[FFmpeg-user] Capturing video using ffmpeg in source format

[Moritz Barsnick]

On Wed, Dec 06, 2017 at 15:58:28 +0000, Dhananjay Umap wrote:

> ffmpeg -f video4linux2 -i /dev/video0 -start_number 1 -vframes 250 -c:v mpeg4 -qscale:v 1 test1.avi
> As I can see below the rawvideo is at 1920x1080 format, but the input to the card is 4K as well as the card is 4K capable.

> If I use the following command, I capture 4K video with 25 fps.
> ./ffmpeg -f video4linux2 -i /dev/video0 -s 3840x2160 -start_number 1 -vframes 250 -c:v mpeg4 -qscale:v 1 test.avi
> In this case scaling is performed by the ffmpeg to 4K which affects
> the KPI of the captured video. This also affect the speed of the
> capture since ffmpeg is processing the video to 4K during capture
> which eventually affects the KPI for the captured video.

You need to tell the v4l2 device to choose the correct input
resolution. The input option "-video_size <resolution>" should work.
(You may also want to choose the correct "-pixel_format".)

Yet even then, ffmpeg will convert 3840x2160 (raw) to 3840x2160
(mpeg4), and take about as much CPU as when it does soft upscaling
1920x1080 -> 3840x2160). Only that the quality will be better. You need
to find encoding settings which are faster (at the cost of quality or
size/compression ratio). With x264, I would try various presets, such
as "-preset fast". I'm not sure what to do for the mpeg4 codec, Google
or others on this list should help you. The default of 200 kb/s is
probably the issue, I'm guessing that the codec many be trying really
hard to compress the hell out of your HD images (and won't succeed).

Cheers,
Gruß nach Nürnberg,
Moritz