Video overlay
From Wikipedia, the free encyclopedia
Video overlay is any technique used to display a video window on a computer display while bypassing the chain of CPU - graphics card - computer monitor. This is done in order to speed up the video display, and it is commonly used, for example, by TV tuner cards and early 3D graphics accelerator cards.
Various methods to achieve video overlay are in use:
- A video overlay device can be connected between the graphics card analog VGA output and the monitor's input forming a "VGA passthrough". The device modifies the VGA signal and interpolates the analog video signal at exactly the right time, so that it will appear on the right spot on screen; the rest of the screen is filled by the signal coming from the graphics card. The driver software informs the video overlay device about the desired position of the video window on screen.
- Some video overlay devices write the digital video signal directly into the graphics card's video memory or provide it to the graphics card's RAMDAC.
- Hardware overlay is a technique implemented by most modern graphics card that allows an application to write to a dedicated part of video memory, rather than to the part shared by all applications. In this way, clipping, moving and scaling of the image can be performed by the graphics hardware rather than by the CPU in software.
