Working with color trapping and overprinting
When colors are trapped, they are intentionally overlapped so that misalignments of print separations are not noticeable. In manual trapping, one color must overprint the other. Overprint trapping works best when the top color is much darker than the underlying color; otherwise, an undesirable third color may result (for example, cyan over yellow results in a green object). In some cases, you might actually want to create a third color; for example, you can overprint two spot colors to create a third color.
How overprinted colors mix depends on the type of colors and ink you are mixing and the types of objects you are overprinting. For example, an object that uses a CMYK color overprints differently from an object that uses a spot color. Bitmaps also overprint differently from vector objects. You can preview a simulation of how overprinted colors will mix by using the Enhanced with overprints viewing mode. For more information about choosing a viewing mode, see “Choosing viewing modes.” Some variation between the preview and the printed version may occur, depending on the printer you use.
When you are ready to print, you can choose to preserve overprint settings if you want to trap objects in a document, or if you want to mix the overlapping colors for effect. You can also choose to knock out the overprinted areas so that only the top color is visible. If you want to print a proof version of the file, you can simulate overprints. Simulating overprints rasterizes the file and it prints using process colors only.
You can set a group of objects to overprint. You can overprint bitmaps; or each vector object’s fill, or outline, or both. You can also overprint specific color separations and specify in which order they will print, as well as whether you want to overprint graphics, or text, or both.
The two methods for color trapping automatically are always overprinting black and auto-spreading. Always overprinting black creates a color trap by causing any object that contains 95% black or more to overprint any underlying objects. This option is useful for artwork containing a lot of black text, but it should be used with caution on artwork with a high graphics content. You can adjust the threshold, if the service bureau recommends a black threshold value other than 95%.
Auto-spreading creates color trapping by assigning an outline to an object that is the same color as the object’s fill and having it overprint underlying objects. Auto-spreading is created for all objects in the file that meet three conditions: they do not already have an outline, are filled with a uniform fill, and have not already been designated to overprint.
For advanced trapping options, see “Specifying In-RIP trapping settings.”
To preserve or ignore overprints when printing |  |
Print.|
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Ignore — does not overprint areas; color on top prints and underlying colors are knocked out
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To print a simulated overprint | |
To overprint selected objects | |
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Select an object.
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You can also set an object to overprint by right-clicking the object and choosing an overprint option from the context-menu.
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To overprint selected color separations | |
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In the Advanced separations settings dialog box, choose a color separation from the Screening technology list box.
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You can change the order in which color separations print by selecting a color separation and choosing an order from the Order list box.
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To trap by always overprinting black | |
To set the overprint black threshold | |
To trap by auto-spreading | |