My GSoC Project

August 26, 2017

The Project

I was involved in GSoC’17 with OpenPrinting under the Linux Foundation working on the project - Common Printing Dialog. The idea behind this project was to ensure that all dialogs on Linux behave similarly and show similar options.

https://summerofcode.withgoogle.com/projects/#6401547484266496

The Idea

The basic idea was to have a server-client architecture where every method of printing, CUPS, Google Cloud Print, etc. behaved as servers and the dialogs behaved like clients. I’ll use client, dialog and frontend interchangeably throughout this blog post.

For achieving the communication, we used dbus which acts as a message bus between the servers and clients and offers us various features like broadcasting “signals” and also abstracting away server endpoints as simple function calls.

So, as soon as a dialog starts it sends out a signal to every backend which registers the frontend and starts processing. They asynchronously emit each and every printer associated with them. The dialog listens to these events and updates its list of printers.

Now, when a user tries to access options of a printer, the dialog queries the relevant backend and fetches the options. Also once the user presses the print button, the dialog sends the options chosen by the user to the backend.

My Job

The work was a group project among five students with: * 2 People working on the QT Dialog * 1 Person working on the CUPS Backend * 1 Person working on the Google Cloud Print Backend * I was working on integrating this in LibreOffice’s print dialogs


LibreOffice is probably used for a lot of printing on Linux these days and thus, it had a full fledged printing dialog ready. Our QT Dialog was being made from scratch, so first of all people had to make the general UI and stuff and then start working on the functionality.

So, I was involved in mostly trying to integrate the new backends with LO dialog with changes to just how the printers were obtained, etc. I worked closely with the backend teams, pointing out limitations or missing features required to get the LO dialog working just as it was before, with the new backends and helped in evolving the interfaces. The final dbus interfaces can be found here: https://github.com/dracarys09/cpd-interface-files

I managed to finally integrate changes in LO such that, if the new backends weren’t installed on a Linux system, it would fall back to the previous implementation.

Work Done

My changes to LO code can be found here: https://gerrit.libreoffice.org/#/c/40565

Other minor stuff I worked on: * Initial sample server and client * Bug Fixes to the CUPS Backend * Bug Fixes to the GCP Backend


Basically, I wrote a Manager, which listened to all printer added/removed events and updated the printers shown accordingly. It also allowed querying a specific printer for advanced options like print quality, paper quality which aren’t necessarily present in all printers. Finally, on pressing Print, the options and the print job were transmitted back to the dialog for printing.

Interesting Issues Faced

One of the main issues I faced was replacing the LO PPDParser. The new method doesn’t rely on PPD files and instead sends all possible settable options as ipp attributes. LO had the system set up such that for any particular printer, it would parse the PPD file and then based on the PPD Keys and Values present, populate the UI.

In this new system, I had to first of all try and ensure that the specific PPDKeys used across LO code such as Duplex (and its corresponding values Simplex, DuplexNoTumble, DuplexTumble) were converted from their corresponding ipp attribute sides (and corresponding values one-sided, two-sided-long-edge, two-sided-short-edge). This needed to be done for a 2-3 more keys and then the rest of the generic keys were easily handled using their ipp names.

Another related issue was that LO required page sizes (in exact mm) for displaying the preview, but in our new system, only page names (like a4) were returned from the backend. To tackle this, first of all, we shifted to sending standard PWG paper names from the backend and in the dialog, I used CUPS utility functions to get the exact paper sizes for the dialog to use.

Targets Left

Mostly the new dialog works and behaves like the older dialog without any big difference. The list of targets left for this are the following: * The current drop-down for the paper sizes uses the PWG names which aren’t very user-friendly. That could be mapped to user-friendly names. * The dialog on closing should emit a Shutdown signal so that the backends can go to sleep if no frontend is active. I have been unable to get that to work on my own. With help from the LO developers, this should be trivial. * I have to get this code merged to main LO codebase so that all future releases of LO can use the newer backends (if available on a particular system).

Acknowledgements

I am thankful to my mentors Danny, Till and Aveek for their guidance throughout the project. The interactions with them and working on this project was a great learning experience and I gained a lot of insight into how something which we take for granted, printers, work on our systems. Special thanks to Caolán McNamara (LibreOffice contributor) who took out some of his time and helped me out with any doubts regarding LO codebase.

Also, thanks to Abhijeet, Anshul, Nilanjana and Rithvik for helping out whenever I was stuck or had a doubt. It was great working with you guys. Cheers!

Conclusion

Overall, this year’s GSoC was a great learning experience for me and I have a learned a lot from this project.


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