[FFmpeg-user] Getting precise start time from wall-clock for capture

Mon May 18 22:41:03 CEST 2015

On Mon, May 18, 2015 at 18:38:18 +0200, Nicolas George wrote:
> As you can see, this is the monotonic clock (and my uptime here is 16 days),
> but lavd can convert for you:
> 
> http://ffmpeg.org/ffmpeg-devices.html#video4linux2_002c-v4l2

I tried that, because I wanted to answer the question as well. It was
non-obvious to me that this, what you call "normalize", would happen:

> With that, you just need to do whatever you want with the timestamps. By
> default, ffmpeg will normalize the timestamps before any filtering or
> encoding, but you can disable that with the "-copyts" option.

I see the timestamp of the first frame "reset" to 0 - is that what you
mean with "normalize"?

>From the description of the v4l2 device, I had expected the wallclock
to be copied to the timestamps (PTS, right?), but they began with 0 for
the first frame. I then tried to add
"-vf setpts=RTCSTART+PTS-STARTPTS"
but that was apparently a long shot - it didn't help.

So I added "-copyts", but again to no avail. The first frame always
shows:
pkt_pts=0
pkt_pts_time=0.000000
pkt_dts=0
pkt_dts_time=0.000000
best_effort_timestamp=0
best_effort_timestamp_time=0.000000

I was hoping to be able to hint the original poster to grab the start
time from the PTS of the first frame.

So: How to get wallclock timestamps into the video?

For reference, my attempt. (The input obviously has the correct
wallclock time: 1431981242.712328.)

$ ffmpeg -f v4l2 -input_format yuv420p -timestamps abs -i /dev/video1 -t 1 -c:v libx264 -copyts ~/tmp/video1.mkv -y
ffmpeg version 2.6.git Copyright (c) 2000-2015 the FFmpeg developers
  built with icc (ICC) 14.0.3 20140422
  configuration: --prefix=/usr/new/tools/video/install/ffmpeg/2015-05-11 --cc=icc --cxx=icpc --enable-gpl --enable-version3 --enable-nonfree --disable-shared --enable-gnutls --enable-libcdio --
enable-libfreetype --enable-libx264 --enable-libmp3lame --enable-openal --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libass --enable-libv
4l2 --enable-libvidstab --enable-libfdk-aac --enable-libsmbclient --enable-libquvi --enable-libzvbi --enable-libzmq --extra-ldflags=-L/usr/new/tools/video/install/fdk-aac/current/lib --extra-cf
lags=-I/usr/new/tools/video/install/fdk-aac/current/include
  libavutil      54. 23.101 / 54. 23.101
  libavcodec     56. 38.100 / 56. 38.100
  libavformat    56. 32.100 / 56. 32.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 16.101 /  5. 16.101
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
[video4linux2,v4l2 @ 0xa60bd00] Detected absolute timestamps
Input #0, video4linux2,v4l2, from '/dev/video1':
  Duration: N/A, start: 1431981242.712328, bitrate: 124416 kb/s
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 720x576, 124416 kb/s, 25 fps, 25 tbr, 1000k tbn, 1000k tbc
[libx264 @ 0xa60de20] using cpu capabilities: MMX2 SSE2 Cache64
[libx264 @ 0xa60de20] profile High, level 3.0
[libx264 @ 0xa60de20] 264 - core 142 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, matroska, to '/home/barsnick/tmp/video1.mkv':
  Metadata:
    encoder         : Lavf56.32.100
    Stream #0:0: Video: h264 (libx264) (H264 / 0x34363248), yuv420p, 720x576, q=-1--1, 25 fps, 1k tbn, 25 tbc
    Metadata:
      encoder         : Lavc56.38.100 libx264
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 0xa60de20] non-strictly-monotonic PTS
    Last message repeated 12 times
[libx264 @ 0xa60de20] non-strictly-monotonic PTS0:00:00.00 bitrate=N/A
    Last message repeated 10 times
frame=   24 fps=2.6 q=-1.0 Lsize=    2129kB time=-23860:55:45.88 bitrate=N/A
video:2128kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.039607%
[libx264 @ 0xa60de20] frame I:1     Avg QP:38.00  size: 90932
[libx264 @ 0xa60de20] frame P:23    Avg QP:38.00  size: 90753
[libx264 @ 0xa60de20] mb I  I16..4:  0.0% 70.0% 30.0%
[libx264 @ 0xa60de20] mb P  I16..4:  0.9% 85.8% 13.1%  P16..4:  0.0%  0.1%  0.1%  0.0%  0.0%    skip: 0.0%
[libx264 @ 0xa60de20] 8x8 transform intra:85.3% inter:65.0%
[libx264 @ 0xa60de20] coded y,uvDC,uvAC intra: 100.0% 0.0% 0.0% inter: 100.0% 0.0% 0.0%
[libx264 @ 0xa60de20] i16 v,h,dc,p:  0%  7% 81% 13%
[libx264 @ 0xa60de20] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu:  1% 42% 29%  2%  3%  2%  8%  2% 11%
[libx264 @ 0xa60de20] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu:  2% 34% 18%  5%  6%  3% 13%  3% 15%
[libx264 @ 0xa60de20] i8c dc,h,v,p: 100%  0%  0%  0%
[libx264 @ 0xa60de20] Weighted P-Frames: Y:4.3% UV:0.0%
[libx264 @ 0xa60de20] ref P L0: 34.2%  8.8% 32.1% 23.8%  1.2%
[libx264 @ 0xa60de20] kb/s:inf

Moritz