Overlay surfaces, casually referred to as overlays, are surfaces with special hardware-supported capabilities. Overlay surfaces are frequently used to display live video, recorded video, or still bitmaps over the primary surface without blitting to the primary surface or changing the primary surface's contents in any way. Overlay surface support is provided entirely by the hardware; DirectDraw supports any capabilities as reported by the display device driver. DirectDraw does not emulate overlay surfaces.
An overlay surface is analogous to a clear piece of plastic that you draw on and place in front of the monitor. When the overlay is in front of the monitor, you can see both the overlay and the contents of the primary surface together, but when you remove it, the primary surface's contents are unchanged. In fact, the mechanics of overlays work much like the clear plastic analogy. When you display an overlay surface, you're telling the device driver where and how you want it to be visible. While the display device paints scan lines to the monitor, it checks the location of each pixel in the primary surface to see if an overlay should be visible there instead. If so, the display device substitutes data from the overlay surface for the corresponding pixel, as shown in the following illustration.
By using this method, the display adapter produces a composite of the primary surface and the overlay on the monitor, providing transparency and stretching effects, without modifying the contents of either surface. The composited surfaces are injected into the video stream and sent directly to the monitor. Because this on-the-fly processing and pixel substitution is handled at the hardware level, no noticeable performance loss occurs when displaying overlays. Additionally, this method makes it possible to seamlessly composite primary and overlay surfaces with different pixel formats.
You create overlay surfaces by calling the IDirectDraw4::CreateSurface method, specifying the DDSCAPS_OVERLAY flag in the associated DDSCAPS2 structure. Overlay surfaces can only be created in video memory, so you must also include the DDSCAPS_VIDEOMEMORY flag. As with other types of surfaces, by including the appropriate flags you can create either a single overlay or a flipping chain made up of multiple overlay surfaces.