KWin release notes for KDE4.0

Introduction

KWin, the standard KDE window manager, in KDE4.0 ships with the first version of built-in support for compositing, making it also a compositing manager. This allows KWin to provide advanced graphical effects, like for example with Compiz, while also providing all the features from previous KDE releases (such as very good intergration with the rest of KDE, advanced configurability, focus stealing prevention, well-tested window manager, robust handling of misbehaving applications/toolkits, etc.). Unlike Compiz, KWin still functions even when no system support for compositing is available, with only compositing features not being available in such case.

Previous KWin versions in later KDE3.x releases included a standalone compositing manager called kompmgr, based on the xcompmgr compositing manager. Kompmgr was only loosely tied with KWin, used only XRender for rendering and provided only basic features like transparency, shadows and fade in/out animations. Compositing manager in KWin in KDE4.0 is integrated with the rest of KWin, can use either OpenGL or XRender for rendering and has a framework for compositing effects, all these allowing KWin to provide a much wider range of features.

Note, however, that compositing support in KWin in KDE4.0 is still considered experimental, for several reasons. System support for compositing is often being problematic (various bugs in X, drivers or other parts of the system), manual configuration of X may be required for proper results (see below), some applications may not be prepared and work well with compositing, the performance may not be adequate, and other problems. Also, while KWin's compositing support is considered usable and reasonably stable, it is relatively new code and has been tested only on a limited range of hardware.

Therefore, compositing support in KWin is disabled by default, and needs to be explicitly enabled. If there will be any problems, you can disable it again (see below for troubleshooting) and report a bug with all relevant information about the problem.

Setting up

Compositing support is enabled in KWin's configuration. Press Alt+F3 and select 'Configure Window Behavior'. In the configuration module, select page 'Desktop Effects' and enable checkbox 'Enable desktop effects'. After accepting the changes, a dialog with a timeout will appear, asking to confirm enabling of compositing support. If you do not confirm within the timeout, compositing support will be disabled again, therefore, if enabling compositing triggers any problems, it should be sufficient to wait several seconds before the changes are reverted. Note that after enabling or disabling compositing it is recommended to restart your KDE session in order to ensure that all applications detect the change.

If you cannot enable desktop effects, it may be because either your KDE is not built with necessary support, or more probably because your system is not capable of providing compositing support. See file COMPOSITE_HOWTO for some instructions on setting up your system. Note that there may be other factors affecting whether you do or do not have compositing support.

Usage

A quick overview of features provided by compositing manager in KWin:

Ctrl+F9 (and Ctrl+F10 for windows from all desktops) shows an overview of all windows and allows activating one of them. The feature can be also activated by moving the mouse into the top-left screen corner. A window can be activated by clicking it or by using arrows and Enter key. You can also type text to filter the list of windows.
Ctrl+F8 shotcut activates a desktop grid - all your virtual desktops will be arranged on the screen (as an enlarged pager) - you can select and activate desktops using a number, a function key, by clicking on it or by using arrows and Enter key, you can move windows by dragging them or by right-clicking on them.
The DesktopGrid effect also provides animations when switching between virtual desktops (can be turned off).
The window switcher (Alt+Tab by default) provides live thumbnails of windows.
Windows blocked by modal dialogs are dimmed.
Screen can be zoomed in and out using Win+<equals>, Win+<minus> and reset using Win+0 (it is currently not possible to use mouse wheel, but this feature is planned). Note that because of input transformation not being yet available in X the zoomed screen has to move around to keep the mouse pointer at the same place like it would be when not zoomed.
Screen can be shown with inverted colors by pressing Ctrl+Win+I (accessibility feature, Invert effect is not enabled by default).
There are fade animations during login and logout.
Windows fade in and out.
Minimize animation to/from taskbar.
Windows have shadows.

There are more features that are not enabled by default and need to be explicitly enabled in the configuration.

There are various videos showing various compositing features of KWin. For example, search for 'kwin_composite' at youtube.com (please keep in mind that many of those windows are old and show testing or demo effects).

Performance

Compositing internally works by redirecting window drawing to offscreen memory and composing it on the screen in an additional drawing pass. This means that in general composited desktop on average has worse performance that non-composited desktop (although in some cases it may perform better, be that real improvement or just perceived one due to animations, better synchronization or similar factors). For example, binding window pixmaps to OpenGL textures (that is, preparing window contents for drawing) can be a relatively costly operation with large windows, making things like animations in Plasma desktop window or page scrolling in a maximized browser window jerky. Heavy system load can also cause the compositing manager not repaint often enough, resulting in lagging or jerky screen redrawing.

KWin in KDE4.0 is also relatively new code and has not been extensively optimized yet, therefore its performance may not be in some areas comparable with performance of other compositing managers. In such cases performance should be improved with newer versions.

Note that current XRender implementations (in X/drivers) often perform rather poorly and therefore the OpenGL mode usually should have much better performance. See below for notes on XRender mode.

Tip: Performance/smoothness with nVidia cards: Smoothness of KWin rendering can be improved by setting env.variable KWIN_NVIDIA_HACK to 1 (e.g. append 'export KWIN_NVIDIA_HACK=1' to your ~/.profile file). This sets '__GL_YIELD=NOTHING' for KWin, letting KWin use more CPU time for OpenGL operations, however at the expense of affecting performance of other applications. This is therefore disabled by default. This setting may be removed in the future if the negative impact becomes insignificant. See section 'OPENGL YIELD BEHAVIOR' in README.txt for nVidia cards.

Troubleshooting

As already said, compositing support in KWin is considered usable and reasonably stable, but due to several reasons it may not work properly for you.

If there are any problems with compositing support, the simplest option is to disable it again. KWin will normally continue functioning, only not providing compositing features. If you cannot normally turn off compositing support (for example because the screen is corrupted), you can turn it off using one of these ways:

run command 'kwriteconfig --file kwinrc --group Compositing --key Enabled false' from the command line
set environment variable 'KWIN_COMPOSE' to 'N' (append 'export KWIN_COMPOSE=N' at the end of your ~/.profile), this affects compositing only temporarily

You will probably need to switch to text mode or start failsafe session from KDM to be able to perform this.

See file COMPOSITE_HOWTO for some issues with various graphics cards.

XRender mode

It is possible to use XRender for compositing instead of the default OpenGL. XRender mode in general has less features, however at the moment it is also considered unstable - it has not received as much testing as OpenGL mode, some features may be incomplete and it is recommended to use the OpenGL mode if possible. Also note that current XRender implementations (in X/drivers) often perform rather poorly.

Developers

KWin provides support for writing compositing effects that may be loaded into KWin as plugins. These effects communicate with KWin core using C++ API specially designed for this purpose, making effects not directly dependent on KWin core and changes in it.

At the time of the KDE4.0 release, since compositing support is still under heavy development, this API is considered unstable and subject to change. If you write your own effect plugin, you may need to recompile it after KWin update. KWin will however detect incompatible versions and will not load such plugins (automatic, you do not need to provide any code for it). As the compositing support will become more stabilized, this API will be kept backwards and binary compatible, just like with other KDE libraries.

At the time of the KDE4.0 release, API for compositing effects is unfortunately only sparsely documented. Developers interested in writing compositing effects for KWin are suggested to use source code of effects shipped with KWin (the Howto effect in test/ directory as the starting point) and/or ask on the KWin mailing list.

Links to various KWin-related documents are available at techbase.kde.org .