gstreamer/docs/design/draft-query.txt
Wim Taymans a7f3844adf docs/design/: Docs updates.
Original commit message from CVS:
* docs/design/draft-ghostpads.txt:
* docs/design/draft-push-pull.txt:
* docs/design/draft-query.txt:
* docs/design/part-overview.txt:
Docs updates.
Added general overview doc.
Added draft ghostpad replacement idea.
2005-05-24 16:47:06 +00:00

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Text

DRAFT Query
-----------
Status
Implemented, move me to design...
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 GstData 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
- start position
- current position
- end position
- length
- GST_QUERY_LATENCY:
- get amount of buffering
- GST_QUERY_CONVERT:
- convert format/value to another format/value pair.
- GST_QUERY_FORMATS:
- return list of supported formats.
Also????
- GST_QUERY_CAPS: