Snapshot of a SDL_Overlay

Hey everyone.

I’m modifing Ffplay ( a simple player which is include with ffmpeg), in order to
make it a preview player for video editing. I did some marker in and out
function, in order to play only a part from a video.
But I would like to capture the markers and save them to image.
I first think to use SDL_SaveBMP() in oder to copy the screen surface. But when
I am doing it, I’am geting a black picture (with the good size) but not the
image I would like.
I guess it is because ffplay use SDL_Overlay system. But I have no idea how I
can get a snasphot of a video in this case.

Is someone know how I can make a picture file from data which came from an
SDL_Overlay, it would be pleasant, to have some advice.