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docs: update synchronization docs
This commit is contained in:
Wim Taymans
parent
2e0ee2e767
commit
24907879a7
1 changed files with 32 additions and 29 deletions
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@ -77,38 +77,38 @@ The following notation is used:
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B: GstBuffer
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- B.timestamp = buffer timestamp (GST_BUFFER_TIMESTAMP)
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NS: SEGMENT event preceeding the buffers.
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- NS.start: start field in the SEGMENT event
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- NS.stop: stop field in the SEGMENT event
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- NS.rate: rate field of SEGMENT event
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- NS.abs_rate: absolute value of rate field of SEGMENT event
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- NS.time: time field in the SEGMENT event
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- NS.accum: total accumulated time of all previous SEGMENT events. This
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field is kept in the GstSegment structure.
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S: SEGMENT event preceeding the buffers.
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- S.start: start field in the SEGMENT event
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- S.stop: stop field in the SEGMENT event
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- S.rate: rate field of SEGMENT event
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- S.abs_rate: absolute value of rate field of SEGMENT event
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- S.time: time field in the SEGMENT event
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- S.base: a base time for the time.
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- S.offset: an offset to apply to S.start or S.stop
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Valid buffers for synchronisation are those with B.timestamp between NS.start
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and NS.stop. All other buffers outside this range should be dropped or clipped
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to these boundaries (see also part-segments.txt).
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Valid buffers for synchronisation are those with B.timestamp between S.start
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and S.stop (after applying the S.offset). All other buffers outside this range
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should be dropped or clipped to these boundaries (see also part-segments.txt).
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The following transformation to running_time exist:
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if (NS.rate > 0.0)
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B.running_time = (B.timestamp - NS.start) / NS.abs_rate + NS.accum
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else
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B.running_time = (NS.stop - B.timestamp) / NS.abs_rate + NS.accum
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if (S.rate > 0.0)
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B.running_time = (B.timestamp - (S.start + S.offset)) / S.abs_rate + S.base
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else
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B.running_time = ((S.stop - S.offset) - B.timestamp) / S.abs_rate + S.base
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We write B.running_time as the running_time obtained from the SEGMENT event
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and the buffers of that segment.
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The first displayable buffer will yield a value of 0 (since B.timestamp ==
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NS.start and NS.accum == 0).
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S.start and S.offset and S.base == 0).
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For NS.rate > 1.0, the timestamps will be scaled down to increase the playback
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For S.rate > 1.0, the timestamps will be scaled down to increase the playback
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rate. Likewise, a rate between 0.0 and 1.0 will slow down playback.
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For negative rates, timestamps are received stop NS.stop to NS.start so that the
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For negative rates, timestamps are received stop S.stop to S.start so that the
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first buffer received will be transformed into B.running_time of 0 (B.timestamp ==
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NS.stop and NS.accum == 0).
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S.stop and S.accum == 0).
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Synchronisation
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@ -123,7 +123,7 @@ As we have seen, we can get a running_time:
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- using the buffer timestamp and the preceeding SEGMENT event as (assuming
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positive playback rate):
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B.running_time = (B.timestamp - NS.start) / NS.abs_rate + NS.accum
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B.running_time = (B.timestamp - (S.start + S.offset)) / S.abs_rate + S.base
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We prefix C. and B. before the two running times to note how they were
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calculated.
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@ -175,9 +175,9 @@ It is the stream time that is used for:
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Stream time is calculated using the buffer times and the preceeding SEGMENT
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event as follows:
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stream_time = (B.timestamp - NS.start) * NS.abs_applied_rate + NS.time
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stream_time = (B.timestamp - S.start) * S.abs_applied_rate + S.time
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For negative rates, B.timestamp will go backwards from NS.stop to NS.start,
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For negative rates, B.timestamp will go backwards from S.stop to S.start,
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making the stream time go backwards.
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In the PLAYING state, it is also possible to use the pipeline clock to derive
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@ -187,21 +187,24 @@ Give the two formulas above to match the clock times with buffer timestamps
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allows us to rewrite the above formula for stream_time (and for positive rates).
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C.running_time = absolute_time - base_time
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B.running_time = (B.timestamp - NS.start) / NS.abs_rate + NS.accum
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B.running_time = (B.timestamp - (S.start + S.offset)) / S.abs_rate + S.base
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=>
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(B.timestamp - NS.start) / NS.abs_rate + NS.accum = absolute_time - base_time;
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(B.timestamp - (S.start + S.offset)) / S.abs_rate + S.base = absolute_time - base_time;
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=>
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(B.timestamp - NS.start) / NS.abs_rate = absolute_time - base_time - NS.accum;
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(B.timestamp - (S.start + S.offset)) / S.abs_rate = absolute_time - base_time - S.base;
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=>
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(B.timestamp - NS.start) = (absolute_time - base_time - NS.accum) * NS.abs_rate
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filling (B.timestamp - NS.start) in the above formule for stream time
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(B.timestamp - (S.start + S.offset)) = (absolute_time - base_time - S.base) * S.abs_rate
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=>
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stream_time = (absolute_time - base_time - NS.accum) * NS.abs_rate * NS.abs_applied_rate + NS.time
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(B.timestamp - S.start) = S.offset + (absolute_time - base_time - S.base) * S.abs_rate
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filling (B.timestamp - S.start) in the above formule for stream time
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=>
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stream_time = (S.offset + (absolute_time - base_time - S.base) * S.abs_rate) * S.abs_applied_rate + S.time
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This last formula is typically used in sinks to report the current position in
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an accurate and efficient way.
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