This is a filter for CUPS -- the Common Unix Printing System -- that will allow to create virtual printers to open the printed file in an application.
The use case it was created for: I have a printer which can only print black and white, and grayscales get pixeladet due to dithering. Having small fonts in not 100% black makes the text hardly readable because not all pixels are printed. With this CUPS filter, I can print from any application into The Gimp -- GNU Image Manipulation Program -- make the document exactly black & white there, and print from The Gimp to the real printer.