How to capture raw video from video card in kernel mode?

Question

linux

Preface. I'm not a systems programmer, I'm just trying to become one.
There is a similar question , which remained unanswered for me, because I did not find an answer to the question of how to implement video capture from the core. Nevertheless, the author managed to capture a raw video stream, but he did not reveal how (which is exactly what interests me!).
I can't even capture from userspace. When I try to read the framebuffer (in my case /dev/fb0) I get a black image.
Exits only via gstreamer -> ximagesrc
How to open an existing framebuffer from the kernel?
Is it possible to capture video not from the framebuffer?
What can be done from the kernel to capture an image from the screen?

Answer 1

To read something from the framebuffer, you must first write something there.
I suspect that you have xes running. And they don't use a framebuffer. So you need to deal with api x.