[FFmpeg-devel] [PATCH] avfilter: Add delay filter

[Meredydd Luff]

On Sat, 26 Jan 2019 at 10:19, Marton Balint <cus at passwd.hu> wrote:
> On Sat, 26 Jan 2019, Meredydd Luff wrote:
> Why do you need an internal queue for this? If I am getting this right,
> you inject blank frames and then pass the input as is.
>
> A possible real world use case would be useful for me to better understand
> what you want to do.

Motivating example: we are capturing live video and audio, where for
reasons beyond our control the audio has been delayed by, say, 10
seconds. We want to delay the video to match. If we just inject blank
frames and adjust tps of subsequent frames (which is what tpad does -
sorry for misstatement in previous message), then this works for short
delays (eg sub-1-second). Beyond that, we get all sorts of sync
weirdness, with the video freezing for extended periods of time or
forever.

My guess is that the plumbing (I'd very speculatively raise my
eyebrows in the direction of the frame_wanted/back-pressure machinery)
isn't dealing well live capture when something's messing with the pts
but otherwise leaving everything in the same buffers. So I built a
filter with an internal one-in-one-out buffer that seems not to have
this problem.

In short, this does not work:
ffmpeg -i /dev/video0 -f pulse -ac 2 -i default -vf
tpad=start_duration=10 -t 20 output.mkv

This does:
ffmpeg -i /dev/video0 -f pulse -ac 2 -i default -vf delay=60 -t 20 output.mkv

Meredydd