gstreamer/docs/design/part-trickmodes.txt

Trickmodes
==========

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:

 1) 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).
    - 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().


 2) Send the 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
    GST_FORMAT_TIME.

    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 NEW_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 NEW_SEGMENT event will
    be set to 1.0 to indicate that no rate adjustment has been done.

    for server side trick mode a NEW_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 NEW_SEGMENT event is sent downstream with a rate
different from 1.0. Plugins receiving the NEW_SEGMENT can decide to perform the 
rate conversion of the media data (retimestamp video frames, resample audio, ...).
A plugin can also decide to drop frames in the case of fast playback or use a more
efficient decoding algorithm (skip B frames, ...). If a plugin decides to resample
or retimestamp, it should modify the NEW_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 NEW_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
information.


client side backwards trickmode 
-------------------------------

For backwards playback the following rules apply:

 - the rate in the NEW_SEGMENT is less than 0.0.
 - the NEW_SEGMENT start position is less than the stop position, playback will
   however happen from stop to start in reverse.
 - the time member in the NEW_SEGMENT is set to the stream time of the start
   position.

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.

 - 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.

 - A decoder decodes and accumulates all frames. If a new keyframe or EOS is received, 
   all accumulated frames are sent downsteam in reverse. 

 - 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.

 - for transcoding, audio and video resamplers can be used to reverse, resample and
   retimestamp the buffers.

Notes:

 - The clock keeps running forward.
 - backwards playback potentially uses a lot of memory as frames and undecoded data
   get buffered.