Trapping

Color Approximation Trapping, (Captrap), is a mechanism for performing configurable bitmap based trapping as part of the Ghostscript rendering process. Embedded within the Ghostscript core, both standard and custom devices can easily take advantage of this innovative functionality.

What is Trapping?

Most printing methods generate color output by mixing separate inks on a single output medium. While solutions do exist that lay down all the inks at the same time, it is far more common for output to happen in several passes, which can result in small unpredictable offsets between each color. The process of modifying a printed page to make it more resistant to such registration errors is known as “trapping”.

“Prebaked” vs “Live” Trapping

Some software that generates files for printing has the ability to modify the source PDL document directly; effectively “baking” trapping fixes into the original file. This technique is fine when using a single output device, but can produce unpredictable registration errors on different printers.

Rather than being confined to a single output device, Captrap traps the final result “live”, so that files can be successfully processed regardless of what application generated them and without dependency on a single printer.

Color Approximation Trapping

Trapping solutions generally work on the principle that the human eye is relatively forgiving of colors blending together at the edges of objects. Where areas of different colors abut one another, Captrap extends one slightly to build in resilience against gaps.

Our solution works by extending the areas filled by lighter inks where they abut darker ones. The exact color around the edge of the darker object is sacrificed for an “approximate” one, but the overall image is far less sensitive to registration errors.

Captrap can be configured to work with different sets of inks, of differing intensities, and for different maximum X and Y offsets. Such changes are run-time rather than compile-time options, enabling a range of different settings to be tried to ensure user satisfaction.

Captrap is built into Ghostscript output devices such as psdcmyk and tiffsep by default, so can easily be experimented with.

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