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Samba HowTo Guide
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The CUPS Filtering Architecture

The core of the CUPS filtering system is based on Ghostscript. In addition to Ghostscript, CUPS uses some other filters of its own. You (or your OS vendor) may have plugged in even more filters. CUPS handles all data file formats under the label of various MIME types. Every incoming print file is subjected to an initial autotyping. The autotyping determines its given MIME type. A given MIME type implies zero or more possible filtering chains relevant to the selected target printer. This section discusses how MIME types recognition and conversion rules interact. They are used by CUPS to automatically set up a working filtering chain for any given input data format.

If CUPS rasterizes a PostScript file natively to a bitmap, this is done in two stages:

  • The first stage uses a Ghostscript device named “cups” (this is since version 1.1.15) and produces a generic raster format called “CUPS raster”.

  • The second stage uses a “raster driver” that converts the generic CUPS raster to a device-specific raster.

Make sure your Ghostscript version has the “cups” device compiled in (check with gs -h | grep cups ). Otherwise you may encounter the dreaded Unable to convert file 0 in your CUPS error_log file. To have “cups” as a device in your Ghostscript, you either need to patch GNU Ghostscript and recompile or use ESP Ghostscript. The superior alternative is ESP Ghostscript. It supports not just CUPS, but 300 other devices (while GNU Ghostscript supports only about 180). Because of this broad output device support, ESP Ghostscript is the first choice for non-CUPS spoolers, too. It is now recommended by for all spoolers.

CUPS printers may be set up to use external rendering paths. One of the most common is provided by the Foomatic/cupsomatic concept from This uses the classical Ghostscript approach, doing everything in one step. It does not use the “cups” device, but one of the many others. However, even for Foomatic/cupsomatic usage, best results and broadest printer model support is provided by ESP Ghostscript (more about Foomatic/cupsomatic, particularly the new version called now foomatic-rip , follows).

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