[FFmpeg-devel] [PATCH 3/3] avdevice: use av_gettime_relative() for timestamps

[Kieran Kunhya]

Sent from my mobile device

On Sun, 7 Feb 2021, 16:31 Nicolas George, <george at nsup.org> wrote:

> Kieran Kunhya (12021-02-07):
> > It is wrong to suggest that the clock of an IP webcam has any relation at
> > all to the PC clock both absolute or relative.
>
> It is not wrong to let users observe that their particular model of IP
> webcam uses Unix timestamps and is correctly synchronized. This is a
> common practice, after all.


This is nonsense. The clock on any device comes from the oscillator onboard
the device. Not from any PC clock on the motherboard. These clocks are
almost certainly not the same model, they have different temperatures etc
so they drift differently.

> This is as usual a major flaw in v4l2. The monotonic clock should always
> be
> > used. It's the least-worst clock.
>
> Care to justify how giving the user an option is worse than not?


Give the user the option not to use the monotonic clock.

Care to explain how you propose to synchronize device captured on
> different computers?


This is not possible without high end capture equipment to synchronise the
clocks on each camera. Everything else is guesswork.

I encourage you to read the section on clocks here:
https://haasn.xyz/posts/2016-12-25-falsehoods-programmers-believe-about-%5Bvideo-stuff%5D.html

Kieran

Regards,
>
> --
>   Nicolas George
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