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. Because of the much greater processing power of modern graphics cards, and the awkwardness of adding additional analog hardware signal processing path, this method is now little used.
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 cards 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.
Overlay advertising is a technique used by online video producers to monetize video content through using an overlay layer to deliver and display an ad unit. This can be in the form of a video advertisement, a product placement or a contextual link, clickable graphic or text that provides information related to the content of the video and/or the target of the link being placed. Examples of this technology include Overlay.TVhttp://Overlay.TV (overlay ads, user-generated contextual links and product placement done post edit), , BubblePlyhttp://BubblePly.com (clickable speech or thought bubbles) and VideoEgghttp://VideoEgg.com (specialized overlay ads on video streams or video ads).
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