[FFmpeg-user] Question about delay when displaying selected keyframes

[Paul B Mahol]

On Tue, Jul 2, 2024 at 5:21 PM Sachin Bhat <sachinbhat694 at gmail.com> wrote:

> Hi,
>
> I have an input ts file that is at 30 fps, and a GOP of 10. I am trying to
> display keyframes at a specific time interval by sending a keyframe plus
> duplicates to a HW decoder until the next desired keyframe appears. .I am
> using duplication because the HW decoder does not handle very low
> framerates well, and I have been led to believe that the duplicate frames
> are just copies of the keyframe, so they do not contribute greatly to
> bandwidth usage after transcoding.  As a result, I get the illusion of a
> low framerate video while actually producing a 30fps stream.
>
> The command I am using to send a keyframe every 10 seconds (every 30th
> keyframe):
> ffmpeg -re -i test-src-30fps.ts -vf "select='eq(pict_type, I)',
> select='not(mod(n,30))'" -vsync cfr -c:v libx264 -f mpegts
> udp://{decoder_ip}:{decoder_port}
>
>
You can combine 2 select filters into one.


> However, when I run this command, the nth keyframe is only displayed once
> the n+1 keyframe is reached. For example, the output log says: frame = 1
> fps=0.1 q=0.0 size=0kB time=00:00:00.00 bitrate=N/A for a certain amount of
> time, then frame=301 fps=27 q=29.0 size=58kb time=00:00:07.33 bitrate =
> 65.2 kb/s dup = 301 speed = 0.85x. As a result every keyframe ends up being
> displayed at a large delay to when it appears in the actual input stream.
> The delay increases if I increase the interval for my desired keyframes. On
> the decoder side I can see that the output is 30fps.
>
> Can someone explain why this is happening, and if there is a way to have a
> fixed delay regardless of how often I want to select a keyframe?
>
> Thanks,
> Sachin
> _______________________________________________
> ffmpeg-user mailing list
> ffmpeg-user at ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-user
>
> To unsubscribe, visit link above, or email
> ffmpeg-user-request at ffmpeg.org with subject "unsubscribe".
>