Color Conversion | Trapping | Color Management
The color gamut of RGB is broader than CMYK and the file size is smaller as well, but CMYK is the better choice for documents that will be used on the Web and for print. CMYK documents work well enough in the RGB environment, but RGB doesn't work as well in the CMYK environment.
Trapping can help compensate for registration problems by slightly expanding one color into another and eliminating the gaps and color shifts between colors. PDF stores the trapping instructions in such a way that the trapped images will look the same on any computer and also when the images are output.
For trapping to be effective, it must compensate for less than perfect registration without forming noticeable lines of unrelated color around objects caused by the overprint. Process colors that share a sufficient percentage of common colors do not require trapping, because any registration problems will reveal a color that is similar to each of the adjacent colors. An example of where trapping would not be necessary would be a printed item containing mostly reds and oranges. If the object on such a page were not in perfect register, the result would not be visually distracting because the colors in the gaps and overprinted areas would be similar to any adjacent colors.
The sequence in which the process colors are printed also influences ink trapping. For standard four color printing, the correct sequence to print the process colors are black, cyan, magenta, and finally yellow. Printing in the correct sequence will help ensure the best results. A change in the standard sequence will result in a change in the ink trapping characteristics.
Device color spaces refer to the color space of the input device such as a scanner and the color space of the output device such as a printer. Data that is scanned may not display the same colors on a monitor and a printer, so a color management system is necessary. There are many differences between color profiles, so there is no guarantee that colors will be consistent throughout the workflow.
The ICC (International Color Consortium) was developed to set standards for device independent color. ICC profiles are electronic files that describe all of the color characteristics of a device. They can be either generic for a certain device such as a scanner or a monitor or they can be customized with profile generation tools. Color profiles are important because the color spaces of devices such as scanners, monitors, printers, presses, and digital cameras vary greatly.
PDF is able to support objects that are specified in different device dependent, device independent, or special color spaces. Acrobat supports Adobe Color Engine (ACE) and ICC Color Management that are in Photoshop and Illustrator. Displaying and printing color documents is more consistent. InDesign does not support this.
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