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

[Marton Balint]

On Sat, 26 Jan 2019, Meredydd Luff wrote:

> 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
>

My guess:

Because of tpad you are not reading fast enough frames from the video 
input, therefore frames are dropped there. Solution is to increase ffmpeg 
input thread queue size with the -thread_queue_size option.

Regards,
Marton