Trapping is a technique used in the printing industry to reduce ink registration issues.
Since CMYK is by definition is a 4 color process, each sheet of paper has to be gripped, registered, and printed four separate times; once for each color. The mechanical process of registering a sheet of paper at four different ink stations is one of hope and optimism. If you are printing on white paper and you have a red area that is surrounded by a black border, any misalignment, red or black, will result in a white gap between the red and black. Trapping introduces an overlap so that misalignment is hidden. The idea is to print lighter colors first and oversized then print with black or dark colors, 'trapping' the lighter colors with the dark color. There is more information in the Photoshop Help Files under...

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Printing (Photoshop) > Creating color traps

After you have converted an image to CMYK, you can adjust the color trap. Trap is the overlap needed to ensure that a slight misalignment or movement of the plates while printing does not affect the final appearance of the print job. If any distinctly different colors in your image touch, you may need to overprint them slightly to prevent tiny gaps from appearing when the image is printed. This technique is known as trapping. In most cases, your print shop will determine if trapping is needed and tell you what values to enter in the Trap dialog box.

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Mr3

"The Cerebral Ass

Ruth

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- Trapping... anyone know anything about it?