The Libre Graphics Meeting is the annual event for open source creative graphics software. It greatly helps in improving the open source software stack through lots of talks, discussions, round tables, work shops and wonderful face to face meetings. There is always a great mixture of developers, artists, writers, translaters and interested people present, who come together in a very friendly and inclusive atmosphere. We had in the past always a OpenICC round table, when I was at LGM, and discussed various topics and planed around colour management. That should happen this year again with many ideas coming up.

To get people from all over the world to Europe, we need your help:

Sirko has created another pledgie:

Some days ago on FOSDEM I gave a presentation about Colour Management in Compositors. At that point is was not very clear how to introduce colour management especially into the upcoming Wayland display server core and thus make it wide spread. The answer from Wayland developers is the same as from Xorg ones. They want a small core and colour management does not fit inside this.

As a result of a discussion between several colour management interested people from wayland, toolkits and me on the wayland IRC channel, we found a smallest common denominator. That will be a per window colour correction mechanism. The advantage is, it will be very easy to implement inside compositors and they can even start today about ICC support. The biggest disadvantage for applications is, they need to colour correct the whole window. That is as well the reason, why I did not like the idea in the past. Anyway, hopefully toolkits will jump in at one point and make that easy. Meanwhile we need to focus on example code, which demonstrates how per window colour correction can work.

The spec can be found as usual in the libXcm git repository. The main new part is the _ICC_COLOR_OUTPUTS atom and XcolorOutput structure.

Colour managed printing under Linux relies on several components to play nicely together. Linux has the great lcms Colour Management Module (CMM) to parse ICC profiles and apply colour transformations based on those. The standard print job PDF can have source ICC profiles attached. CUPS knows about per print queue server side configured output ICC profiles. If feet with the correct settings by the according colour managing print filters, Ghostscript does a great job with the provided information at producing colour corrected raster output using lcms. That output is further processed by the printer driver and spooled by CUPS to the physical device.

PDF contains most often colour values defined in DeviceRGB, which is a very short way to specify some colour. And you know programmers are lazy and simply use that. So Ghostscript does a trick to colour manage these documents nonetheless and assumes DeviceRGB to be meant as sRGB, which is in this situation kind of the best it can do.

But DeviceRGB being handled as sRGB blocks practically two important use cases.

1. Advanced application might want to do colour management early inside the application.Think of proofing and other specialised tasks done by designers and PrePress studios.
2. Profilers, the applications which create ICC in the first place need targets to be printed without any colour correction. This case is vital to being able to setup colour management at all for new devices, media and drivers by creating valid ICC profiles. It affects owners of colour measurement devices for printers and if they publish their ICC profiles most other users too.

Fortunately there is a way to specify a output device profile per job, which is the way CUPS is designed to be used from client side. Comparably a per session based user device profile introduces a high risk to interfere with standard profile selection mechanisms and concurrenting sessions and is pretty limited in scope. The PDF/X standard allows to embed a output profile inside the document. That way all colour management is completely defined inside the PDF per job and can bypass any unwanted server side magic. The mechanism is called OutputIntent. Applications and print dialogs can use the OutputIntent in order to reliably send device Rgb or Cmyk through a colour management wise non intercepted printing path.

However, manipulation of existing PDF files is not that easy. Thankfully Joseph Simon has put some work into a project called Color-Managed Printing eXtension or short libCmpx. The library handles the harder parts of embedding a ICC output profile into a PDF/X and assists with profile selection. His primary design goal for the libCmpx library is to help enabling colour management in print dialogs. The origin of the project lays in the XCPD Google Summer of Code 2011 project for the OpenICC group.