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Desktop Window Manager (DWM, previously Desktop Compositing Engine or DCE) is the window manager in Windows Vista, Windows 7, and Windows 8^{[citation needed]} 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.^[1]

Architecture[link]

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 XP 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^[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.

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 bitmaps. 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.^[3] 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.^[4] 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.^[5]

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.^[6] 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. ^[7] 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.^[8] 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.^[3]

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 WM_PAINT message, as was the case with pre-Vista Windows OSs.^[6] DWM uses double-buffered graphics to prevent flickering and tearing during window moves.^[3][6] The compositing engine uses optimizations such as culling to improve performance, as well as not redrawing areas that haven't changed.^[8] Because the compositor is multi-monitor aware, the DWM natively supports that too.^[8]

During full-screen applications, such as games, the DWM does not perform window compositing and therefore performance will not appreciably decrease.

Redirection[link]

For rendering techniques that are not DWM-aware, output must be redirected to the DWM buffers. With Windows, either GDI 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.^[9]

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.^[9] 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.^[9] To prevent this, DWM is temporarily turned off, as long as an application which mixes GDI and DirectX in the same window is running.

Hardware requirements[link]

In Windows Vista, DWM requires compatible physical or virtual hardware:^[10]

• A GPU that supports the Windows Display Driver Model (WDDM)
• Direct3D 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 Unknown extension tag "syntaxhighlight" ^[11] 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.

See also[link]

• Windows Aero

References[link]

External links[link]

• Desktop Window Manager
• APIs in the Desktop Window Manager
• Using the DWM APIs
• What is DWM.exe is it a virus?
• dwm.exe

A window manager is system software that controls the placement and appearance of windows within a windowing system in a graphical user interface.^[1] Most window managers are designed to help provide a desktop environment. They work in conjunction with the underlying graphical system that provides required functionality—support for graphics hardware, pointing devices, and a keyboard, and are often written and created using a widget toolkit.^[2]

Few window managers are designed with a clear distinction between the windowing system and the window manager. Every graphical user interface based on a windows metaphor has some form of window management. In practice, the elements of this functionality vary greatly.^[3] Elements usually associated with window managers allow the user to open, close, minimize, maximize, move, resize, and keep track of running windows, including window decorators. Many window managers also come with various utilities and features: e.g. docks, task bars, program launchers, desktop icons, and wallpaper.

X window managers[link]

On systems using the X window system, there is a clear distinction between the window manager and the windowing system. Strictly speaking, an X window manager does not directly interact with video hardware, mice, or keyboards – that is the responsibility of the X server.

Users of the X Window System have the ability to easily use many different window managers – Metacity, used in GNOME, and KWin, used in KDE Plasma Workspaces, and many others. Since many window managers are modular, people can use others, such as Compiz (a 3D compositing window manager), which replaces the window manager. Sawfish and awesome on the other hand are extensible window managers offering exacting window control. Components of different window managers can even be mixed and matched; for example, the window decorations from KWin can be used with the desktop and dock components of GNOME.

X window managers also have the ability to re-parent applications, meaning that, while initially all applications are adopted by the root window (essentially the whole screen), an application started within the root window can be adopted by (i.e., put inside of) another window. Window managers under the X window system adopt applications from the root window and re-parent them to window decorations (for example, adding a title bar). Re-parenting can also be used to add the contents of one window to another. For example, a flash player application can be re-parented to a browser window, and can appear to the user as supposedly being part of that program. Re-parenting window managers can therefore arrange one or more programs into the same window, and can easily combine tiling and stacking in various ways.

Microsoft Windows[link]

This section requires expansion.

Microsoft Windows has provided an integrated stacking window manager since Windows 2.0; Windows Vista introduced the compositing dwm.exe as an optional hardware-accelerated alternative. In Windows, the role of the window manager is tightly coupled with the kernel's graphical subsystems and is largely non-replaceable, although third-party utilities can be used to simulate a Tiling window manager on top of such systems.

Windows Explorer (explorer.exe) is used by default in modern Windows systems to provide a panel and file manager, along with many functions of a window manager; aspects of Windows can be modified through the provided configuration utilities, modifying the Windows registry or with 3rd party tools, such as WindowBlinds or resource editors.

The Windows window manager can also act as an X window manager through Cygwin/X in multiwindow mode (and, possibly, other X window implementations).

Note that Microsoft and X Window System use different terms to describe similar concepts. For example, there is no specific word for window manager functionality in Windows (shell is sometimes used in this context, but its sense is fuzzy); probably, lacking such a term, Microsoft avoids advertising of third-party window managers.^{[citation needed]}

Types of window managers[link]

Window managers are often divided into three+ classes, which describe how windows are drawn and updated.

Compositing window managers[link]

Compositing window managers let all windows be created and drawn separately and then put together and displayed in various 2D and 3D environments. The most advanced compositing window managers allow for a great deal of variety in interface look and feel, and for the presence of advanced 2D and 3D visual effects.

Stacking window managers[link]

All window managers that have overlapping windows and are not compositing window managers are stacking window managers, although it is possible that not all use the same methods. Stacking window managers allow windows to overlap by drawing background windows first, which is referred to as painter's algorithm. Changes sometimes require that all windows be re-stacked or repainted, which usually involves redrawing every window. However, to bring a background window to the front usually only requires that one window be redrawn, since background windows may have bits of other windows painted over them, effectively erasing the areas that are covered.

Tiling window manager[link]

Tiling window managers paint all windows on-screen by placing them side by side or above and below each other, so that no window ever covers another. Microsoft Windows 1.0 used tiling, and a variety of tiling window managers for X are available.

Dynamic window manager[link]

Dynamic window managers can dynamically switch between tiling or floating window layout. A variety of dynamic window managers for X are available.

Features and facilities of window managers[link]

Autohide

An autohide facility enables menubars to disappear when the pointer is moved away from the edge of the screen.

Borders

A border is a window decoration component provided by some window managers, that appears around the active window. Some window managers may also display a border around background windows.

Context Menu

Some window managers provide a context menu that appears when an alternative click event is applied to a desktop component.

Desktop Wallpaper

Some window managers provide a desktop wallpaper facility that displays a background picture in the root window.

Focus Stealing

Focus stealing is a facility some window managers provide. It allows an application not in focus to suddenly gain focus and steal user input intended for the previously focused application.

Iconification

An iconification facility lets users minimize running applications to a desktop icon or taskpanel icon.

Joined Windows

Some window managers provide a joined windows facility that lets user join application window frames together.

Keyboard Equivalents

Some window managers provide keyboard equivalents that enables the keyboard to replicate mouse functionality.

Menubar

A menubar provides the facility to launch programs via a menu and may contain additional facilities including a start button, a taskbar, and a system tray.

Menu Panel

A menu panel a component of some window managers that provides the facility to launch programs using a menu. A menu panel is similar to a menubar, but appears as a floating panel, rather than a horizontal or vertical bar.

The menu panel may contain additional facilities including a start button, a task panel, and a system tray.

Mouse focus

The mouse focus model determines how the pointing device affects the input focus within the window manager. The focus model determine which component of the graphical user interface is currently selected to receive input as the pointer is moved around the screen.

Mouse warping

Mouse warping is a facility that centres the pointer on the current application as it is made active.

Multiple Desktops

A window manager may provide a multiple desktops facility. This enables switching between several root window desktops. This prevents clutter of the root window, because applications can run on different desktops.

Pager

Some window managers provide a pager tool that provides the facility to switch between multiple desktops. The pager may appear as an onscreen window or as a gadget in the taskbar or taskpanel.

Plugins

Some window managers have a modular construction that enables plug-in modules to provide features as required.

Rollup

A rollup facility enables windows to appear as just a titlebar on the desktop.

Root Menu

Some window managers provide a root menu, which appears when the root window or desktop background is touched.

Shortcuts

Some window managers provide a shortcut facility that lets users place icons on the root window that access specific programs or facilities.

Tabbed Windows

Some window managers provide a tabbed windows facility that groups applications together in common frames.

Task Switching

The window manager may provide various task switching facilities that let the user change the currently focused application, including:

• Changing the mouse focus using a pointing device
• Keyboard task switching facilities (for example, by pressing Alt-Tab)
• Clicking on the task in a taskbar or taskpanel

Taskbar

Some window managers provide a taskbar that shows running applications. The taskbar may show all applications that are running including those that have been minimized, and may provide the facility to switch focus between them. The taskbar may be incorporated into a menubar on some window managers.

Task Panel

A task panel is similar to a taskbar, but appears as a floating panel, rather than a horizontal or vertical bar.

Start Button

A start button is a desktop widget that provides a menu of programs that can be launched. The start button is typically placed on a menubar at the bottom of the screen.

Notification Area

A Notification Area is used to display icons for system and program features that have no desktop window. It contains mainly icons to indicate status information and notifications such as arrival of a new mail message. Some systems may also show a clock in the Notification Area.

Title Bars

A titlebar is a window decoration component some window managers provide at the top of each window. The titlebar is typically used to display the name of the application, or the name of the open document, and may provide title bar buttons for minimizing, maximizing, closing or rolling up of application windows.

Title Bar Buttons

Title bar buttons are included in the titlebar of some window managers, and provide the facility to minimize, maximize, rollup or close application windows. Some window managers may display the titlebar buttons in the taskbar or task panel, rather than in a titlebar.

Virtual Desktop

A virtual desktop (also called a scrolling desktop) is a facility some window managers provided that lets the desktop be larger than the actual screen

History[link]

In the 1970s, the Xerox Alto became the first computer shipped with a working WIMP GUI. It used a stacking window manager that allowed overlapping windows.^[4] While it is unclear if Microsoft Windows contains designs copied from Apple's Mac OS, it is clear that neither was the first to produce a GUI using stacking windows. In the early 1980s, the Xerox Star, successor to the Alto, used tiling for most main application windows, and used overlapping only for dialogue boxes, removing most of the need for stacking.^[5]

GEM 1.1 was a window manager that supported the desktop metaphor, and used stacking, allowing all windows to overlap. It was released in the early 1980s.^[6] GEM is famous for having been included as the main GUI used on the Atari ST, which ran Atari TOS, and was also a popular GUI for MS-DOS prior to the widespread use of Microsoft Windows. As a result of a lawsuit by Apple, GEM was forced to remove the stacking capabilities, making it a tiling window manager.^[7]

Mac OS was one of the earliest commercially successful examples of a GUI that used a sort of stacking window management via QuickDraw. Currently Mac OS X uses a somewhat more advanced window manager that has supported compositing since Mac OS X 10.0, and was updated in Mac OS X 10.2 to support hardware accelerated compositing via the Quartz Compositor.^[8]

During the mid 1980s, Amiga OS contained an early example of a compositing window manager called Intuition (one of the low-level libraries of AmigaOS, which was present in Amiga system ROMs), capable of recognizing which windows or portions of them were covered, and which windows were in the foreground and fully visible, so it could draw only parts of the screen that required refresh. Additionally, Intuition supported compositing. Applications could first request a region of memory outside the current display region for use as bitmap. The Amiga windowing system would then use a series of bit blits using the system's hardware blitter to build a composite of these applications' bitmaps, along with buttons and sliders, in display memory, without requiring these applications to redraw any of their bitmaps.

Intuition also anticipated the choices of the user by recognizing the position of the pointer floating over other elements of the screen (title bars of windows, their close and resizing gadgets, whole icons), and thus it was capable of granting nearly a zero-wait state experience to the use of the Workbench window manager.

Noteworthy to mention is the fact that Workbench was the only window manager that eventually inspired an entire family of descendant and successors: Ambient in MorphOS, Zune/Wanderer in AROS, Workbench NG (New Generation in AmigaOS 4.0 and 4.1. Workbench 4.1 was enhanced by 2D vector interface powered by Cairo libraries, and presenting a modern Porter-Duff 3D based Compositing Engine.

In 1988, Presentation Manager became the default shell in OS/2, which, in its first version, only used a command line interface (CLI). IBM and Microsoft designed OS/2 as a successor to DOS and Windows for DOS. After the success of the Windows 3.10, however, Microsoft abandoned the project in favor of Windows. After that, the Microsoft project for a future OS/2 version 3 became Windows NT, and IBM made a complete redesign of the shell of OS/2, substituting the Presentation Manager of OS/2 1.x for the object-oriented Workplace Shell that made its debut in the OS/2 2.0.^[9]

References[link]