Position tracking and seeking

So far, we've looked at how to create a pipeline to do media processing and how to make it run. Most application developers will be interested in providing feedback to the user on media progress. Media players, for example, will want to show a slider showing the progress in the song, and usually also a label indicating stream length. Transcoding applications will want to show a progress bar on how much percent of the task is done. GStreamer has built-in support for doing all this using a concept known as querying. Since seeking is very similar, it will be discussed here as well. Seeking is done using the concept of events.

Querying: getting the position or length of a stream

Querying is defined as requesting a specific stream property related to progress tracking. This includes getting the length of a stream (if available) or getting the current position. Those stream properties can be retrieved in various formats such as time, audio samples, video frames or bytes. The function most commonly used for this is gst_element_query (), although some convenience wrappers are provided as well (such as gst_element_query_position () and gst_element_query_duration ()). You can generally query the pipeline directly, and it'll figure out the internal details for you, like which element to query.

Internally, queries will be sent to the sinks, and “dispatched” backwards until one element can handle it; that result will be sent back to the function caller. Usually, that is the demuxer, although with live sources (from a webcam), it is the source itself.

#include <gst/gst.h>

static gboolean
cb_print_position (GstElement *pipeline)
  gint64 pos, len;

  if (gst_element_query_position (pipeline, GST_FORMAT_TIME, &pos)
    && gst_element_query_duration (pipeline, GST_FORMAT_TIME, &len)) {
    g_print ("Time: %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",
         GST_TIME_ARGS (pos), GST_TIME_ARGS (len));

  /* call me again */
  return TRUE;

main (gint   argc,
      gchar *argv[])
  GstElement *pipeline;


  /* run pipeline */
  g_timeout_add (200, (GSourceFunc) cb_print_position, pipeline);
  g_main_loop_run (loop);



Events: seeking (and more)

Events work in a very similar way as queries. Dispatching, for example, works exactly the same for events (and also has the same limitations), and they can similarly be sent to the toplevel pipeline and it will figure out everything for you. Although there are more ways in which applications and elements can interact using events, we will only focus on seeking here. This is done using the seek-event. A seek-event contains a playback rate, a seek offset format (which is the unit of the offsets to follow, e.g. time, audio samples, video frames or bytes), optionally a set of seeking-related flags (e.g. whether internal buffers should be flushed), a seek method (which indicates relative to what the offset was given), and seek offsets.

The first offset (start) is the new position to seek to, while the second offset (stop) is optional and specifies a position where streaming is supposed to stop. Usually it is fine to just specify GST_SEEK_TYPE_NONE as stop_type and GST_CLOCK_TIME_NONE as stop offset.

In case of reverse playback (rate < 0) the meaning of start and stop is reversed and stop is the position to seek to.

The behaviour of a seek is also wrapped in the gst_element_seek() and gst_element_seek_simple() and you would usually use those functions to initiate a seek on a pipeline.

static void
seek_to_time (GstElement *pipeline,
          gint64      time_nanoseconds)
  if (!gst_element_seek (pipeline, 1.0, GST_FORMAT_TIME, GST_SEEK_FLAG_FLUSH,
                         GST_SEEK_TYPE_SET, time_nanoseconds,
                         GST_SEEK_TYPE_NONE, GST_CLOCK_TIME_NONE)) {
    g_print ("Seek failed!\n");

Seeks with the GST_SEEK_FLAG_FLUSH should be done when the pipeline is in PAUSED or PLAYING state. The pipeline will automatically go to preroll state until the new data after the seek will cause the pipeline to preroll again. After the pipeline is prerolled, it will go back to the state (PAUSED or PLAYING) it was in when the seek was executed. You can wait (blocking) for the seek to complete with gst_element_get_state() or by waiting for the ASYNC_DONE message to appear on the bus.

Seeks without the GST_SEEK_FLAG_FLUSH should only be done when the pipeline is in the PLAYING state. Executing a non-flushing seek in the PAUSED state might deadlock because the pipeline streaming threads might be blocked in the sinks.

It is important to realise that seeks will not happen instantly in the sense that they are finished when the function gst_element_seek () returns. Depending on the specific elements involved, the actual seeking might be done later in another thread (the streaming thread), and it might take a short time until buffers from the new seek position will reach downstream elements such as sinks (if the seek was non-flushing then it might take a bit longer).

It is possible to do multiple seeks in short time-intervals, such as a direct response to slider movement. After a seek, internally, the pipeline will be paused (if it was playing), the position will be re-set internally, the demuxers and decoders will decode from the new position onwards and this will continue until all sinks have data again. If it was playing originally, it will be set to playing again, too. Since the new position is immediately available in a video output, you will see the new frame, even if your pipeline is not in the playing state.

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