You need to set some video related config variables, e.g.,
video_source v4l2,/dev/video
video_display x11,nil
module avcodec.so
module vp8.so
module vp9.so
...
module selfview.so
module v4l2.so
module x11.so
video_selfview window
and then in your account, e.g.,
;video_codecs=vp9,vp8,h264
-- Juha