$  ffmpeg -loglevel debug -s 1366x768 -f x11grab   -i :0.0 -c libx264 -crf 28 -preset ultrafast /tmp/output.mp4
ffmpeg version 3.1.4-1~bpo8+1 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.9.2 (Debian 4.9.2-10)
  configuration: --prefix=/usr --extra-version='1~bpo8+1' --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --enable-shared --disable-libtesseract --disable-stripping --disable-decoder=libschroedinger --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --disable-libebur128 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librubberband --enable-libschroedinger --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-openal --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-frei0r --enable-chromaprint --enable-libopencv --enable-libx264
  libavutil      55. 28.100 / 55. 28.100
  libavcodec     57. 48.101 / 57. 48.101
  libavformat    57. 41.100 / 57. 41.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 47.100 /  6. 47.100
  libavresample   3.  0.  0 /  3.  0.  0
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
  libpostproc    54.  0.100 / 54.  0.100
Splitting the commandline.
Reading option '-loglevel' ... matched as option 'loglevel' (set logging level) with argument 'debug'.
Reading option '-s' ... matched as option 's' (set frame size (WxH or abbreviation)) with argument '1366x768'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'x11grab'.
Reading option '-i' ... matched as input file with argument ':0.0'.
Reading option '-c' ... matched as option 'c' (codec name) with argument 'libx264'.
Reading option '-crf' ... matched as AVOption 'crf' with argument '28'.
Reading option '-preset' ... matched as AVOption 'preset' with argument 'ultrafast'.
Reading option '/tmp/output.mp4' ... matched as output file.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option loglevel (set logging level) with argument debug.
Successfully parsed a group of options.
Parsing a group of options: input file :0.0.
Applying option s (set frame size (WxH or abbreviation)) with argument 1366x768.
Applying option f (force format) with argument x11grab.
Successfully parsed a group of options.
Opening an input file: :0.0.
[x11grab @ 0x17cba00] Probe buffer size limit of 5000000 bytes reached
[x11grab @ 0x17cba00] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0.0':
  Duration: N/A, bitrate: N/A
    Stream #0:0, 2, 1/1000000: Video: rawvideo, 1 reference frame (BGR[0] / 0x524742), bgr0, 1366x768, 0/1, 29.97 fps, 1000k tbr, 1000k tbn, 1000k tbc
Successfully opened the file.
Parsing a group of options: output file /tmp/output.mp4.
Applying option c (codec name) with argument libx264.
Successfully parsed a group of options.
Opening an output file: /tmp/output.mp4.
[file @ 0x17d8580] Setting default whitelist 'file,crypto'
Successfully opened the file.
detected 4 logical cores
[graph 0 input from stream 0:0 @ 0x17cb820] Setting 'video_size' to value '1366x768'
[graph 0 input from stream 0:0 @ 0x17cb820] Setting 'pix_fmt' to value '298'
[graph 0 input from stream 0:0 @ 0x17cb820] Setting 'time_base' to value '1/1000000'
[graph 0 input from stream 0:0 @ 0x17cb820] Setting 'pixel_aspect' to value '0/1'
[graph 0 input from stream 0:0 @ 0x17cb820] Setting 'sws_param' to value 'flags=2'
[graph 0 input from stream 0:0 @ 0x17cb820] Setting 'frame_rate' to value '30000/1001'
[graph 0 input from stream 0:0 @ 0x17cb820] w:1366 h:768 pixfmt:bgr0 tb:1/1000000 fr:30000/1001 sar:0/1 sws_param:flags=2
[format @ 0x17d6500] compat: called with args=[yuv420p|yuvj420p|yuv422p|yuvj422p|yuv444p|yuvj444p|nv12|nv16]
[format @ 0x17d6500] Setting 'pix_fmts' to value 'yuv420p|yuvj420p|yuv422p|yuvj422p|yuv444p|yuvj444p|nv12|nv16'
[auto-inserted scaler 0 @ 0x17e0f80] Setting 'flags' to value 'bicubic'
[auto-inserted scaler 0 @ 0x17e0f80] w:iw h:ih flags:'bicubic' interl:0
[format @ 0x17d6500] auto-inserting filter 'auto-inserted scaler 0' between the filter 'Parsed_null_0' and the filter 'format'
[AVFilterGraph @ 0x17d79c0] query_formats: 4 queried, 2 merged, 1 already done, 0 delayed
[auto-inserted scaler 0 @ 0x17e0f80] picking yuv444p out of 7 ref:bgr0 alpha:0
[auto-inserted scaler 0 @ 0x17e0f80] w:1366 h:768 fmt:bgr0 sar:0/1 -> w:1366 h:768 fmt:yuv444p sar:0/1 flags:0x4
No pixel format specified, yuv444p for H.264 encoding chosen.
Use -pix_fmt yuv420p for compatibility with outdated media players.
[libx264 @ 0x17d52e0] using mv_range_thread = 56
[libx264 @ 0x17d52e0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX AVX2 FMA3 LZCNT BMI2
[libx264 @ 0x17d52e0] profile High 4:4:4 Predictive, level 3.2, 4:4:4 8-bit
[libx264 @ 0x17d52e0] 264 - core 142 r2431 a5831aa - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=6 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=25 scenecut=0 intra_refresh=0 rc=crf mbtree=0 crf=28.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=0
[mp4 @ 0x17d3c20] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
Output #0, mp4, to '/tmp/output.mp4':
  Metadata:
    encoder         : Lavf57.41.100
    Stream #0:0, 0, 1/30000: Video: h264 (libx264), 1 reference frame ([33][0][0][0] / 0x0021), yuv444p, 1366x768, 0/1, q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc
    Metadata:
      encoder         : Lavc57.48.101 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))
Press [q] to stop, [?] for help
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
[swscaler @ 0x17e1140] Warning: data is not aligned! This can lead to a speedloss
Clipping frame in rate conversion by 0.000008
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
Clipping frame in rate conversion by 0.000023
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)op=9 speed=   0x
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
cur_dts is invalid (this is harmless if it occurs once at the start per stream)
[rawvideo @ 0x17d37a0] PACKET SIZE: 4196352, STRIDE: 5464
*** dropping frame 3 from stream 0 at ts 0
[libx264 @ 0x17d52e0] frame=   0 QP=25.00 NAL=3 Slice:I Poc:0   I:4128 P:0    SKIP:0    size=178793 bytes
[libx264 @ 0x17d52e0] frame=   1 QP=30.00 NAL=2 Slice:P Poc:2   I:0    P:0    SKIP:4128 size=11 bytes
[libx264 @ 0x17d52e0] frame=   2 QP=31.00 NAL=2 Slice:P Poc:4   I:2569 P:468  SKIP:1091 size=88387 bytes
frame=    3 fps=2.7 q=-1.0 Lsize=     262kB time=00:00:00.06 bitrate=32186.0kbits/s dup=0 drop=20 speed=0.061x
video:262kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.309944%
Input file #0 (:0.0):
  Input stream #0:0 (video): 23 packets read (96516096 bytes); 23 frames decoded;
  Total: 23 packets (96516096 bytes) demuxed
Output file #0 (/tmp/output.mp4):
  Output stream #0:0 (video): 3 frames encoded; 3 packets muxed (267790 bytes);
  Total: 3 packets (267790 bytes) muxed
23 frames successfully decoded, 0 decoding errors
[AVIOContext @ 0x17d83e0] Statistics: 34 seeks, 36 writeouts
[libx264 @ 0x17d52e0] frame I:1     Avg QP:25.00  size:178793
[libx264 @ 0x17d52e0] frame P:2     Avg QP:30.50  size: 44199
[libx264 @ 0x17d52e0] mb I  I16..4: 100.0%  0.0%  0.0%
[libx264 @ 0x17d52e0] mb P  I16..4: 31.1%  0.0%  0.0%  P16..4:  5.7%  0.0%  0.0%  0.0%  0.0%    skip:63.2%
[libx264 @ 0x17d52e0] coded y,u,v intra: 26.6% 25.0% 24.7% inter: 5.1% 4.5% 4.8%
[libx264 @ 0x17d52e0] i16 v,h,dc,p: 63% 32%  3%  1%
[libx264 @ 0x17d52e0] kb/s:21353.93
Exiting normally, received signal 2.