Desktop Window Manager

It is also included with Windows Server 2008, but requires the "Desktop Experience" feature and compatible graphics drivers to be installed.

This is because Windows 7 supports (limited) hardware acceleration for GDI[2] and in doing so does not need to keep a copy of the buffer in system RAM so that the CPU can write to it.

DWM-aware rendering technologies like WPF directly make the internal data structures available in a DWM-compatible format.

[3] With Windows Vista, the transitions are limited to the set of built-in shaders that implement the transformations.

DWM exposes a public API that allows applications to access these thumbnail representations.

The composition tree represents the desktop and all the windows hosted in it, which are then rendered by MIL from the back of the scene to the front.

In pre-Vista Windows OSs, background applications had to be requested to re-render themselves by sending them the WM_PAINT message.

[3][6] The compositing engine uses optimizations such as culling to improve performance, as well as not redrawing areas that have not changed.

It is possible to prevent DWM from restarting temporarily in Windows 8, which causes the desktop to turn black, the taskbar grey, and break the start screen/modern apps, but desktop apps will continue to function and appear just like Windows 7 and Vista's Basic theme, based on the single-buffer renderer used by XP.

This allows DWM to function without compatible drivers, but not at the same level of performance as with a normal graphics card.

Under DWM, GDI calls are redirected to use the Canonical Display Driver (cdd.dll), a software renderer.

Another buffer is allocated in the video memory to represent the DirectX surface, which is used as the texture for the window meshes.

Writing the output of GDI to system memory is not hardware accelerated, nor is conversion to the DirectX surface.

[11] To prevent this, DWM is temporarily turned off, as long as an application that mixes GDI and DirectX in the same window is running.

In addition, Windows Virtual PC allows composition using the Remote Desktop Protocol.