Chapter 11. Position tracking and seeking

Table of Contents

Querying: getting the position or length of a stream
Events: seeking (and more)

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);