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docs/design/part-synchronisation.txt: Some more info on how the stream_time in GstBaseSink is done.

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Original commit message from CVS:
* docs/design/part-synchronisation.txt:
Some more info on how the stream_time in GstBaseSink is done.
This commit is contained in:
Wim Taymans 2007-12-31 19:11:39 +00:00
parent 5abff12ba7
commit d8bea77230
2 changed files with 29 additions and 0 deletions
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ChangeLog
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@ -1,3 +1,8 @@
2007-12-31 Wim Taymans <wim.taymans@collabora.co.uk>
* docs/design/part-synchronisation.txt:
Some more info on how the stream_time in GstBaseSink is done.
2007-12-30 Tim-Philipp Müller <tim at centricular dot net>
* tests/check/generic/sinks.c: (gst_sinks_suite):

24
docs/design/part-synchronisation.txt
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@ -180,5 +180,29 @@ event as follows:
For negative rates, B.timestamp will go backwards from NS.stop to NS.start,
making the stream time go backwards.
In the PLAYING state, it is also possible to use the pipeline clock to derive
the current stream_time.
Give the two formulas above to match the clock times with buffer timestamps
allows us to rewrite the above formula for stream_time (and for positive rates).
C.running_time = absolute_time - base_time
B.running_time = (B.timestamp - NS.start) / NS.abs_rate + NS.accum
=>
absolute_time - base_time = (B.timestamp - NS.start) / NS.abs_rate + NS.accum
=>
absolute_time - base_time = (B.timestamp - NS.start) / NS.abs_rate + NS.accum
=>
(B.timestamp - NS.start) = (absolute_time - base_time - NS.accum) * NS.abs_rate
=>
stream_time = (absolute_time - base_time - NS.accum) * NS.abs_rate * NS.abs_applied_rate + NS.time
This last formula is typically used in sinks to report the current position in
an accurate and efficient way.
Note that the stream time is never used for synchronisation against the clock.