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Query
-----
NOTE: this is implemented as proposed
Purpose
~~~~~~~
Queries are used to get information about the stream.
A query is started on a specific pad and travels up or downstream.
Types of queries
~~~~~~~~~~~~~~~~
- get length of stream
- get position in stream
- get seeking capabilities
- get latency
- convert one value to another
- query supported formats
- query internal links.
Current implementation
~~~~~~~~~~~~~~~~~~~~~~
The current implementation of query requires pads to implement the
following functions:
gboolean (*GstPadConvertFunction) (GstPad *pad,
GstFormat src_format, gint64 src_value,
GstFormat *dest_format, gint64 *dest_value);
gboolean (*GstPadQueryFunction) (GstPad *pad, GstQueryType type,
GstFormat *format, gint64 *value);
GList* (*GstPadIntLinkFunction) (GstPad *pad);
const GstFormat* (*GstPadFormatsFunction) (GstPad *pad);
const GstEventMask* (*GstPadEventMaskFunction) (GstPad *pad);
const GstQueryType* (*GstPadQueryTypeFunction) (GstPad *pad);
Most of these functions are not very extensible in particular,
the queryfunction can only return one value.
Requirements
~~~~~~~~~~~~
- multiple return values, grouped together when they make sense.
- one pad function to perform the query
- extensible queries.
Proposition
~~~~~~~~~~~
- define GstQuery extending GstMiniObject and containing a GstStructure (see GstMessage)
- define standard query types (see proposed types)
- define methods to create a parse the results in the GstQuery.
- define pad method:
gboolean (*GstPadQueryFunction) (GstPad *pad, GstQuery *query);
pad returns result in query structure and TRUE as result or FALSE when
query is not supported.
Proposed types
~~~~~~~~~~~~~~
- 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_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.
- 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.
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