PPA (Printing Performance Architecture) is a closed, proprietary protocol developed by Hewlett Packard for a short-lived series of DeskJet printers. In essence, the PPA protocol moves the low-level processing of the data to the host computer rather than the printer. This allows for a low-cost (to produce) printer with a small amount of memory and computing power. However, in practice the printer was often as expensive as more capable printers and HP has since discontinued the use of PPA in favour of returning to PCL3e in their latest USB-based printers.
This project is a testament to open source persistence to develop a workable solution for a proprietary platform despite the original manufacturer being extremely unhelpful.
Disclaimer: This project is not in any way associated with Hewlett Packard. All trademarks are the property by their appropriate owners. Their use here is not intended to dispute the ownership of these marks.