gstreamer/docs/design/draft-framestep.txt

Frame step
----------

This document is a draft that lists some ideas and purely informational.

This document outlines the details of the frame stepping functionality in
GStreamer.

The stepping functionality operates on the current playback segment, position
and rate as it was configured with a regular seek event. In contrast to the seek
event, it operates very closely to the sink and thus has a very low latency and
is not slowed down by queues and does not actually perform any seeking logic.
For this reason we want to include a new API instead of reusing the seek API.

The following requirements are needed:

 - The ability to walk forwards and backwards in the stream.
 - Arbitrary increments in any supported format (time, frames, bytes ...)
 - High speed, minimal overhead. This mechanism is not more expensive than
   simple playback.
 - switching between forwards and backwards stepping should be fast.
 - Maintain synchronisation between streams.
 - Get feedback of the amount of skipped data.
 - Ability to play a certain amount of data at an arbitrary speed.

We want a system where we can step frames in PAUSED as well as play short
segments of data in PLAYING.


Use Cases
---------

 * frame stepping in video only pipeline in PAUSED

    .-----.    .-------.              .------.    .-------.
    | src |    | demux |    .-----.   | vdec |    | vsink |
    |    src->sink    src1->|queue|->sink   src->sink     |
    '-----'    '-------'    '-----'   '------'    '-------'

    - app sets the pipeline to PAUSED to block on the preroll picture
    - app seeks to required position in the stream. This can be done with a
      positive or negative rate depending on the required frame stepping
      direction.
    - app steps frames (in GST_FORMAT_DEFAULT or GST_FORMAT_BUFFER). The
      pipeline loses its PAUSED state until the required number of frames have
      been skipped, it then prerolls again. This skipping is purely done in
      the sink.
    - sink posts STEP_DONE with amount of frames stepped and corresponding time
      interval.

 * frame stepping in audio/video pipeline in PAUSED

    .-----.    .-------.              .------.    .-------.
    | src |    | demux |    .-----.   | vdec |    | vsink |
    |    src->sink    src1->|queue|->sink   src->sink     |
    '-----'    |       |    '-----'   '------'    '-------'
               |       |              .------.    .-------.
               |       |    .-----.   | adec |    | asink |
               |      src2->|queue|->sink   src->sink     |
               '-------'    '-----'   '------'    '-------'


    - app sets the pipeline to PAUSED to block on the preroll picture
    - app seeks to required position in the stream. This can be done with a
      positive or negative rate depending on the required frame stepping
      direction.
    - app steps frames (in GST_FORMAT_DEFAULT or GST_FORMAT_BUFFER) or an amount
      of time on the video sink. The pipeline loses its PAUSED state until the
      required number of frames have been skipped, it then prerolls again.
      This skipping is purely done in the sink.
    - sink posts STEP_DONE with amount of frames stepped and corresponding time
      interval.
    - the app skips the same amount of time on the audiosink to align the
      streams again. When huge amount of video frames are skipped, there needs
      to be enough queueing in the pipeline to compensate for the accumulated
      audio.

 * frame stepping in audio/video pipeline in PLAYING

    - app sets the pipeline to PAUSED to block on the preroll picture
    - app seeks to required position in the stream. This can be done with a
      positive or negative rate depending on the required frame stepping
      direction.
    - app configures frames steps (in GST_FORMAT_DEFAULT or GST_FORMAT_BUFFER) or
      an amount of time on the sink. The step event has a flag indicating live
      stepping so that the stepping will only happens in PLAYING. 
    - app sets pipeline to PLAYING. The pipeline continues PLAYING until it
      consumed the amount of time.
    - sink posts STEP_DONE with amount of frames stepped and corresponding time
      interval. The sink will then wait for another step event. Since the
      STEP_DONE message was emited by the sink when it handed off the buffer to
      the device, there is usually sufficient time to queue a new STEP event so
      that one can seamlessly continue stepping.


events
------

 A new GST_EVENT_STEP event is introduced to start the step operation.  
 The step event is created with the following fields in the structure:

  "format", GST_TYPE_FORMAT
     The format of the step units

  "amount", G_TYPE_UINT64
     The amount of units to step. -1 resumes normal non-stepping behaviour to
     the end of the segment.

  "rate", G_TYPE_DOUBLE
     The rate and direction at which the frames should be stepped in PLAYING
     mode. 1.0 is the normal playback speed and direction of the segment, 2.0
     is double speed. A negative rate will step backwards. A speed of 0.0 is
     not allowed. When performing a flushing step, the speed is not relevant. 

  "flush", G_TYPE_BOOLEAN
     when flushing is TRUE, the step is performed immediatly: 

      - In the PAUSED state the pipeline loses the PAUSED state, the requested
        amount of data is skipped and the pipeline prerolls again. When the
	pipeline was stepping while the event is sent, the current step
	operation is updated with the new amount and format. The sink will do a
	best effort to comply with the new amount.
      - In the PLAYING state, the requested amount of data is skipped (not
	rendered) from the previous STEP request or from the position of the
	last PAUSED if no previous STEP operation was performed.

     When flushing is FALSE, the step will be performed later.

      - In the PAUSED state the step will be done when going to PLAYING. Any
	previous step operation will be overridden with the new STEP event.
      - In the PLAYING state the step operation will be performed after the
	current step operation completes. If there was no previous step
	operation, the step operation will be performed from the position of the
	last PAUSED state.
 
 The application will create a STEP event to start or stop the stepping
 operation. Both stepping in PAUSED and PLAYING can be performed by means of 
 the flush flag.

 The event is usually sent to the pipeline, which will typically distribute the
 event to all of its sinks. For some use cases, like frame stepping on video
 frames only, the event should only be sent to the video sink and upon reception
 of the STEP_DONE message, one can step the other sinks to align the streams
 again.

 Since the step event does not update the base_time of any of the elements, the
 sinks should keep track of the amount of stepped data in order to remain
 synchronized against the clock.


messages
--------

 A new GST_MESSAGE_STEP_DONE message is created. It contains the following
 fields:

  "format", GST_TYPE_FORMAT
     The format of the step units that completed.

  "amount", G_TYPE_UINT64
     The amount of units that were stepped.

  "rate", G_TYPE_DOUBLE
     The rate and direction at which the frames were stepped.

  "duration", G_TYPE_UINT64
     The total duration of the stepped units in GST_FORMAT_TIME.

 The message is emited by the element that performs the step operation.


Direction switch
----------------

 When quickly switching between a forwards and a backwards step of, for example,
 one video frame, we need either:

  a) issue a new seek to change the direction from the current position.
  b) cache a certain number of stepped frames and walk the cache.

 option a) might be very slow.
 For option b) we would ideally like to offload this caching functionality to a
 separate element, which means that we need to forward the STEP event upstream.
 It's unclear how this could work in a generic way. What is a demuxer supposed
 to do when it received a step event? a flushing seek to what stream position?