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106 lines
4.7 KiB
Text
106 lines
4.7 KiB
Text
DRAFT Sparse Streams
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--------------------
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Introduction
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~~~~~~~~~~~~
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In 0.8, there was some support for Sparse Streams through the use of
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FILLER events. These were used to mark gaps between buffers so that downstream
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elements could know not to expect any more data for that gap.
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In 0.10, segment information conveyed through SEGMENT events can be used
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for the same purpose.
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In 1.0, there is a GAP event that works in a similar fashion as the FILLER
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event in 0.8.
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Use cases
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~~~~~~~~~
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1) Sub-title streams
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Sub-title information from muxed formats such as Matroska or MPEG consist of
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irregular buffers spaced far apart compared to the other streams
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(audio and video). Since these usually only appear when someone speaks or
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some other action in the video/audio needs describing, they can be anywhere
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from 1-2 seconds to several minutes apart.
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Downstream elements that want to mix sub-titles and video (and muxers)
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have no way of knowing whether to process a video packet or wait a moment
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for a corresponding sub-title to be delivered on another pad.
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2) Still frame/menu support
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In DVDs (and other formats), there are still-frame regions where the current
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video frame should be retained and no audio played for a period. In DVD,
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these are described either as a fixed duration, or infinite duration still
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frame.
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3) Avoiding processing silence from audio generators
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Imagine a source that from time to time produces empty buffers (silence
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or blank images). If the pipeline has many elements next, it is better to
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optimise the obsolete data processing in this case. Examples for such sources
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are sound-generators (simsyn in gst-buzztard) or a source in a voip
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application that uses noise-gating (to save bandwith).
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Details
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~~~~~~~
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1) Sub-title streams
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The main requirement here is to avoid stalling the pipeline between sub-title
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packets, and is effectively updating the minimum-timestamp for that stream.
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A demuxer can do this by sending an 'update' SEGMENT with a new start time
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to the subtitle pad. For example, every time the SCR in MPEG data
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advances more than 0.5 seconds, the MPEG demuxer can issue a SEGMENT with
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(update=TRUE, start=SCR ). Downstream elements can then be aware not to
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expect any data older than the new start time.
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The same holds true for any element that knows the current position in the
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stream - once the element knows that there is no more data to be presented
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until time 'n' it can advance the start time of the current segment to 'n'.
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This technique can also be used, for example, to represent a stream of
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MIDI events spaced to a clock period. When there is no event present for
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a clock time, a SEGMENT update can be sent in its place.
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2) Still frame/menu support
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Still frames in DVD menus are not the same, in that they do not introduce
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a gap in the timestamps of the data. Instead, they represent a pause in the
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presentation of a stream. Correctly performing the wait requires some
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synchronisation with downstream elements.
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In this scenario, an upstream element that wants to execute a still frame
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performs the following steps:
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* Send all data before the still frame wait
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* Send a DRAIN event to ensure that all data has been played downstream.
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* wait on the clock for the required duration, possibly interrupting
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if necessary due to an intervening activity (such as a user navigation)
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* FLUSH the pipeline using a normal flush sequence (FLUSH_START,
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chain-lock, FLUSH_STOP)
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* Send a SEGMENT to restart playback with the next timestamp in the
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stream.
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The upstream element performing the wait must only do so when in the PLAYING
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state. During PAUSED, the clock will not be running, and may not even have
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been distributed to the element yet.
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DRAIN is a new event that will block on a src pad until all data downstream
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has been played out.
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Flushing after completing the still wait is to ensure that data after the wait
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is played correctly. Without it, sinks will consider the first buffers
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(x seconds, where x is the duration of the wait that occurred) to be
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arriving late at the sink, and they will be discarded instead of played.
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3) For audio, 3) is the same case as 1) - there is a 'gap' in the audio data
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that needs to be presented, and this can be done by sending a SEGMENT
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update that moves the start time of the segment to the next timestamp when
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data will be sent.
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For video, however it is slightly different. Video frames are typically
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treated at the moment as continuing to be displayed after their indicated
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duration if no new frame arrives. In 3), it is desired to display a blank
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frame instead, in which case at least one blank frame should be sent before
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updating the start time of the segment.
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