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Sahil Arora
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Add PCLm support to cups-filters

Introduction

This project aims at writing the first implementation of the PCLm format convertor, which is a strict subset of the PDF format and lets printers low on memory print the files without any issues. This project was done during the Google Summer of Code 2017 program: Add support to CUPS-FILTERS for enabling conversion to and printing of PCLm files supported by IPP everywhere printers, under the mentorship of The Linux Foundation as guiding organization.

Mentors

  1. Till Kamppeter: Till Kamppeter is the project leader for OpenPrinting. He was invited in 2006 to work for the Free Standards Group (now The Linux Foundation) merging http://linuxprinting.org into OpenPrinting and leading the OpenPrinting project full time. With OpenPrinting, he leads the development of new printing architectures, technologies, printing infrastructure, and interface standards for Linux and Unix-style operating systems. For this he is in contact with the leading printer manufacturers, all relevant free software projects, and the distribution vendors. Till is also a distinguished Linux Foundation Fellow.
  • Contact: till [dot] kamppeter [at] gmail [dot] com
  • Github: tillkamppeter
  1. Jay Berkenbilt: Jay is the creator and maintainer of the widely used QPDF library. He is currently the Principal Release Manager at Appian.

Background

CUPS and CUPS-filters

CUPS is the standards-based, open source printing system developed by Apple Inc. for macOS® and other UNIX®-like operating systems. CUPS uses the Internet Printing Protocol (IPP) to support printing to local and network printers. CUPS is the primary printing software. CUPS-FILTERS is a software which is shipped with CUPS and is mainly responsible for, as the name suggests, filtering the data that goes to the printer, for example converting the print file to a format supported by the printer, getting IPP attributes from the printer, etc.

Internet Printing Protocol (IPP)

The Internet Printing Protocol is an Internet protocol for communication between computers and printers (or print server). It allows programs to send one or more print jobs to the printer or print server and perform administration such as querying the status of a printer, obtaining the status of print jobs, or cancelling individual print jobs.

Printers have always come with a driver installation when you buy it, which contained all the drivers and necessary software to print a file. The drivers contained instructions for how the operating system should interact with the printer, and what language the printer understands. So the printer manufactures had no rules or conventions on how to make a printer, what languages should it support, etc. All they did was put the CD with the printer. But then came smartphones, iPads, etc., devices where printer manufactures could not put the CDs. Hence came into picture Internet Printing Protocol or the IPP. This was a protocol as to how the printers should be made in order to support printing without a manufacturer specific software, or basically driverlessly. This is very similar to HTTP, the protocol you use to get a web page. When you click on a web page, your browser first contact the DNS provider which gives the equivalent IP address of the server, then your browser sends a GET Request to that IP of the server to get data. Similarly, in modern day wireless printers, you do an IPP GET request. You have a software which contacts a DNS service on your machine to get the IP of the printer, and then asks for the printer for all the capabilities it has, for instance does it support 2-side printing, duplex printing, etc. The with the same process you transfer a file to the printer to print, via IPP, along with the options we feel, like color_printing = false, and you have your print in the printer. This is IPP, the future of printing.

About PCLm

PCLm is a file format which was created by HP, and is a light-weight, strict subset of PDF which makes printing fast and efficient. That means that it can be viewed by a normal PDF viewer. It is streamable, i.e. can be printed page by page unlike PDFs, where PDF files require them to be loaded completely to the printer memory to be printed. Many cheap printers which are low on memory have this limitation of printing large PDF files, and here PCLm comes to the rescue. It also makes printing faster for high end printers. PCLm is also one of the standard file formats supported by Wifi-Direct and Mopria printing standards.

Problem at hand

Until now, there were no open source softwares which could create a PCLm file. The only software which is paid is PCLWorks, which is a proprietary software. Other implementations of PCLm include the softwares included by the printer manufacturers, but they are not available for linux operating systems. Hence, this project is an attempt at converting an input file to a valid PCLm file for the printer to print it.

Impact

There are millions of printers today which are certified by Wifi-Direct and/or Mopria printing standards. All those printers support PCLm, and the success of this project means that all printers designed for driverless printing (typically the ones where the manufacturer tells you can print from a phone or tablet on it) should print via CUPS. Most modern network printers do this, many USB printers, too.

Challenges

The biggest challenge was lack of resources. Since there were no previous implementations of PCLm, everything had to be done from scratch. There was one implementation here, but it was an obsolete android application which could not build. Hence, for about one and a half month we had no idea if we could pull it off or not. Also, this implementation was very heavy and would have increased the size of the cups-filter software by manifolds and would have been extremely unmaintainable.

Since PCLm was a subset of PDF, we decided we would modify an already existing filter which converts PWG Raster file format to PCLm. The filter was already using QPDF, hence we decided we would use QPDF to do our task.

However, mid-July we realized that it was impossible for QPDF to do some tasks which were required by PCLm. To facilitate that, we had to try making changes to QPDF as well so that it support PCLm conversion.

The other big challenge was debugging PCLm files. There was a time when they were displayed correctly by the PDF viewer, but the printer would say that it encountered a **document-format-error** while printing the file. No error logs, no data, nothing. Hence debugging was also a very difficult task here.

Achievements

The code changes made and features added in this project have made it to cups-filters v1.17.0. Also, QPDF has also been released to v7.0.0 and facilitates PCLm creation mode. PCLm compatibility will be first introduced as a new feature in Ubuntu 17.10 Artful! This will make Ubuntu Artful be as capable as many mobile devices when it comes to printing.

  1. The cups-filters source does not use Git, but Bazaar. The source can be found here: http://bzr.linuxfoundation.org/loggerhead/openprinting/`CUPS`-filters/changes
  2. My commits in the bazaar repository of CUPS-filters can be found her
  1. Since all the development of cups-filter happens in bazaar and I have chosen git to work with, all the changes in the commits in this repository(except 39aefc, d686a9, ccb47b and 964e63) have been done by me. You can find the commits here: https://github.com/sahilarora535/`CUPS`-filters/commits/master.
  2. I also had to make changes to QPDF for completing this project. The commit for which can be found here: https://github.com/qpdf/qpdf/commit/b19210fa7dbab7efa7a3cd65653be4ff2f6e08e9
  3. To test the code, I created a tool to create some sample PCLm files from raster images. You can also find some saple PCLm files here. This has also been completely written by me, and can be found here: http://github.com/sahilarora535/raster-to-pclm/.

Footnotes

cups-filters

CUPS-filters got split out of CUPS for CUPS version 1.6.x, containing the filters and backends which Apple does not need for Mac OS X and therefore did not want to maintain any more. Till Kamppeter had overtaken this part as an OpenPrinting project named CUPS-filters. He added CUPS-browsed as CUPS itself did not automatically make remote CUPS queues available locally any more. He also took maintainership on all CUPS features which Apple has given up. With the time, CUPS-filters got improved CUPS-filters, especially switched to a PDF-based printing workflow, added legacy CUPS broadcasting/browsing, sophisticated filtering of remote printers, auto setup of remote IPP printers, driverless printing, etc., and all the time kept it compatible with new CUPS features.

Internet Printing Protocol

IPP was created in the late 1990s and was base of CUPS which got 1.0 in 2000, well before smartphones appeared, but IPP 2.0 added things like the possibility to poll the full set of printer capabilities to allow driverless printing.

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