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108 lines
3.1 KiB
Markdown
108 lines
3.1 KiB
Markdown
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# Tagreading
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The tagreading (metadata reading) use case for mediacenter applications
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is not too well supported by the current GStreamer architecture. It uses
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demuxers on the files, which generally said takes too long (building
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seek-index, prerolling). What we want is specialized elements / parsing
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modes that just do the tag-reading.
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The idea is to define a TagReadIFace. Tag-demuxers, classic demuxers and
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decoder plugins can just implement the interface or provide a separate
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element that implements the interface.
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In addition we need a tagreadbin, that similar to decodebin does a
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typefind and then plugs the right tagread element(s). If will only look
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at elements that implement the interface. It can plug serval if
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possible.
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For optimal performance typefind and tagread could share the list of
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already peeked buffers (a queue element after sink, but that would
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change pull to push).
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## Design
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The plan is that applications can do the following: pipeline = "filesrc
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\! tagbin" for (file\_path in list\_of\_files) {
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filesrc.location=file\_path pipeline.set\_state(PAUSED) // wait for TAGS
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& EOS pipeline.set\_state(READY) }
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- it should have one sinkpad of type ANY
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- it should send EOS when all metadata has been read "done"-signal
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from all tagread-elements
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- special tagread-elements should have RANK\_NONE to be not
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autoplugged by decodebin
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## Interface
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- gboolean iface property "tag-reading" Switches the element to
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tagreading mode. Needed if normal element implement that behaviour.
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Elements will skip parsing unneeded data, don’t build a seeking
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index, etc.
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- signal "done" Equivalent of EOS.
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## Use Cases
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- mp3 with id3- and apetags
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- plug id3demux \! apedemux
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- avi with vorbis audio
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- plug avidemux
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- new pad → audio/vorbis
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- plug vorbisdec or special vorbiscomment reader
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## Additional Thoughts
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- would it make sense to have 2-phase tag-reading (property on tagbin
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and/or tagread elements)
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- 1st phase: get tag-data that are directly embedded in the data
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- 2nd phase: get tag-data that has to be generated
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- e.g. album-art via web, video-thumbnails
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- what about caching backends
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- it would be good to allow applications to supply tagbin with a
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tagcache- object instance. Whenever tagbin gets a *location* to
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tagread, it consults the cache first. whenever there is a cache-miss
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it will tag-read and then store in the
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cache
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``` c
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GstTagList *gst_tag_cache_load_tag_data (GstTagCache *self, const gchar *uri);
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gst_tag_cache_store_tag_data (GstTagCache *self, const gchar *uri, GstTagList *tags);
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```
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## Tests
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- write a generic test for parsers/demuxers to ensure they send tags
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until they reached PAUSED (elements need to parse file for
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prerolling anyway): set pipeline to paused, check for tags, set to
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playing, error out if tags come after paused
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## Code Locations
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- tagreadbin → gst-plugins-base/gst/tagread
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- tagreaderiface → gst-plugins-base/gst-libs/gst/tag
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## Reuse
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- ogg : gst-plugins-base/ext/ogg
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- avi : gst-plugins-good/gst/avi
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- mp3 : gst-plugins-good/gst/id3demux
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- wav : gst-plugins-good/gst/wavparse
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- qt : gst-plugins-bad/gst/qtdemux
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