On Mon, 15 Mar 2021, Hassan wrote:
Hello,
I am using ffmpeg on a Windows 10 machine and I want to record the desktop
at a high frame rate while appending accurate timestamps to each frame.
I am recording my desktop using the following command:
ffmpeg -f gdigrab -framerate 60 -i desktop -vf "settb=AVTB,
setpts='trunc(PTS/1K)*1K+st(1,trunc(RTCTIME/1K))-1K*trunc(ld(1)/1K)',
drawtext=fontfile=ArialBold.ttf:fontsize=40:fontcolor=white:text='%{localtime}.%{eif\:1M*t-1K*trunc(t*1K)\:d}:box=1:boxborderw=20:[email protected]:x=10:y=10'"
-c:v libx264rgb -crf 0 -preset ultrafast output.mkv
The long text next to -vf flag is used to append timestamp (date and
current time in milliseconds) on the top left corner of the frame with
black background.
The issue is that, ideally, when I am recording at 60 FPS, each subsequent
frame should have a timestamp with an increment of 16.66 msec. However, the
timestamp is not incremented as such. Instead, it stays the same on a lot
of frames and then changes.
For example, when I break the video into frames, the frame titled
"img0428.png" has the timestamp 18:44:16.828 (hh:mm:ss.millisec)
[image: image.png].
Then until "next 40 frames, it says the same. On file "img0469.png", the
timestamp changes and becomes 18:44:17.510.
[image: image.png]
So, the timestamp changed after 41 frames and the time difference is 682
milliseconds. Ideally, each of the 40 frames between these two frames
should carry an incremental timestamp by a step size of 16.66 msec but this
is not happening.
Therefore, my questions are as follows:
1. Am I using the right method to append timestamps to the recorded frames?
No. If you use a filter to set timestamps, that definitely will cause
jitter in the output.
2. What is the reason that the timestamping on the frames is not correct?
Because you are setting time timestamps in a filter. If you want something
like 10 millisecond accuracy, filter processing delays, or encoding
latencies are not negligable at all.
3. How can I fix this issue?
Use the original timestamps which are returned by gdigrab. Also use
-copyts to pass the timestamps to the filters unmodified. Also use
-thread_queue_size parameter to enable threaded reading of input. And use
'%{pts\:localtime}' variable expansion to get the date based on frame
timestamp.
4. What are the alternate methods to append accurate epoch timestamps to
each of the recorded frames?
As stated above. Note that it will still not be very accurate. In order to
make it better I think a more accurate av_usleep() implementation is
needed on Windows. With that, it should be decent enought.
Regards,
Marton
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