adam@blackbox:~/Downloads/ffmpeg$ ./ffmpeg -v verbose -f x11grab -video_size 2560x1440 -i :0.0+0,0 -c:v nvenc output.avi
ffmpeg version N-75677-g16aac9a Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.9.2 (Debian 4.9.2-10)
  configuration: --prefix=/ffmpeg/build --bindir=/ffmpeg/bin --enable-nvenc --enable-nonfree --extra-cflags=-I/usr/include/nvenc
  libavutil      55.  2.100 / 55.  2.100
  libavcodec     57.  3.100 / 57.  3.100
  libavformat    57.  2.102 / 57.  2.102
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6.  9.101 /  6.  9.101
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
[x11grab @ 0x24be460] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, x11grab, from ':0.0+0,0':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: rawvideo, 1 reference frame (BGR[0] / 0x524742), bgr0, 2560x1440, 29.97 fps, 29.97 tbr, 1000k tbn, 29.97 tbc
File 'output.avi' already exists. Overwrite ? [y/N] y
[graph 0 input from stream 0:0 @ 0x24ca700] w:2560 h:1440 pixfmt:bgr0 tb:1/1000000 fr:30000/1001 sar:0/1 sws_param:flags=2
[auto-inserted scaler 0 @ 0x24d4600] w:iw h:ih flags:'bicubic' interl:0
[format @ 0x24d3620] auto-inserting filter 'auto-inserted scaler 0' between the filter 'Parsed_null_0' and the filter 'format'
[auto-inserted scaler 0 @ 0x24d4600] w:2560 h:1440 fmt:bgr0 sar:0/1 -> w:2560 h:1440 fmt:yuv444p sar:0/1 flags:0x4
[nvenc @ 0x24c7a20] 2 CUDA capable devices found
[nvenc @ 0x24c7a20] [ GPU #0 - < GeForce GTX 660 > has Compute SM 3.0, NVENC Not Available ]
[nvenc @ 0x24c7a20] [ GPU #1 - < GeForce GTX 660 > has Compute SM 3.0, NVENC Not Available ]
[nvenc @ 0x24c7a20] No NVENC capable devices found
Output #0, avi, to 'output.avi':
    Stream #0:0: Video: h264, 1 reference frame, none, q=2-31, 128 kb/s, 29.97 fps
    Metadata:
      encoder         : Lavc57.3.100 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