X11R7.7 will bring multi-touch, friction, and better synchronization

The Wayland display server may grab more headlines these days, but that does not mean that X is sitting still. In fact, the X.org Foundation is due to release X11R7.7 shortly, with a host of new input and display features.

The last major release of the X Window System was X11R7.6, from December 2010. The modular design of X releases (adopted with X11R7.0) means that different parts of the source tree evolve at different paces; consequently it can be a little work to sort out precisely what improvements land with each full release. Release manager Alan Coopersmith recently posted a draft of the upcoming release notes, though, which gives a concise summary of the changes slated to arrive.

The XInput2 extension (XI2) is a highly-visible example of important, user-facing changes that land via extensions rather than in the core. XI2 2.2 added multi-touch input earlier this year; a long-awaited feature implemented by Peter Hutterer that enables the X server to recognize multiple input points from a single hardware device, thus allowing applications to interpret simultaneous input events, such as gestures. It also defines touch event sequences and cleanly differentiates between touch-sensitive input devices like trackpads and touch-sensitive screens. Both are enhancements that will prove useful as application toolkits like GTK+ and Qt make moves into the consumer tablet space.

But XI2 2.1 was also released between X11R7.6 and now, bringing with it a large patch set from Daniel Stone that improves the smooth-scrolling behavior. Other changes added a way for applications to track raw device events (largely of interest to game controllers), and added several missing defines to the XI2 API. The latter issue was that XI2 provided some-but-not-all defines in its own XI* namespace, and programmers were forced to fall back on core defines to keep track of other constants, such as AnyPropertyType or GrabSuccess. Falling back on such core defines is bad style, particularly because it was easy to accidentally do so when a relevant XI2 define did exist (such as using AnyModifier instead of XIAnyModifier), which would cause the program to break.

New features for client applications

The X Fixes extension adds the notion of "pointer barriers" in this release. The feature is derived from applications' desire to set easy-to-reach targets as active regions on the screen — such as the upper-left-corner of the display, which is used by several desktop environments to activate a "dashboard" style overlay. While monitoring the upper-left corner is child's play in a single monitor setup, the task becomes trickier with multiple displays. Some developers might want the upper-left-corner of the right-hand display — which is physically in the middle of the combined screen — to retain its active target status and still be easily reachable. Pointer barriers allow them to do so, by giving the desired target region some "friction" (for lack of a better term) that constrains pointer motion; the result is that the cursor slows down when it hits the target area, making it more difficult to overshoot.

The X Synchronization extension (XSync) now allows client applications to create "fence" objects that they can use to synchronize different rendering back-ends in use at the same time. This is important because not all renderers function at the same speed — such as when OpenGL is used to render only one portion of the client window. X is normally oblivious to this reality, but, as Aaron Plattner explained it when the work began, the application can create a fence (initially in an un-triggered state), and tell the OpenGL renderer to wait until the fence is triggered. When the rest of the X screen is finished rendering, the fence is triggered, and OpenGL updates its portion of the screen immediately.

The font system receives several improvements in this release. The first is that the location of installed X11 fonts can be changed at configuration time; the example given in the draft release notes moves them to /usr/share/fonts/X11/, which would put them closer to other system-installed fonts than the old location, /usr/X11R6/lib/X11/fonts/. The second change is the deprecation of some old code. Previous X releases included a bundled PostScript Type1 font backend; in X11R7.7 this backend is removed and FreeType is used instead. Support for the now-obsolete CID-keyed font format has been replaced entirely, as its functionality has been taken over by the OpenType format.

Under the hood

Primarily for debugging purposes, the X Resource extension now allows client applications to request more information about other clients, including their process IDs and the size of other clients' allocated resources. There are also several new debugging functions in the X server that can be bound to key sequences — although to prevent tragic mishaps, they are not bound by default. XF86LogGrabInfo will write the current set of input grabs to the X log file, and XF86LogWindowTree will write the current window tree. XF86Ungrab will release all active input grabs, while XF86ClearGrab will kill the connections of all active input grabs. These two potentially-dangerous functions were the source of CVE-2012-0064, which allowed attackers to bypass password-protected screensavers. Leaving the functions unbound offers partial protection only, so the new release also requires users to explicitly turn on support for the functions in their X Keyboard extension (XKB) configuration file.

Also of potential interest to developers is progress being made on the X protocol C Bindings (XCB). XCB is intended to one day completely replace the aging Xlib, which is already accessed primarily through compatibility layers provided by higher-level toolkits. Although it remains a work in progress, XCB developers have begun to add support for the OpenGL X extension (GLX) and XKB.

Several components are now marked as deprecated, specifically the nested and virtual X servers Xvfb, Xnest, Xephyr, and Xfake. All are designed to either display their contents inside another X server or to render content into a virtual framebuffer. The plan is for both types of functionality to eventually be replaced by substitute video drivers, xf86-video-nested and xf86-video-dummy. The functionality will remain unchanged, but with less maintenance demanded of the project.

Last but by no means least, the project has migrated its documentation to DocBook. That documentation includes the X protocol as well as the APIs and ABIs. Previously the documentation existed in a variety of different formats; the standardization on DocBook not only unifies it, but thanks to DocBook's XML underpinnings, allows it to be cross-linked for reference.

Of course, there are always long lists of changes and bugfixes made to the video drivers, input drivers, and modules that do not bring new functionality, but still provide a better experience. The project has a massive combined change-log document online that provides an entry point into every fix released during this development cycle. Between that and the new DocBook documentation, X11 fans have lots of reading to look forward to.