values for the Stroke as appear in the Fill box. You can change the stroke’s color values by selecting the stroke and then adjusting its color values in the Color panel. This method enlarges the object by stroking its boundaries with the same color as the object’s fill.
Object stroked with fill color
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Draw a line along the edge or edges that you want to trap. If the object is complex, use the Direct Selection tool
to
select the edges to be trapped, copy them, and choose Edit >
Paste In Front to paste the copy directly on top of the original.
Drop shadow with a trap (left) is based on the line drawn where the object and its drop shadow meet (right).