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design: update the framestep draft

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Update the docs a little.
Add property to allow incremental stepping so that we can reduce excessive
queueing.
This commit is contained in:
Wim Taymans 2009-02-27 12:34:29 +01:00
parent df620c9cb6
commit 013f4a58d5
1 changed files with 28 additions and 8 deletions
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36
docs/design/draft-framestep.txt
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@ -17,7 +17,7 @@ The following requirements are needed:
- The ability to walk forwards and backwards in the stream. - The ability to walk forwards and backwards in the stream.
- Arbitrary increments in any supported format (time, frames, bytes ...) - Arbitrary increments in any supported format (time, frames, bytes ...)
- High speed, minimal overhead. This mechanism is not more expensive than - High speed, minimal overhead. This mechanism is not more expensive than
simple playback. simple playback.
- switching between forwards and backwards stepping should be fast. - switching between forwards and backwards stepping should be fast.
- Maintain synchronisation between streams. - Maintain synchronisation between streams.
- Get feedback of the amount of skipped data. - Get feedback of the amount of skipped data.
@ -107,17 +107,29 @@ events
the end of the segment. the end of the segment.
"rate", G_TYPE_DOUBLE "rate", G_TYPE_DOUBLE
The rate and direction at which the frames should be stepped in PLAYING The rate at which the frames should be stepped in PLAYING mode. 1.0 is
mode. 1.0 is the normal playback speed and direction of the segment, 2.0 the normal playback speed and direction of the segment, 2.0
is double speed. A negative rate will step backwards. A speed of 0.0 is is double speed. A speed of 0.0 is not allowed. When performing a
not allowed. When performing a flushing step, the speed is not relevant. flushing step, the speed is not relevant. Note that we don't allow negative
rates here, use a seek with a negative rate first to reverse the playback
direction.
"intermediate", G_TYPE_BOOLEAN
Signal that this step operation is an intermediate step, part of a series
of step operations. It is mostly interesting for stepping in the PAUSED state
because the sink will only perform a preroll after a non-intermediate step
operation completes. Intermediate steps are usefull to flush out data from
other sinks in order to not cause excessive queueing. In the PLAYING state
the intermediate flag has no visual effect. In all states, the intermediate
flag is passed to the corresponding GST_MESSAGE_STEP_DONE.
"flush", G_TYPE_BOOLEAN "flush", G_TYPE_BOOLEAN
when flushing is TRUE, the step is performed immediatly: when flushing is TRUE, the step is performed immediatly:
- In the PAUSED state the pipeline loses the PAUSED state, the requested - In the PAUSED state the pipeline loses the PAUSED state, the requested
amount of data is skipped and the pipeline prerolls again. When the amount of data is skipped and the pipeline prerolls again when a
pipeline was stepping while the event is sent, the current step non-intermediate step completes.
When the pipeline was stepping while the event is sent, the current step
operation is updated with the new amount and format. The sink will do a operation is updated with the new amount and format. The sink will do a
best effort to comply with the new amount. best effort to comply with the new amount.
- In the PLAYING state, the requested amount of data is skipped (not - In the PLAYING state, the requested amount of data is skipped (not
@ -132,7 +144,8 @@ events
current step operation completes. If there was no previous step current step operation completes. If there was no previous step
operation, the step operation will be performed from the position of the operation, the step operation will be performed from the position of the
last PAUSED state. last PAUSED state.
The application will create a STEP event to start or stop the stepping The application will create a STEP event to start or stop the stepping
operation. Both stepping in PAUSED and PLAYING can be performed by means of operation. Both stepping in PAUSED and PLAYING can be performed by means of
the flush flag. the flush flag.
@ -143,6 +156,10 @@ events
of the STEP_DONE message, one can step the other sinks to align the streams of the STEP_DONE message, one can step the other sinks to align the streams
again. again.
For large stepping amounts, there needs to be enough queueing in front of all
the sinks. If large steps need to be performed, they can be split up into
smaller step operations using the "intermediate" flag on the step.
Since the step event does not update the base_time of any of the elements, the Since the step event does not update the base_time of any of the elements, the
sinks should keep track of the amount of stepped data in order to remain sinks should keep track of the amount of stepped data in order to remain
synchronized against the clock. synchronized against the clock.
@ -163,6 +180,9 @@ messages
"rate", G_TYPE_DOUBLE "rate", G_TYPE_DOUBLE
The rate and direction at which the frames were stepped. The rate and direction at which the frames were stepped.
"intermediate", G_TYPE_BOOLEAN
If this is an intermediate step operation that completed.
"duration", G_TYPE_UINT64 "duration", G_TYPE_UINT64
The total duration of the stepped units in GST_FORMAT_TIME. The total duration of the stepped units in GST_FORMAT_TIME.