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Query
-----
Purpose
~~~~~~~
Queries are used to get information about the stream.
A query is started on a specific pad and travels up or downstream.
Requirements
~~~~~~~~~~~~
- multiple return values, grouped together when they make sense.
- one pad function to perform the query
- extensible queries.
Implementation
~~~~~~~~~~~~~~
- GstQuery extends GstMiniObject and contains a GstStructure (see GstMessage)
- some standard query types are defined below
- methods to create and parse the results in the GstQuery.
- define pad method:
gboolean (*GstPadQueryFunction) (GstPad *pad,
GstObject *parent,
GstQuery *query);
pad returns result in query structure and TRUE as result or FALSE when
query is not supported.
Query types
~~~~~~~~~~~
- GST_QUERY_POSITION:
get info on current position of the stream in stream_time.
- GST_QUERY_DURATION:
get info on the total duration of the stream.
- GST_QUERY_LATENCY:
get amount of latency introduced in the pipeline. (See part-latency.txt)
- GST_QUERY_RATE:
get the current playback rate of the pipeline
- GST_QUERY_SEEKING:
get info on how seeking can be done
- getrange, with/without offset/size
- ranges where seeking is efficient (for caching network sources)
- flags describing seeking behaviour (forward, backward, segments,
play backwards, ...)
- GST_QUERY_SEGMENT:
get info about the currently configured playback segment.
- GST_QUERY_CONVERT:
convert format/value to another format/value pair.
- GST_QUERY_FORMATS:
return list of supported formats that can be used for GST_QUERY_CONVERT.
- GST_QUERY_BUFFERING:
query available media for efficient seeking (See part-buffering.txt)
- GST_QUERY_CUSTOM:
a custom query, the name of the query defines the properties of the query.
- GST_QUERY_URI:
query the uri of the source or sink element
- GST_QUERY_ALLOCATION:
the buffer allocation properties (See part-bufferpool.txt)
- GST_QUERY_SCHEDULING:
the scheduling properties (See part-scheduling.txt)
- GST_QUERY_ACCEPT_CAPS:
check if caps are supported (See part-negotiation.txt)
- GST_QUERY_CAPS:
get the possible caps (See part-negotiation.txt)
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