~$ ffmpeg -f x11grab -r 30 -s 1920x1200 -i :0.0 -vcodec nvenc -b:v 4500k ~/test.mkv
ffmpeg version N-76470-g3692d85 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04)
  configuration: --prefix=/home/user/ffmpeg_build --extra-cflags=-I/home/user/ffmpeg_build/include --extra-ldflags=-L/home/user/ffmpeg_build/lib --bindir=/home/user/bin --enable-gpl --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-nonfree --enable-nvenc
  libavutil      55.  5.100 / 55.  5.100
  libavcodec     57. 14.100 / 57. 14.100
  libavformat    57. 14.100 / 57. 14.100
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 14.101 /  6. 14.101
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, x11grab, from ':0.0':
  Duration: N/A, start: 1446798427.504237, bitrate: N/A
    Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 1920x1200, 30 fps, 30 tbr, 1000k tbn, 30 tbc
File '/home/user/test.mkv' already exists. Overwrite ? [y/N] y
[nvenc @ 0x2242200] No NVENC capable devices found
Output #0, matroska, to '/home/user/test.mkv':
    Stream #0:0: Video: h264, none, q=2-31, 128 kb/s, 30 fps
    Metadata:
      encoder         : Lavc57.14.100 nvenc
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (nvenc))
Error while opening encoder for output stream #0:0 - maybe incorrect parameters such as bit_rate, rate, width or height


~$ glxinfo | grep command
    GL_NV_blend_equation_advanced, GL_NV_blend_square, GL_NV_command_list,
    GL_NV_blend_equation_advanced, GL_NV_blend_square, GL_NV_command_list,

~$ dpkg -l | grep ^ii | grep -i nvidia
ii  bbswitch-dkms                                               0.7-2ubuntu1                                        amd64        Interface for toggling the power on nVidia Optimus video cards
ii  boinc-nvidia-cuda                                           7.2.42+dfsg-1                                       amd64        metapackage for CUDA-savvy BOINC client and manager
ii  libcuda1-346                                                346.96-0ubuntu0.0.1                                 amd64        NVIDIA CUDA runtime library
ii  nvidia-346                                                  346.96-0ubuntu0.0.1                                 amd64        NVIDIA binary driver - version 346.96
ii  nvidia-libopencl1-346                                       346.96-0ubuntu0.0.1                                 amd64        NVIDIA OpenCL Driver and ICD Loader library
ii  nvidia-modprobe                                             340.24-1~ubuntu14.04.1                              amd64        utility to load NVIDIA kernel modules and create device nodes
ii  nvidia-opencl-icd-346                                       346.96-0ubuntu0.0.1                                 amd64        NVIDIA OpenCL ICD
ii  nvidia-prime                                                0.6.2                                               amd64        Tools to enable NVIDIA's Prime
ii  nvidia-settings                                             331.20-0ubuntu8                                     amd64        Tool for configuring the NVIDIA graphics driver