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316 lines
12 KiB
Text
316 lines
12 KiB
Text
Events
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------
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Events are objects passed around in parallel to the buffer dataflow to
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notify elements of various events.
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Events are received on pads using the event function. Some events should
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be interleaved with the data stream so they require taking the STREAM_LOCK,
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others don't.
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Different types of events exist to implement various functionalities.
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GST_EVENT_FLUSH_START: data is to be discarded
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GST_EVENT_FLUSH_STOP: data is allowed again
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GST_EVENT_CAPS: Format information about the following buffers
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GST_EVENT_STREAM_CONFIG: Stream config: stream headers and codec setup data
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GST_EVENT_SEGMENT: Timing information for the following buffers
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GST_EVENT_TAG: Stream metadata.
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GST_EVENT_BUFFERSIZE: Buffer size requirements
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GST_EVENT_SINK_MESSAGE: An event turned into a message by sinks
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GST_EVENT_EOS: no more data is to be expected on a pad.
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GST_EVENT_QOS: A notification of the quality of service of the stream
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GST_EVENT_SEEK: A seek should be performed to a new position in the stream
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GST_EVENT_NAVIGATION: A navigation event.
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GST_EVENT_LATENCY: Configure the latency in a pipeline
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GST_EVENT_STEP: Stepping event
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GST_EVENT_RECONFIGURE: stream reconfigure event
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* GST_EVENT_DRAIN: Play all data downstream before returning.
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* not yet implemented, under investigation, might be needed to do still frames
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in DVD.
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src pads
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--------
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A gst_pad_push_event() on a srcpad will first store the sticky event in the
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sticky array before sending the event to the peer pad. If there is no peer pad
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and the event was not stored in the sticky array, FALSE is returned.
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Flushing pads will refuse the events and will not store the sticky events.
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sink pads
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---------
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A gst_pad_send_event() on a sinkpad will call the event function on the pad. If
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the event function returns success, the sticky event is stored in the sticky
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event array and the event is marked for update.
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When the pad is flushing, the _send_event() function returns FALSE immediately.
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When the next data item is pushed, the pending events are pushed first.
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This ensures that the event function is never called for flushing pads and that
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the sticky array only contains events for which the event function returned
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success.
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pad link
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--------
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When linking pads, the srcpad sticky events are marked for update when they are
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different from the sinkpad events. The next buffer push will push the events to
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the sinkpad.
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FLUSH_START/STOP
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~~~~~~~~~~~~~~~~
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A flush event is sent both downstream and upstream to clear any pending data
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from the pipeline. This might be needed to make the graph more responsive
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when the normal dataflow gets interrupted by for example a seek event.
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Flushing happens in two stages.
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1) a source element sends the FLUSH_START event to the downstream peer element.
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The downstream element starts rejecting buffers from the upstream elements. It
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sends the flush event further downstream and discards any buffers it is
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holding as well as return from the chain function as soon as possible.
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This makes sure that all upstream elements get unblocked.
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This event is not synchronized with the STREAM_LOCK and can be done in the
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application thread.
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2) a source element sends the FLUSH_STOP event to indicate
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that the downstream element can accept buffers again. The downstream
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element sends the flush event to its peer elements. After this step dataflow
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continues. The FLUSH_STOP call is synchronized with the STREAM_LOCK so any
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data used by the chain function can safely freed here if needed. Any
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pending EOS events should be discarded too.
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After the flush completes the second stage, data is flowing again in the pipeline
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and all buffers are more recent than those before the flush.
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For elements that use the pullrange function, they send both flush events to
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the upstream pads in the same way to make sure that the pullrange function
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unlocks and any pending buffers are cleared in the upstream elements.
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A FLUSH_START may instruct the pipeline to distribute a new base_time to
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elements so that the running_time is reset to 0.
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(see part-clocks.txt and part-synchronisation.txt).
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EOS
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~~~
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The EOS event can only be sent on a sinkpad. It is typically emited by the
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source element when it has finished sending data. This event is mainly sent
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in the streaming thread but can also be sent from the application thread.
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The downstream element should forward the EOS event to its downstream peer
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elements. This way the event will eventually reach the sinks which should
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then post an EOS message on the bus when in PLAYING.
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An element might want to flush its internally queued data before forwarding
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the EOS event downstream. This flushing can be done in the same thread as
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the one handling the EOS event.
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For elements with multiple sink pads it might be possible to wait for EOS on
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all the pads before forwarding the event.
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The EOS event should always be interleaved with the data flow, therefore the
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GStreamer core will take the STREAM_LOCK.
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Sometimes the EOS event is generated by another element than the source, for
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example a demuxer element can generate an EOS event before the source element.
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This is not a problem, the demuxer does not send an EOS event to the upstream
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element but returns GST_FLOW_EOS, causing the source element to stop
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sending data.
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An element that sends EOS on a pad should stop sending data on that pad. Source
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elements typically pause() their task for that purpose.
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By default, a GstBin collects all EOS messages from all its sinks before
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posting the EOS message to its parent.
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The EOS is only posted on the bus by the sink elements in the PLAYING state. If
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the EOS event is received in the PAUSED state, it is queued until the element
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goes to PLAYING.
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A FLUSH_STOP event on an element flushes the EOS state and all pending EOS messages.
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GST_EVENT_STREAM_CONFIG
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~~~~~~~~~~~~~~~~~~~~~~~
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A stream config event is sent downstream by an element to pass stream headers
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or codec/stream setup data to elements downstream.
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Stream headers are buffers that are to be pre-pended to a stream to create
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a valid decodable bitstream. This is useful for e.g. network elements who
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will send such stream headers first when a new client connects in the middle
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of a streaming session. The stream headers and the current data will then
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create a valid decodable stream. Stream headers are usually also sent as
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buffers at the beginning of a stream in addition to the rest of the stream
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data.
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Setup data is codec config data that must be communicated outside of the
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data stream and is required by the consumer / downstream element in order
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to interpret the data stream correctly. Prepending it to the data stream is
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usually not allowed and will not yield a valid stream.
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SEGMENT
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~~~~~~~
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A segment event is sent downstream by an element to indicate that the following
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group of buffers start and end at the specified positions. The newsegment event
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also contains the playback speed and the applied rate of the stream.
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Since the stream time is always set to 0 at start and after a seek, a 0
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point for all next buffer's timestamps has to be propagated through the
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pipeline using the SEGMENT event.
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Before sending buffers, an element must send a SEGMENT event. An element is
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free to refuse buffers if they were not preceeded by a SEGMENT event.
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Elements that sync to the clock should store the SEGMENT start and end values
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and subtract the start value from the buffer timestamp before comparing
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it against the stream time (see part-clocks.txt).
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An element is allowed to send out buffers with the SEGMENT start time already
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subtracted from the timestamp. If it does so, it needs to send a corrected
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SEGMENT downstream, ie, one with start time 0.
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A SEGMENT event should be generated as soon as possible in the pipeline and
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is usually generated by a demuxer or source. The event is generated before
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pushing the first buffer and after a seek, right before pushing the new buffer.
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The SEGMENT event should be sent from the streaming thread and should be
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serialized with the buffers.
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Buffers should be clipped within the range indicated by the newsegment event
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start and stop values. Sinks must drop buffers with timestamps out of the
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indicated segment range.
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TAG
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~~~
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The tag event is sent downstream when an element has discovered metadata
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tags in a media file. Encoders can use this event to adjust their tagging
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system. A tag is serialized with buffers.
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BUFFERSIZE
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~~~~~~~~~~
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NOTE: This event is not yet implemented.
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An element can suggest a buffersize for downstream elements. This is
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typically done by elements that produce data on multiple source pads
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such as demuxers.
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QOS
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~~~
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A QOS, or quality of service message, is generated in an element to report
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to the upstream elements about the current quality of real-time performance
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of the stream. This is typically done by the sinks that measure the amount
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of framedrops they have. (see part-qos.txt)
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SEEK
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~~~~
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A seek event is issued by the application to configure the playback range
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of a stream. It is called form the application thread and travels upstream.
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The seek event contains the new start and stop position of playback
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after the seek is performed. Optionally the stop position can be left
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at -1 to continue playback to the end of the stream. The seek event
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also contains the new playback rate of the stream, 1.0 is normal playback,
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2.0 double speed and negative values mean backwards playback.
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A seek usually flushes the graph to minimize latency after the seek. This
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behaviour is triggered by using the SEEK_FLUSH flag on the seek event.
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The seek event usually starts from the sink elements and travels upstream
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from element to element until it reaches an element that can perform the
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seek. No intermediate element is allowed to assume that a seek to this
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location will happen. It is allowed to modify the start and stop times if it
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needs to do so. this is typically the case if a seek is requested for a
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non-time position.
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The actual seek is performed in the application thread so that success
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or failure can be reported as a return value of the seek event. It is
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therefore important that before executing the seek, the element acquires
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the STREAM_LOCK so that the streaming thread and the seek get serialized.
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The general flow of executing the seek with FLUSH is as follows:
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1) unblock the streaming threads, they could be blocked in a chain
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function. This is done by sending a FLUSH_START on all srcpads or by pausing
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the streaming task, depending on the seek FLUSH flag.
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The flush will make sure that all downstream elements unlock and
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that control will return to this element chain/loop function.
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We cannot lock the STREAM_LOCK before doing this since it might
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cause a deadlock.
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2) acquire the STREAM_LOCK. This will work since the chain/loop function
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was unlocked/paused in step 1).
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3) perform the seek. since the STREAM_LOCK is held, the streaming thread
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will wait for the seek to complete. Most likely, the stream thread
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will pause because the peer elements are flushing.
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4) send a FLUSH_STOP event to all peer elements to allow streaming again.
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5) create a NEWSEGMENT event to signal the new buffer timestamp base time.
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This event must be queued to be sent by the streaming thread.
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6) start stopped tasks and unlock the STREAM_LOCK, dataflow will continue
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now from the new position.
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More information about the different seek types can be found in
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part-seeking.txt.
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NAVIGATION
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~~~~~~~~~~~
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A navigation event is generated by a sink element to signal the elements
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of a navigation event such as a mouse movement or button click.
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Navigation events travel upstream.
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LATENCY
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~~~~~~~
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A latency event is used to configure a certain latency in the pipeline. It
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contains a single GstClockTime with the required latency. The latency value is
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calculated by the pipeline and distributed to all sink elements before they are
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set to PLAYING. The sinks will add the configured latency value to the
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timestamps of the buffer in order to delay their presentation.
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(See also part-latency.txt).
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DRAIN
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~~~~~
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NOTE: This event is not yet implemented.
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Drain event indicates that upstream is about to perform a real-time event, such
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as pausing to present an interactive menu or such, and needs to wait for all
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data it has sent to be played-out in the sink.
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Drain should only be used by live elements, as it may otherwise occur during
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prerolling.
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Usually after draining the pipeline, an element either needs to modify timestamps,
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or FLUSH to prevent subsequent data being discarded at the sinks for arriving
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late (only applies during playback scenarios).
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