Video capture device drivers define the four logical video channels shown in the following table. Each channel represents a portion of the data path between video hardware and system memory.
|
Channel |
Constant |
Definition |
|
External In |
VIDEO_EXTERNALIN |
Data path from an external video source, such as a TV, VCR, or camera, into a frame buffer. |
|
Video In |
VIDEO_IN |
Data path from the frame buffer to system memory. |
|
Video Out |
VIDEO_OUT |
Data path from system memory to the frame buffer. |
|
External Out |
VIDEO_EXTERNALOUT |
Data path from the frame buffer to an output display, such as an overlay window on a user’s screen. |
User-mode video capture drivers implement the concept of logical video channels, and clients can open the channels they need to create a complete input and/or output data path. Logical channels do not exist within kernel-mode drivers.
The sample video capture drivers provided with the Windows NT DDK support the VIDEO_EXTERNALIN, VIDEO_IN, and VIDEO_EXTERNALOUT channels. Support for these channels means that users employing the AVIcap window class or the Video For Windows API can capture video images from an external source, store the images in memory or a file, and view the captured images as they are received. The sample user-mode video capture drivers do not support the VIDEO_OUT channel, so users cannot use them to play back recorded images. Instead, user applications call the Video Compression Manager’s API to send drawing requests to Video Compression Manager drivers.
For more information about video channels, see Opening Video Channels and Configuring Video Channels.