The technique aims to improve the display of a fast-moving video image — such as a computer game, a DVD, or the signal from a TV card.
In addition, the graphics processing unit (GPU) provides an efficient way to scale the video in size and often performs color-format conversions (such as MPEG-2's YCbCr into RGB).
To get the image from the separate video memory to display in tandem with the remaining shared elements on the display, the graphical subsystem associates a certain attribute (for example, a particular color) as a "mask" for that overlay, which the graphics card understands to mean that it is to draw from the separate overlay buffer onto the screen.
If another window moves over top of the purple area and obscures a part of it, then the graphics hardware performs the clipping by itself.
Many newer[update] graphics cards can support more than one monitor and/or a TV screen as output devices.
However, some graphics cards have the option to completely redirect hardware overlay to the TV screen.
To improve performance, each program draws to its own independent memory buffer instead of to a slow graphical subsystem.
With modern GPUs capable of advanced 3D graphics (as a consequence of the video game industry), operating systems can apply computationally intensive motion, scaling, and lighting effects to normal 2D windows.
Some dedicated hardware overlay devices use embedded Linux as an operating system, for example the video logger [1] by Racelogic uses a Texas Instruments DM355 micro-controller to blend a frame buffer containing graphics onto a live video feed, and then store the result in MPEG-4 format on a flash card.