The (Re)Architecture of the X Window System

The X Window System, Version 11, is the standard window system on Linux and UNIX systems. X11, designed in 1987, was “state of the art” at that time. From its inception, X has been a network transparent window system in which X client applications can run on any machine in a network using an X server running on any display. While there have been some significant extensions to X over its history (e.g. OpenGL support), X's design lay fallow over much of the 1990's. With the increasing interest in open source systems, it was no longer sufficient for modern applications and a significant overhaul is now well underway. This paper describes revisions to the architecture of the window system used in a growing fraction of desktops and embedded systems.

While part of this work is “good citizenship” required by open source, some of the architectural problems solved ease the ability of open source applications to print their results, and some of the techniques developed are believed to be in advance of the commercial computer industry.

The challenges being faced include:

o X's fundamentally flawed font architecture made it difficult to implement good WYSIWYG systems

o Inadequate 2D graphics, which had always been intended to be augmented and/or replaced

o Developers are loathe to adopt any new technology that limits the distribution of their applications

o Legal requirements for accessibility for screen magnifiers are difficult to implement

o Modern user interface eye candy, which sport translucent graphics, windows, drop shadows, etc.

o Full integration of applications into 3 D environments

o Collaborative shared use of X (e.g. multiple simultaneous use of projector walls or other shared applications)

While some of this work has been published elsewhere, there has never been any overview paper describing this work as an integrated whole, and the compositing manager work described below is novel as of fall 2003. This work represents a long term effort that started in 1999, and will continue for several years more.