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

wolverin wolverin82 at mail.ru
Mon Jan 2 13:16:46 EET 2023

>On Thu, Dec 29, 2022 at 9:45 AM wolverin via Libav-user < libav-user at ffmpeg.org > wrote:
>>I'm transcoding live video from MJPEG to H264 using ffmpeg library in my C/C++ project
>>Help me figure out how to calculate the pts/dts correctly, using various av_rescale* functions did not give the correct values and I tried my simple version, but the number of frames per second is different all the time and of course these values are approximate
>>int64_t * pcnt
>>pPktOut->pos = (*pcnt);
>>pPktOut->pts = pPktOut->pos * pFmtCtxOut->streams[0]->time_base.den  / pCdcCtxOut->time_base.den;
>This does not make any sense.
>Time base is rational and thus both numerator and denumerator should be used.
Thanks for your reply.
Yes, you are right, but I have time_base.num = 1 (for out stream 1/90000, for codeccontext 1/15 — 15 fps is the desired frequency, but in reality it varies between 16-20)
Here is an example of my time base dumps
Input #0, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 532397.028007, bitrate: N/A
  Stream #0:0: Video: mjpeg (Baseline), yuvj422p(pc, bt470bg/unknown/unknown), 1280x720, 30 fps, 30 tbr, 1000k tbn
[libx264 @ 0x14ef370] using cpu capabilities: ARMv6 NEON
[libx264 @ 0x14ef370] profile High 4:2:2, level 3.1, 4:2:2 8-bit
[libx264 @ 0x14ef370] 264 - core 148 r2643 5c65704 - H.264/MPEG-4 AVC codec - Copyleft 2003-2015 -  http://www.videolan.org/html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chrom1 trellis=0 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=0 threads=4 lookahead_threads=4 sliced_threads=1 sl4 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=15 scenecut=0 _refresh=0 rc_lookahead=0 rc=cbr mbtree=0 bitrate=500 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 vbv_maxrate=500 vbv_ze=1000 nal_hrd=none filler=0 ip_ratio=1.40 aq=0
Output #0, rtsp, to 'rtsp://':
  Stream #0:0: Video: h264, yuv422p(tv, bt470bg/unknown/unknown), 1280x720, q=2-31, 500 kb/s, 15 tbr, 90k tbn
I compared such an approximate calculation of pts/dts in ffprobe for my code and ffmpeg and they match until the real fps becomes different from the calculated one, then I see either delays or frame losses in VLC.
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