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When the video stream is added to the output, the graphics card can simply scan the co-ordinates of the canvas.When it encounters green, it knows it has found a visible portion of the video window, and only draws those portions of the video.
While in (video) memory, individual windows can be transformed separately, and accelerated video may be added at this stage using a texture filter, before the window is composited and drawn.In the event that the window manager doesn't directly support compositing, it is more difficult to isolate where the video stream should be rendered, because by the time it can be accelerated the output has already been turned into a single image.The only way to do this is usually to employ a post processed hardware overlay, using chroma keying.After all of the windows have already been drawn, the only pieces of information we have available are the size and position of the video window's canvas.A third piece of information is required to indicate which parts of the video window's canvas are obscured by other windows and which are not.Under X, how video is finally drawn depends largely on the X window manager in use.
With properly installed drivers, and GPU hardware such as supported Intel, ATI, and n Vidia chip sets, some window managers, called compositing window managers, allow windows to be separately processed and then rendered (or composited).
Therefore, the video player draws its canvas using a solid color (we'll say green), and this color becomes a makeshift third dimension.
When all windows have been drawn, windows covering the video player will block out the green color.
In order for this to work, three things have to come together: Most modern video controllers provide the functions required for XVideo; this feature is known as hardware scaling and YUV acceleration or sometimes as 2D hardware acceleration.
The XFree86 X display server has implemented XVideo since version 4.0.2.
The X video extension, often abbreviated as XVideo or Xv, is a video output mechanism for the X Window System.