You can't do it from an X11 session, which uses DRM/KMS instead of fbdev, hence you will get the compatibility fbdev buffer, which will be all 0s. You need to b

You can't do it from an X11 session, which uses DRM/KMS instead of fbdev, hence you will get the compatibility fbdev buffer, which will be all 0s. You need to be VT switched away (hit Ctrl+Alt+F3 or similar).