Comment 17 for bug 1753509

Mathias Weyland (launchpad-weyland), thanks for the link to the patched GTK, thispatch is generally a good idea as it is not a good idea of a print dialog talking directly to IPP printers, circumventing CUPS.

The problems are the following:

1. A typical desktop application sends the data to get printed in PDF format, but not all IPP printers understand PDF. The driverless IPP standards only require one of the four print data formats PDF, Apple Raster, PWG Raster, and PCLm. As PDF is a complex vector graphics format and the other 3 are simple raster formats, cheap printers usually do not understand PDF, so the dialog sending a job directly to the printer would not work here. When printing through a CUPS queue, CUPS detects which format the printer understands and converts if needed.

2. An essential functionality when printing from a desktop application is spooling, storing the print job on disk when receiving it, so the the dialog can close quickly and allow the user continuing to work in his application. Many, especially cheaper IPP printers do not have such capability, the dialog itself also does not spool AFAIK. Spooling is usually a task of CUPS (CUPS is a so-called print spooler). Therefoe printing should always go through CUPS.