[Libav-user] bitrate and file size issue with nvidia nvenv encoder

[Sreenath BH]

Hi all,

I am trying to use nvidia's nvenc codec to transcode video files.
ffmpeg has been built
with both libx264 and nvenc codecs enabled.

ffmpeg when run using nvenc codec creates a file that is almost double
the size of input
mp4 file and bitrate almost double the size of input file.

This is output of ffprobe on the input file:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'large.mp4':
 Metadata:
 Duration: 01:49:12.30, start: 0.000000, bitrate: 391 kb/s
  Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
768x432 [SAR 1:1 DAR 16:9], 292 kb/s, 30 fps, 30 tbr, 30 tbn, 60 tbc
(default)
  Metadata:
    creation_time   : 2013-11-15 18:04:27
    handler_name    : Apple Video Media Handler

-----     Command used with libx264:
ffmpeg -i large.mp4 -loglevel quiet -c:a aac -c:v libx264 -pix_fmt
yuv420p -movflags +faststart out.mp4

output file has size of : 313818971  bytes
ffprobe output on the output file:

 Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out.mp4':
 Metadata:
   encoder         : Lavf57.25.100
   Duration: 01:49:12.30, start: 0.023220, bitrate: 383 kb/s
  Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuv420p,
768x432 [SAR 1:1 DAR 16:9], 245 kb/s, 30 fps, 30 tbr, 15360 tbn, 60
tbc (default)
 Metadata:
  handler_name    : VideoHandler

--------  Command using nvenv codec:
ffmpeg -i large.mp4 -loglevel quiet -c:a aac -c:v nvenc -pix_fmt
yuv420p -movflags +faststart out.mp4

Size of output file:
646598462  bytes (640 MB)

ffprobe output on the output files are as follows:

  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'out.mp4':
  Metadata:
   encoder         : Lavf57.25.100
   Duration: 01:49:12.30, start: 0.023220, bitrate: 789 kb/s
   Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661),
yuv420p(tv), 768x432 [SAR 1:1 DAR 16:9], 654 kb/s, 30 fps, 30 tbr,
15360 tbn, 60 tbc (default)
  Metadata:
   handler_name    : VideoHandler

So the differences between the two appear to be:

(1) pixel format :  yuv420p(tv) with nvidia whereas it is yuv420p with libx264
(2) h264 (Main)  with nvidia whereas it is Video: h264 (High)
(3) bitrate: 789 kb/s with nvidia whereas it is  383 kb/s with lib264.

What is causing nvidia codec to double the bitrate? As can be seen, I
am not specifying any bitrate to use. So I expect ffmpeg to retain the
same bitrate.

I would like to retain the same bitrate and it would be good is the
file size if not so large.

Any help appreciated,

thanks,
Sreenath