Desktop Window Manager
Encyclopedia
Desktop Window Manager (DWM, previously Desktop Compositing Engine or DCE) is the window manager
in Windows Vista
and Windows 7 that enables the Windows Aero
graphical user interface
and visual theme. The Desktop Window Manager requires video cards supporting DirectX
9.0 and Shader Model 2.0. DWM is not included with Windows Vista Starter edition. It is included with Windows Vista Home Basic edition, but with some aspects of the Windows Aero interface (such as transparent Glass and Flip 3D) disabled. It is also included with Windows Server 2008, but requires the "Desktop Experience" feature and compatible graphics drivers to be installed.
and earlier (and Windows Vista and Windows 7 with Windows Aero disabled), which has each program writing to the same main buffer.
DWM works in different ways based on if it is the Windows 7 DWM or the Windows Vista DWM and if the graphics drivers it is using are WDDM 1.0 or 1.1.
Under Windows 7 and with WDDM 1.1 drivers, DWM only writes the program's buffer to the video RAM, even if it is a GDI program, this is because Windows 7 supports (limited) hardware acceleration for GDI 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.
Because the compositor has access to the graphics of all applications, it easily allows visual effects that string together visuals from multiple applications, such as transparency. The DWM uses DirectX 9
to perform the function of compositing and rendering in the GPU, freeing the CPU of the task of managing the rendering from the off-screen buffers to the display. However, it does not affect applications painting to the off-screen buffers; depending on the technologies used for that, it might still be CPU-bound. DWM-agnostic rendering techniques like GDI
are redirected to the buffers by rendering the UI as bitmap
s. DWM-aware rendering technologies like WPF
directly make the internal data structures available in a DWM-compatible format. The window contents in the buffers are then converted to DirectX textures.
The desktop itself is a full-screen Direct3D
surface, with windows being represented as a mesh consisting of two adjacent (and mutually-inverted) triangles, which are transformed to represent a 2D rectangle. The texture, representing the UI chrome, is then mapped onto these rectangles. Window transitions are implemented as transformations of the meshes, using shader
programs. With Windows Vista, the transitions are limited to the set of built-in shaders that implement the transformations. Greg Schechter, a developer at Microsoft has suggested that it might be opened up for developers and users to plug in their own effects in a future release. The DWM only maps the primary desktop object
as a 3D surface; other desktop objects, including virtual desktops as well as the secure desktop used by User Account Control
are not.
Since all applications render to an off-screen buffer, they can be read off the buffer embedded in other applications as well. Since the off-screen buffer is constantly updated by the application, the embedded rendering will be a dynamic representation of the application window and not a static rendering. This is how the live thumbnail previews and Windows Flip work in Windows Vista
. The DWM exposes a public API that allows applications to access these thumbnail representations. The size of the thumbnail is not fixed; applications can request the thumbnails at any size - smaller than the original window, at the same size or even larger - and DWM will scale them properly before returning. Windows Flip 3D does not use the public thumbnail APIs, as they do not allow for directly accessing the Direct3D textures. Instead, Flip 3D is implemented directly in the DWM engine.
The Desktop Window Manager uses Media Integration Layer (MIL), the unmanaged compositor which it shares with Windows Presentation Foundation
, to represent the windows as composition nodes in a composition tree, representing the desktop and all the windows hosted in it, which are then rendered by MIL from back of the scene to the front. Since all the windows contribute to the final image, the color of a resultant pixel can be decided by more than one window. This is used to implement effects such as per-pixel transparency. DWM allows custom shaders to be invoked to control how pixels from multiple applications are used to create the resultant pixel. The DWM includes built-in Pixel shader 2.0 programs that computes the color of a pixel in a window by averaging the color of the pixel as determined by the window behind it and its neighboring pixels. These shaders are used by DWM to achieve the blur effect in the window borders of windows managed by DWM, and optionally for the areas where it is requested by the application.
Since MIL provides a retained mode
graphics system by caching the composition trees, the job of repainting and refreshing the screen when windows are moved is handled by DWM and MIL, freeing the application of the responsibility. The background data is already in the composition tree and the off-screen buffers, that is directly used to render the background, without having the background applications to re-render themselves by sending them the
graphics to prevent flickering and tearing during window moves. The compositing engine uses optimizations such as culling
to improve performance, as well as not redrawing areas that haven't changed. Because the compositor is multi-monitor aware, the DWM natively supports that too.
During full-screen applications, such as games, the DWM does not perform window compositing and therefore performance will not appreciably decrease.
or DirectX
can be used for rendering. To make these two work with DWM, redirection techniques for both are provided with DWM.
With GDI, which is the most used UI rendering technique in Microsoft Windows
, each application window is notified when it or a part of it comes in view and it is the job of the application to render itself. Without DWM, the rendering rasterizes
the UI in a buffer in video memory, from where it is rendered to the screen. Under DWM, a buffer equal to the size of the window is allocated in system memory. GDI calls are redirected to write their outputs to this buffer, rather than the video memory. Another buffer is allocated in the video memory to represent the DirectX surface, which is used as the texture for the Window meshes. The system memory buffer is converted to the DirectX surface separately, and kept in sync. This round-about route is required as GDI cannot output directly in DirectX pixel format. The surface is read by the compositor and is composited to the desktop in video memory. Writing the output of GDI to system memory is not hardware accelerated, nor is conversion to DirectX surface. When a GDI window is minimized, by the limitations of GDI, the buffer is no longer updated. So, DWM uses the last bitmap rendered to the buffer before the application was minimized.
For applications using DirectX
to write to a 3D surface, the DirectX implementation in Windows Vista
uses WDDM
to share the surface with DWM. DWM then uses the surface directly and maps it on to the window meshes. For WPF applications, which are DirectX applications, the compositor renders to such shared surfaces, which are then composited into the final desktop. Applications can mix either rendering technique across multiple child windows, as long as both GDI and DirectX are not used to render the same window. In that case, the ordering between DirectX and GDI rendering cannot be guaranteed, and as such it cannot be guaranteed whether the GDI bitmap from the system memory has been translated to the video memory surface. So, the final composition may not contain the GDI-rendered elements. To prevent this, DWM is temporarily turned off, as long as an application which mixes GDI
and DirectX
in the same window is running.
In Windows 7, the Desktop Window Manager has been reworked to use Direct3D 10.1, but the hardware requirements remain the same as in Windows Vista; Direct3D 9 hardware is supported with device type introduced with the Direct3D 11 runtime.
Until recently, virtualization
software could not emulate the shaders required for DWM to be functional. Now, VirtualBox
4.1, VMware Fusion
3.0, and VMware Workstation
7.0 onwards support it. In addition, Windows Virtual PC
allows composition using the Remote Desktop Protocol
.
Window manager
A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface. Most window managers are designed to help provide a desktop environment...
in Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
and Windows 7 that enables the Windows Aero
Windows Aero
Windows Aero is the graphical user interface and the default theme in most editions of Windows Vista and Windows 7, operating systems released by Microsoft. It is also available in Windows Server 2008, but is not enabled by default. Its name is a backronym for Authentic, Energetic, Reflective and...
graphical user interface
Graphical user interface
In computing, a graphical user interface is a type of user interface that allows users to interact with electronic devices with images rather than text commands. GUIs can be used in computers, hand-held devices such as MP3 players, portable media players or gaming devices, household appliances and...
and visual theme. The Desktop Window Manager requires video cards supporting DirectX
DirectX
Microsoft DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay,...
9.0 and Shader Model 2.0. DWM is not included with Windows Vista Starter edition. It is included with Windows Vista Home Basic edition, but with some aspects of the Windows Aero interface (such as transparent Glass and Flip 3D) disabled. It is also included with Windows Server 2008, but requires the "Desktop Experience" feature and compatible graphics drivers to be installed.
Architecture
The Desktop Window Manager is a Compositing window manager, each program has a buffer that it writes data to, the DWM then composites each program's buffer into a final image, compared to the stacking window manager in Windows XPWindows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
and earlier (and Windows Vista and Windows 7 with Windows Aero disabled), which has each program writing to the same main buffer.
DWM works in different ways based on if it is the Windows 7 DWM or the Windows Vista DWM and if the graphics drivers it is using are WDDM 1.0 or 1.1.
Under Windows 7 and with WDDM 1.1 drivers, DWM only writes the program's buffer to the video RAM, even if it is a GDI program, this is because Windows 7 supports (limited) hardware acceleration for GDI 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.
Because the compositor has access to the graphics of all applications, it easily allows visual effects that string together visuals from multiple applications, such as transparency. The DWM uses DirectX 9
DirectX
Microsoft DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay,...
to perform the function of compositing and rendering in the GPU, freeing the CPU of the task of managing the rendering from the off-screen buffers to the display. However, it does not affect applications painting to the off-screen buffers; depending on the technologies used for that, it might still be CPU-bound. DWM-agnostic rendering techniques like GDI
Graphics Device Interface
The Graphics Device Interface is a Microsoft Windows application programming interface and core operating system component responsible for representing graphical objects and transmitting them to output devices such as monitors and printers....
are redirected to the buffers by rendering the UI as bitmap
Bitmap
In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
s. DWM-aware rendering technologies like WPF
Windows Presentation Foundation
Developed by Microsoft, the Windows Presentation Foundation is a computer-software graphical subsystem for rendering user interfaces in Windows-based applications. WPF, previously known as "Avalon", was initially released as part of .NET Framework 3.0. Rather than relying on the older GDI...
directly make the internal data structures available in a DWM-compatible format. The window contents in the buffers are then converted to DirectX textures.
The desktop itself is a full-screen Direct3D
Direct3D
Direct3D is part of Microsoft's DirectX application programming interface . Direct3D is available for Microsoft Windows operating systems , and for other platforms through the open source software Wine. It is the base for the graphics API on the Xbox and Xbox 360 console systems...
surface, with windows being represented as a mesh consisting of two adjacent (and mutually-inverted) triangles, which are transformed to represent a 2D rectangle. The texture, representing the UI chrome, is then mapped onto these rectangles. Window transitions are implemented as transformations of the meshes, using shader
Shader
In the field of computer graphics, a shader is a computer program that is used primarily to calculate rendering effects on graphics hardware with a high degree of flexibility...
programs. With Windows Vista, the transitions are limited to the set of built-in shaders that implement the transformations. Greg Schechter, a developer at Microsoft has suggested that it might be opened up for developers and users to plug in their own effects in a future release. The DWM only maps the primary desktop object
Object Manager (Windows)
Object Manager is a subsystem implemented as part of the Windows Executive which manages Windows resources. Each resource, which are surfaced as logical objects, resides in a namespace for categorization. Resources can be physical devices, files or folders on volumes, Registry entries or even...
as a 3D surface; other desktop objects, including virtual desktops as well as the secure desktop used by User Account Control
User Account Control
User Account Control is a technology and security infrastructure introduced with Microsoft's Windows Vista and Windows Server 2008 operating systems, with a more relaxed version also present in Windows 7 and Windows Server 2008 R2...
are not.
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
. The DWM exposes a public API that allows applications to access these thumbnail representations. The size of the thumbnail is not fixed; applications can request the thumbnails at any size - smaller than the original window, at the same size or even larger - and DWM will scale them properly before returning. Windows Flip 3D does not use the public thumbnail APIs, as they do not allow for directly accessing the Direct3D textures. Instead, Flip 3D is implemented directly in the DWM engine.
The Desktop Window Manager uses Media Integration Layer (MIL), the unmanaged compositor which it shares with Windows Presentation Foundation
Windows Presentation Foundation
Developed by Microsoft, the Windows Presentation Foundation is a computer-software graphical subsystem for rendering user interfaces in Windows-based applications. WPF, previously known as "Avalon", was initially released as part of .NET Framework 3.0. Rather than relying on the older GDI...
, to represent the windows as composition nodes in a composition tree, representing the desktop and all the windows hosted in it, which are then rendered by MIL from back of the scene to the front. Since all the windows contribute to the final image, the color of a resultant pixel can be decided by more than one window. This is used to implement effects such as per-pixel transparency. DWM allows custom shaders to be invoked to control how pixels from multiple applications are used to create the resultant pixel. The DWM includes built-in Pixel shader 2.0 programs that computes the color of a pixel in a window by averaging the color of the pixel as determined by the window behind it and its neighboring pixels. These shaders are used by DWM to achieve the blur effect in the window borders of windows managed by DWM, and optionally for the areas where it is requested by the application.
Since MIL provides a retained mode
Retained mode
In computing, retained mode rendering is a style for application programming interfaces of graphics libraries, in which the libraries retain a complete model of the objects to be rendered.-Overview:...
graphics system by caching the composition trees, the job of repainting and refreshing the screen when windows are moved is handled by DWM and MIL, freeing the application of the responsibility. The background data is already in the composition tree and the off-screen buffers, that is directly used to render the background, without having the background applications to re-render themselves by sending them the
WM_PAINT message, as was the case with pre-Vista Windows OSs. DWM uses double-bufferedDouble buffering
In computer science, multiple buffering is the use of more than one buffer to hold a block of data, so that a "reader" will see a complete version of the data, rather than a partially-updated version of the data being created by a "writer"...
graphics to prevent flickering and tearing during window moves. The compositing engine uses optimizations such as culling
Back-face culling
In computer graphics, back-face culling determines whether a polygon of a graphical object is visible. It is a step in the graphical pipeline that tests whether the points in the polygon appear in clockwise or counter-clockwise order when projected onto the screen...
to improve performance, as well as not redrawing areas that haven't changed. Because the compositor is multi-monitor aware, the DWM natively supports that too.
During full-screen applications, such as games, the DWM does not perform window compositing and therefore performance will not appreciably decrease.
Redirection
For rendering techniques that are not DWM-aware, output must be redirected to the DWM buffers. With Windows, either GDIGraphics Device Interface
The Graphics Device Interface is a Microsoft Windows application programming interface and core operating system component responsible for representing graphical objects and transmitting them to output devices such as monitors and printers....
or DirectX
DirectX
Microsoft DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay,...
can be used for rendering. To make these two work with DWM, redirection techniques for both are provided with DWM.
With GDI, which is the most used UI rendering technique in Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, each application window is notified when it or a part of it comes in view and it is the job of the application to render itself. Without DWM, the rendering rasterizes
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
the UI in a buffer in video memory, from where it is rendered to the screen. Under DWM, a buffer equal to the size of the window is allocated in system memory. GDI calls are redirected to write their outputs to this buffer, rather than the video memory. Another buffer is allocated in the video memory to represent the DirectX surface, which is used as the texture for the Window meshes. The system memory buffer is converted to the DirectX surface separately, and kept in sync. This round-about route is required as GDI cannot output directly in DirectX pixel format. The surface is read by the compositor and is composited to the desktop in video memory. Writing the output of GDI to system memory is not hardware accelerated, nor is conversion to DirectX surface. When a GDI window is minimized, by the limitations of GDI, the buffer is no longer updated. So, DWM uses the last bitmap rendered to the buffer before the application was minimized.
For applications using DirectX
DirectX
Microsoft DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay,...
to write to a 3D surface, the DirectX implementation in Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
uses WDDM
Windows Display Driver Model
Windows Display Driver Model is the graphic driver architecture for video card drivers running Microsoft Windows versions beginning with Windows Vista....
to share the surface with DWM. DWM then uses the surface directly and maps it on to the window meshes. For WPF applications, which are DirectX applications, the compositor renders to such shared surfaces, which are then composited into the final desktop. Applications can mix either rendering technique across multiple child windows, as long as both GDI and DirectX are not used to render the same window. In that case, the ordering between DirectX and GDI rendering cannot be guaranteed, and as such it cannot be guaranteed whether the GDI bitmap from the system memory has been translated to the video memory surface. So, the final composition may not contain the GDI-rendered elements. To prevent this, DWM is temporarily turned off, as long as an application which mixes GDI
Graphics Device Interface
The Graphics Device Interface is a Microsoft Windows application programming interface and core operating system component responsible for representing graphical objects and transmitting them to output devices such as monitors and printers....
and DirectX
DirectX
Microsoft DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms. Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay,...
in the same window is running.
Hardware requirements
In Windows Vista, DWM requires compatible physical or virtual hardware:- A GPUGraphics processing unitA graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...
that supports the Windows Display Driver ModelWindows Display Driver ModelWindows Display Driver Model is the graphic driver architecture for video card drivers running Microsoft Windows versions beginning with Windows Vista....
(WDDM) - Direct3DDirect3DDirect3D is part of Microsoft's DirectX application programming interface . Direct3D is available for Microsoft Windows operating systems , and for other platforms through the open source software Wine. It is the base for the graphics API on the Xbox and Xbox 360 console systems...
9 support - Pixel Shader 2.0 support,
- Support for 32 bits per pixel
- Passes the Windows Aero acceptance test in the Windows Driver Kit (WDK)
In Windows 7, the Desktop Window Manager has been reworked to use Direct3D 10.1, but the hardware requirements remain the same as in Windows Vista; Direct3D 9 hardware is supported with device type introduced with the Direct3D 11 runtime.
Until recently, virtualization
Virtualization
Virtualization, in computing, is the creation of a virtual version of something, such as a hardware platform, operating system, a storage device or network resources....
software could not emulate the shaders required for DWM to be functional. Now, VirtualBox
VirtualBox
Oracle VM VirtualBox is an x86 virtualization software package, originally created by software company Innotek GmbH, purchased by Sun Microsystems, and now developed by Oracle Corporation as part of its family of virtualization products...
4.1, VMware Fusion
VMware Fusion
VMware Fusion is a virtual machine software product developed by VMware for Macintosh computers with Intel processors. Fusion allows Intel-based Macs to run x86 and x86-64 "guest" operating systems, such as Microsoft Windows, Linux, NetWare and Solaris as virtual machines simultaneously with Mac OS...
3.0, and VMware Workstation
VMware Workstation
VMware Workstation is a virtual machine software suite for x86 and x86-64 computers from VMware, a division of EMC Corporation, which allows users to set up multiple x86 and x86-64 virtual machines and use one or more of these virtual machines simultaneously with the hosting operating system...
7.0 onwards support it. In addition, Windows Virtual PC
Windows Virtual PC
Windows Virtual PC is a virtualization program for Microsoft Windows. In July 2006 Microsoft released the Windows-hosted version as a free product...
allows composition using the Remote Desktop Protocol
Remote Desktop Protocol
Remote Desktop Protocol is a proprietary protocol developed by Microsoft, which provides a user with a graphical interface to another computer. The protocol is an extension of the ITU-T T.128 application sharing protocol. Clients exist for most versions of Microsoft Windows , Linux, Unix, Mac OS...
.