OpenGL rendering over NTSC video stream !!!

Hi folks !

We have this need to produce in real-time, video images on which OpenGL graphic elements are painted.
We thought of rendering these images in the framebuffer over the “video pixels” brought by the video capture card.
In other words, can we render images in a framebuffer without erasing the entire image that was already there ?

An example would be a software that recognize a red hat over a green grass background in a video stream. The software then paint, using OpenGL, a red rectangle around the red Hat. This new video stream is then send to the a Display device. (TV, PC screen…etc)

Thanks in advance !