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>Chapter 11. Position tracking and seeking</H1
><P
> 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. <SPAN
CLASS="application"
>GStreamer</SPAN
> has built-in support for doing all this
using a concept known as <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>querying</I
></SPAN
>. Since seeking
is very similar, it will be discussed here as well. Seeking is done
using the concept of <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>events</I
></SPAN
>.
</P
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NAME="section-querying"
>11.1. Querying: getting the position or length of a stream</A
></H1
><P
> 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
<CODE
CLASS="function"
>gst_element_query ()</CODE
>, although some convenience
wrappers are provided as well (such as
<CODE
CLASS="function"
>gst_element_query_position ()</CODE
> and
<CODE
CLASS="function"
>gst_element_query_duration ()</CODE
>). You can generally
query the pipeline directly, and it'll figure out the internal details
for you, like which element to query.
</P
><P
> Internally, queries will be sent to the sinks, and
<SPAN
CLASS="QUOTE"
>"dispatched"</SPAN
> 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.
</P
><PRE
CLASS="programlisting"
>
#include <gst/gst.h>
static gboolean
cb_print_position (GstElement *pipeline)
{
GstFormat fmt = GST_FORMAT_TIME;
gint64 pos, len;
if (gst_element_query_position (pipeline, &fmt, &pos)
&& gst_element_query_duration (pipeline, &fmt, &len)) {
g_print ("Time: %" GST_TIME_FORMAT " / %" GST_TIME_FORMAT "\r",
GST_TIME_ARGS (pos), GST_TIME_ARGS (len));
}
/* call me again */
return TRUE;
}
gint
main (gint argc,
gchar *argv[])
{
GstElement *pipeline;
[..]
/* run pipeline */
g_timeout_add (200, (GSourceFunc) cb_print_position, pipeline);
g_main_loop_run (loop);
[..]
}
</PRE
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