Watermarking printouts in the Linux world

Printouts most often are a waste of paper.

But much worse is obsolete stationary. It’s not only as a waste of paper, but a waste of money, too, as companies typically have it printed for money.

With today’s high quality color laser printers in many offices, a better solution can be created by watermarking your printouts accordingly. But while many Windows printer drivers support it right out of the box, Linux and its “Common Unix Printer Subsystem” (CUPS) is surprisingly unprepared for this. Fortunately, as it is Linux, you can create your own solution rather easily.

A really short abstract on CUPS printing

I won’t be going into details here, because learning CUPS is worth an article series all by itself. But to cut things short, it’s sufficient to keep the following facts in mind:

• When creating your printing infrastructure with Linux, you’re handling Postscript most of the time. Or rather, CUPS is doing so.
• You can easily create a print server on any Linux box and have all your client stations learn the available printer queues from that box (and you can even have “MS Windows” clients use those queues, too).
• Clients send their print jobs to their local CUPS instance, which forwards the jobs to the selected printer queue on the print server.
• A “filter” for that printer queue will handle the conversion from Postscript to the printer-specific format.

Again, there’s no out-of-the-box watermarking component in this picture. But with a few tools (typically available for your Linux distro, too) you can add this functionality yourself.

The goal

We wanted to have various printer queues, one for each combination of printer and type of stationary, with the print server doing all the work – especially mixing in the stationary layout.

By selecting the proper queue, the user would have their print job “watermarked” with what makes up that specific form or letter paper. The printers only need a single paper bin and feed on plain, white paper. Of course, if you require different paper sizes or weight, you’ll still have to have those multiple sources.

The solution

To achieve this, we had to do the following:

1. Create a PDF that contains the stationary’s design. This was no extra work, as this is already the output of the original process and would be needed for separate printing, too.
2. Create a tool chain to merge the “watermark” PDF and the actual print job.
3. Integrate the tool chain into the CUPS processing.

We selected “pdftk” as our central tool to add the watermark to the print job. And since CUPS is using PostScript, which is the same “language” that PDFs use, all we had to do is convert the print job to a PDF, use “pdftk” to watermark, and convert the resulting PDF back to Postscript. All this packed can be packed into a shell script like the following:
#!/bin/bash

# clean-up
rm -rf $tempdir

“pdftk” wouldn’t read stdin, so you have to take a slight detour – which helps during debugging, too. And of course there’s plenty that could be improved… starting with a way to specify the watermark PDF via some option, rather than hard-coding it into the script. Also note the seemingly unnecessary “filename” variable – it’s just there to illustrate what to look out for when reworking this sample script.

You may have noticed that while we use “ps2pdf” to convert the input stream, we don’t use “pdf2ps” to convert back. We experienced significant print quality problems and color changes with “pdf2ps”, while “pdftops” handled the job quite nicely. And looking at Stefaan Lippen’s blog page on the subject, there may be other severe drawbacks when using pdf2ps, too. Especially the significant increase in output file size may i.e. impact memory consumption and network traffic for networked Postscript printers.

Place that script in CUPS’ filter directory, which is “/usr/lib64/cups/filter/” in our case (obviously, a server running a 64bit variant of Linux – SLES11 SP1).

Now how to invoke this script? When creating your queue, CUPS creates a PPD (“Postscript Printer Definition”) file from the printer-specific PPD and saves it under /etc/cups/ppd/<queuename>.ppd. All you have to do is add a single line to that file, somewhere in the head section:

*cupsFilter: “application/vnd.cups-postscript 100 watermarkpdf”

The documentation of that command can be found on the CUPS web site. “watermarkpdf” was the name I gave to the script when testing this, so you may need to replace this with your own script’s name.

With a script per (watermarked) queue, you’re ready to rumble. Oh, and if anyone has an easy way of including the watermark file name in individual print jobs or at least in the queue’s PPD, please drop me a note in the comments section. Thanks!