ffmpeg -y -vaapi_device /dev/dri/renderD128 -f x11grab -video_size 1920x1080 -i :44 -vf 'hwupload,scale_vaapi=format=nv12' -c:v h264_vaapi -qp 24 output.mp4
ffmpeg version N-89674-g57d0c24132 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 7.2.1 (GCC) 20171128
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-avresample --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libass --enable-libbluray --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxvid --enable-shared --enable-version3
  libavutil      56.  7.100 / 56.  7.100
  libavcodec     58.  9.100 / 58.  9.100
  libavformat    58.  3.100 / 58.  3.100
  libavdevice    58.  0.100 / 58.  0.100
  libavfilter     7.  8.100 /  7.  8.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  0.101 /  5.  0.101
  libswresample   3.  0.101 /  3.  0.101
  libpostproc    55.  0.100 / 55.  0.100
[x11grab @ 0x556af556b340] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':44':
  Duration: N/A, start: 1514998190.199428, bitrate: N/A
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1920x1080, 29.97 fps, 1000k tbr, 1000k tbn, 1000k tbc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (h264_vaapi))
Press [q] to stop, [?] for help
[h264_vaapi @ 0x556af5577240] B frames are not supported (0x1).
Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height
Conversion failed!