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All these new subsystems get you the ability to render windows independently and perform a compositing step on them prior to rendering them onto the desktop. Some cool uses of this are visible in Windows Vista and some of the updated applications that ship with it. The two I'm going to show you how to use are the Aero™ glass effect and thumbnails. The glass effect is available only when running the Aero Glass scheme and compositing is turned on. It isn't available for Aero Basic.

Since each window is created in its own section of video memory, the DWM gets to do the final compositing of that window onto the desktop. This means the DWM has access to the image that's on the desktop and can blend it with your window's rendering, creating a rendering that's a composition of both. This is most noticeable in any areas of a window that are blended with the existing desktop image to create a frosted glass effect. Since each window is rendered to an intermediate off-screen surface, this means the DWM is the only program involved in updating the glass effect. When you move a window with a glass effect, no underlying window needs to be invalidated. The DWM handles updating the visible translucent image to the new coordinates. It's possible to tell the DWM to add some of the client area of a window to be rendered with glass-thus letting you create glass areas for your own use.

This off-screen compositing makes the desktop more responsive. Since each window is now rendered separately from the desktop, the problem you often see in slow-to-update applications like Web browsers is eliminated. In previous version of Windows, it wasn't uncommon to see a display like Figure 2. You could move a window around on top of another application and get this tearing effect because the window underneath took too long to update itself. With desktop composition, you'll no longer see this artifact.