Comment by actionfromafar

What I wonder is, how do you get the video frames to be compressed from the video card into the encoder?

The only frame capture APIs I know, take the image from the GPU, to CPU RAM, then you can put it back into the GPU for encoding.

Are there APIs which can sidestep the "load to CPU RAM" part?

Or is it implied, that a game streaming codec has to be implemented with custom GPU drivers?