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238 lines
9 KiB
Markdown
238 lines
9 KiB
Markdown
# Trickmodes
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GStreamer provides API for performing various trickmode playback. This
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includes:
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- server side trickmodes
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- client side fast/slow forward playback
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- client side fast/slow backwards playback
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Server side trickmodes mean that a source (network source) can provide a
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stream with different playback speed and direction. The client does not
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have to perform any special algorithms to decode this stream.
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Client side trickmodes mean that the decoding client (GStreamer)
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performs the needed algorithms to change the direction and speed of the
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media file.
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Seeking can both be done in a playback pipeline and a transcoding
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pipeline.
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## General seeking overview
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Consider a typical playback pipeline:
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```
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+---------+ +------+
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+-------+ | decoder |->| sink |
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+--------+ | |-->+---------+ +------+
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| source |->| demux |
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+--------+ | |-->+---------+ +------+
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+-------+ | decoder |->| sink |
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+---------+ +------+
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```
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The pipeline is initially configured to play back at speed 1.0 starting
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from position 0 and stopping at the total duration of the file.
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When performing a seek, the following steps have to be taken by the
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application:
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### Create a seek event
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The seek event contains:
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- various `GstSeekFlags` flags describing:
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- where to seek to (`KEY_UNIT`)
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- how accurate the seek should be (`ACCURATE`)
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- how to perform the seek (`FLUSH`)
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- what to do when the stop position is reached (`SEGMENT`).
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- extra playback options (`SKIP`)
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- a format to seek in, this can be time, bytes, units (frames,
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samples), …
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- a playback rate, 1.0 is normal playback speed, positive values
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bigger than 1.0 mean fast playback. negative values mean reverse
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playback. A playback speed of 0.0 is not allowed (but is equivalent
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to PAUSING the pipeline).
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- a start position, this value has to be between 0 and the total
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duration of the file. It can also be relative to the previously
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configured start value.
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- a stop position, this value has to be between 0 and the total
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duration. It can also be relative to the previously configured stop
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value.
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See also `gst_event_new_seek()`.
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### Send the seek event
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Send the new seek event to the pipeline with
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`gst_element_send_event()`.
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By default the pipeline will send the event to all sink elements. By
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default an element will forward the event upstream on all sinkpads.
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Elements can modify the format of the seek event. The most common format
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is `GST_FORMAT_TIME`.
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One element will actually perform the seek, this is usually the demuxer
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or source element. For more information on how to perform the different
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seek types see [seeking](additional/design/seeking.md).
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For client side trickmode a `SEGMENT` event will be sent downstream with
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the new rate and start/stop positions. All elements prepare themselves
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to handle the rate (see below). The applied rate of the SEGMENT event
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will be set to 1.0 to indicate that no rate adjustment has been done.
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for server side trick mode a `SEGMENT` event is sent downstream with a
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rate of 1.0 and the start/stop positions. The elements will configure
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themselves for normal playback speed since the server will perform the
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rate conversions. The applied rate will be set to the rate that will be
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applied by the server. This is done to insure that the position
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reporting performed in the sink is aware of the trick mode.
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When the seek succeeds, the `_send_event()` function will return TRUE.
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## Server side trickmode
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The source element operates in push mode. It can reopen a server
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connection requesting a new byte or time position and a new playback
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speed. The capabilities can be queried from the server when the
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connection is opened.
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We assume the source element is derived from the `GstPushSrc` base class.
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The base source should be configured with:
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```c
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gst_base_src_set_format (src, GST_FORMAT_TIME);
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```
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The `do_seek()` method will be called on the `GstPushSrc` subclass with the
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seek information passed in the `GstSegment` argument.
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The rate value in the segment should be used to reopen the connection to
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the server requesting data at the new speed and possibly a new playback
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position.
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When the server connection was successfully reopened, set the rate of
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the segment to 1.0 so that the client side trickmode is not enabled. The
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applied rate in the segment is set to the rate transformation done by
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the server.
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Alternatively a combination of client side and serverside trickmode can
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be used, for example if the server does not support certain rates, the
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client can perform rate conversion for the remainder.
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```
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source server
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do_seek | |
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----------->| |
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| reopen connection |
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|-------------------->|
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| .
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| success .
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|<--------------------|
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modify | |
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rate to 1.0 | |
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return | |
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TRUE | |
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```
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After performing the seek, the source will inform the downstream
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elements of the new segment that is to be played back. Since the segment
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will have a rate of 1.0, no client side trick modes are enabled. The
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segment will have an applied rate different from 1.0 to indicate that
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the media contains data with non-standard playback speed or direction.
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## client side forward trickmodes
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The seek happens as stated above. a `SEGMENT` event is sent downstream
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with a rate different from 1.0. Plugins receiving the `SEGMENT` can decide
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to perform the rate conversion of the media data (retimestamp video
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frames, resample audio, …).
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If a plugin decides to resample or retimestamp, it should modify the
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`SEGMENT` with a rate of 1.0 and update the applied rate so that
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downstream elements don’t resample again but are aware that the media
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has been modified.
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The GStreamer base audio and video sinks will resample automatically if
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they receive a SEGMENT event with a rate different from 1.0. The
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position reporting in the base audio and video sinks will also depend on
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the applied rate of the segment information.
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When the `SKIP` flag is set, frames can be dropped in the elements. If S
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is the speedup factor, a good algorithm for implementing frame skipping
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is to send audio in chunks of Nms (usually 300ms is good) and then skip
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((S-1) \* Nns) of audio data. For the video we send only the keyframes
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in the (S \* Nns) interval. In this case, the demuxer would scale the
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timestamps and would set an applied rate of S.
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## client side backwards trickmode
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For backwards playback the following rules apply:
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- the rate in the `SEGMENT` is less than 0.0.
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- the `SEGMENT` start position is less than the stop position, playback
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will however happen from stop to start in reverse.
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- the time member in the `SEGMENT` is set to the stream time of the
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start position.
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For plugins the following rules apply:
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- A source plugin sends data in chunks starting from the last chunk of
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the file. The actual bytes are not reversed. Each chunk that is not
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forward continuous with the previous chunk is marked with a `DISCONT`
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flag.
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- A demuxer accumulates the chunks. As soon as a keyframe is found,
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everything starting from the keyframe up to the accumulated data is
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sent downstream. Timestamps on the buffers are set starting from the
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stop position to start, effectively going backwards. Chunks are
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marked with `DISCONT` when they are not forward continuous with the
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previous buffer.
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- A video decoder decodes and accumulates all decoded frames. If a
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buffer with a `DISCONT`, `SEGMENT` or `EOS` is received, all accumulated
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frames are sent downsteam in reverse.
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- An audio decoder decodes and accumulates all decoded audio. If a
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buffer with a `DISCONT`, `SEGMENT` or `EOS` is received, all accumulated
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audio is sent downstream in reverse order. Some audio codecs need
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the previous data buffer to decode the current one, in that case,
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the previous DISCONT buffer needs to be combined with the last
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non-DISCONT buffer to generate the last bit of output.
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- A sink reverses (for audio) and retimestamps (audio, video) the
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buffers before playing them back. Retimestamping occurs relative to
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the stop position, making the timestamps increase again and suitable
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for synchronizing against the clock. Audio sinks also have to
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perform simple resampling before playing the samples.
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- for transcoding, audio and video resamplers can be used to reverse,
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resample and retimestamp the buffers. Any rate adjustments performed
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on the media must be added to the `applied_rate` and subtracted from
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the rate members in the `SEGMENT`
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event.
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In `SKIP` mode, the same algorithm as for forward `SKIP` mode can be used.
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## Notes
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- The clock/`running_time` keeps running forward.
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- backwards playback potentially uses a lot of memory as frames and
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undecoded data gets buffered.
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