[Bug 793683] gstclockoverlay: Add ability to specify time to be displayed by GstClockOverlay

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=793683

--- Comment #5 from Nicolas Dufresne (stormer) <nicolas at ndufresne.ca> ---
(In reply to Abhinav from comment #4)
> Well - thanks for explaining the intent and difference between clockoverlay
> and timeoverlay. While, what you explained makes sense - as A novice it was
> hard for me(and probably others) to get this interpretation correct by
> looking at plugin names "timeoverlay" vs "clockoverlay".
> 
> Anyways, we used clockoverlay and found that it is not same as "Wall Clock"
> value of stream which is being recorded and there is a difference (more than
> few seconds) in our pipeline. We realized that it is because clockoverlay
> uses "current time" and has no direct relation with "Wall Clock" of stream
> capture time. 

It renders the wall clock time of when the buffer passed through clockoverlay,
so depending on your pipeline it can be way in the past when it reaches your
display.

> 
> I am not sure, whats the use of such wall clock for end users, if this wall
> clock doesn't have any predictable relation with stream time. Then we
> realized that there is a need for user to specify "What time he wants to
> display in wall clock format" ? Is it the wall clock time of buffer arrival
> at clockoverlay ? Is it the wall clock time of recording ? or any other.

It's what I just said before, the users decides the when by placing the
clockoverlay at specific place in it's pipeline. If you have a live capture,
and want something close to the capture time, place it right next to your
capture source.

> IMHO, most users would be interested in Wall Clock which corresponds to
> buffer.

I'm not sure what you want exactly. In GStreamer, there is no relationship
between buffer time and wall clock. There is though a relation ship between
buffer timestamp / segment and the pipeline clock. The pipeline clock can be
the system monotonic clock (which is not a wall clock) or an clock derived from
audio ring buffer, which has event less relationship with a wall block.

timeoverlay currently don't have a mode that display the pipeline clock of a
buffer, this is running-time + base_time, though it's not a useful value, the
running-time is much nicers, it's common to all synchronized buffers and starts
at 0.

> 
> IMHO, clockoverlay needs a property for users to be able to specify which
> "Wall Clock" time they are interested in ? For many users, if wall clock has
> no relation with buffer, it may not be useful enough.

I'm sorry, I don't understand what you mean by "which wallclock", do you mean
Time Zone support ? I have never had a clock on my wall that runs at different
rate, so it's hard to think of a different wall clock (outside timezone of
course, which could be fine addition).

That being said, documentation update are always welcome.

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