Accessing the Video buffer of a capture card

This is somesort of Video Processing need !
How can we fill a color buffer or accumulation buffer with the images of a video stream provided by a capture card and connected to a camera video. In fact, how can we access the video buffer of the accelerator card knowing that the card has such a device.

We would need to render graphical elements in that newly filled color buffer to produce video images in which OpenGL rendered elements can be seen.

Thanks in advance !