[Libav-user] How to calculate pts/dts ???

wolverin wolverin82 at mail.ru
Thu Jan 5 16:43:27 EET 2023

>On Mon, Jan 2, 2023 at 4:51 PM wolverin via Libav-user < libav-user at ffmpeg.org > wrote:
>>>Did I understand you correctly that I need to multiply time_base and system time between adjacent frames and add the resulting time to the previous PTS?
>>>PTS is just timebase * seconds represented as int64 number
>>>There are no additions with next/prev pts values.
>>Then pts is the time of one frame?
>Yes, when used with tied time_base AVRational.
Thank you very much, you helped me a lot, it significantly improved the smoothness of playback, but I still see 1 or 2 incomprehensible pauses at the beginning when watching live video in VLC.
Currently, only the output AVStream->time_base is used to calculate pts/dts.
But then what is the outgoing AVCodecContext->time_base needed for and what values? Do I need to initialize AVStream->avg_frame_rate or AVStream->r_frame_rate? Their change does not affect almost the video, but for some reason it increases traffic.
>>avcodec_send_packet(pCdcCtxInp, pPktInp);
>>avcodec_receive_frame(pCdcCtxInp, pFrm);
>>pFrm->pts = av_rescale_q(pPktInp->pts, pFmtCtxInp->streams[0]->time_base, pCdcCtxInp->time_base);
>>avcodec_send_frame(pCdcCtxOut, pFrm);
>>avcodec_receive_packet(pCdcCtxOut, pPktOut);
>>pPktOut->pts = av_rescale_q(pFrm->pts, pCdcCtxInp->time_base, pCdcCtxOut->time_base);
>>pPktOut->dts = pPktOut->pts;
>Inspect values of input/output pts and used timebases(with printfs or other ways...) and you can figure it where PTS becomes nonsense.
As I understand it, the input stream from the usb camera has a constant FR, transcoded I get a variable FR + 
I can start reading earlier  frames from the camera additionally for writing to disk, so I can't scale timestamps directly.

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