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docs: update TOC design docs a little

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Tim-Philipp Müller 2012-07-28 11:02:30 +01:00
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54
docs/design/part-toc.txt
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@ -58,15 +58,63 @@ timestamps for each item using gst_toc_entry_set_start_stop().
The best way to process already created GstToc is to recursively go through The best way to process already created GstToc is to recursively go through
the 'entries' and 'subentries' fields. the 'entries' and 'subentries' fields.
2. Working with GstMessage
2. TOC scope: global and current
There are two main consumers for TOC information: applications and elements
in the pipeline that are TOC writers (such as e.g. matroskamux).
Applications typically want to know the entire table of contents (TOC) with
all entries that can possibly be selected.
TOC writers in the pipeline, however, would not want to write a TOC for all
possible/available streams, but only for the current stream.
When transcoding a title from a DVD, for example, the application would still
want to know the entire TOC, with all titles, the chapters for each title,
and the available angles. When transcoding to a file, we only want the TOC
information that is relevant to the transcoded stream to be written into
the file structure, e.g. the chapters of the title being transcoded (or
possibly only chapters 5-7 if only those have been selected for playback/
transcoding).
This is why we may need to create two different TOCs for those two types
of consumers.
Elements that extract TOC information should send TOC events downstream.
Like with tags, sinks will post a TOC message on the bus for the application
with the global TOC, once a global TOC event reaches the sink.
3. Working with GstMessage
If a table of contents is available, applications will receive a TOC message
on the pipeline's GstBus.
A TOC message will be posted on the bus by sinks when the receive a TOC event
containing a TOC with global scope. Elements extracting TOCs should not post
a TOC message themselves, but send a TOC event downstream.
The reason for this is that there may be cascades of TOCs (e.g. a zip archive
containing multiple matroska files, each with a TOC).
GstMessage with GstToc can be created using gst_message_new_toc() and parsed GstMessage with GstToc can be created using gst_message_new_toc() and parsed
with gst_message_parse_toc(). The 'updated' parameter in these methods indicates with gst_message_parse_toc(). The 'updated' parameter in these methods indicates
whether the TOC was just discovered (set to false) or TOC was already found and whether the TOC was just discovered (set to false) or TOC was already found and
have been updated (set to true). The common usage for such message is to post it have been updated (set to true). This message will typically be posted by sinks
to pipeline in case you have discovered TOC data within your element. to pipeline in case you have discovered TOC data within your element.
3. Working with GstEvent
4. Working with GstEvent
There are two types of TOC-related events:
- downstream TOC events that contain TOC information and
travel downstream
- toc-select events that travel upstream and can be used to select
a certain TOC entry for playback (similar to seek events)
GstToc supports select event through GstEvent infrastructure. The idea is the GstToc supports select event through GstEvent infrastructure. The idea is the
following: when you receive TOC select event, parse it with following: when you receive TOC select event, parse it with