[FFmpeg-user] ffmpeg screencapture from a virtual xserver larger than the physical screen

[Zenny]

Hi,

I have a situation. I do not have a 4K screen, but I need to
screencapture at 4K using ffmpeg to test something.

I tried with:

$ ffmpeg -report -y -f x11grab -r 25 -s 3840x2160 -i :0 -pix_fmt
yuv420p -vcodec libx264 -ar 48000 -qscale 1 -r 24 4k_screecapture.mp4

but got an very obvious error as my screen is only full HD:

[x11grab @ 0x20a29c0] Capture area 3840x2160 at position 0.0 outside
the screen size 1920x1080
:0: Invalid argument

Appreciate any inputs on how to:

1. create a virtual xserver of 4k resolution without any physical
display, and pipe to the physical screen (downsizing to FullHD ).

2. I would like to the 4K steam to a virtual device (v4l2 loopback).

Thanks!