Trickmodes
==========
+GStreamer provides API for performing various trickmode playback. This includes:
-forward playback
-----------------
+ - 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.
-backwards playback
-------------------
+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.
+ - 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).
+
+ 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.
+
+ 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. 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.
+
+
+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 so that
+downstream elements don't resample again.
+
+The GStreamer base audio and video sinks will resample automatically if they receive
+a NEW_SEGMENT event with a rate different from 1.0.
+
+
+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.
+
+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 is received,
+ all accumulated frames are sent backwards.
+
+ - 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.
+
projects / platform / upstream / gstreamer.git / commitdiff