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