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gstreamer/markdown/design/draft-metadata.md

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Metadata

This draft recaps the current metadata handling in GStreamer and proposes some additions.

Supported Metadata standards

The paragraphs below list supported native metadata standards sorted by type and then in alphabetical order. Some standards have been extended to support additional metadata. GStreamer already supports all of those to some extend. This is showns in the table below as either [--],

r-

Audio

Video

Images

further Links:

http://age.hobba.nl/audio/tag_frame_reference.html http://wiki.creativecommons.org/Tracker_CC_Indexing

Current Metadata handling

When reading files, demuxers or parsers extract the metadata. It will be sent a GST_EVENT_TAG to downstream elements. When a sink element receives a tag event, it will post a GST_MESSAGE_TAG message on the bus with the contents of the tag event.

Elements receiving GST_EVENT_TAG events can mangle them, mux them into the buffers they send or just pass them through. Usually is muxers that will format the tag data into the form required by the format they mux. Such elements would also implement the GstTagSetter interface to receive tags from the application.

 +----------+
 | demux    |
sink       src --> GstEvent(tag) over GstPad to downstream element
 +----------+

           method call over GstTagSetter interface from application
                                                          |
                                                          v
                                                    +----------+
                                                    | mux      |
GstEvent(tag) over GstPad from upstream element --> sink       src
                                                    +----------+

The data used in all those interfaces is GstTagList. It is based on a GstStructure which is like a hash table with differently typed entries. The key is always a string/GQuark. Many keys are predefined in GStreamer core. More keys are defined in gst-plugins-base/gst-libs/gst/tag/tag.h. If elements and applications use predefined types, it is possible to transcode a file from one format into another while preserving all known and mapped metadata.

Issues

Unknown/Unmapped metadata

Right now GStreamer can lose metadata when transcoding, remuxing content. This can happend as we don’t map all metadata fields to generic ones.

We should probably also add the whole metadata blob to the GstTagList. We would need a GST_TAG_SYSTEM_xxx define (e.g. GST_TAG_SYSTEM_ID3V2) for each standard. The content is not printable and should be treated as binary if not known. The tag is not mergeable - call gst_tag_register() with GstTagMergeFunc=NULL. Also the tag data is only useful for upstream elements, not for the application.

A muxer would first scan a taglist for known system tags. Unknown tags are ignored as already. It would first populate its own metadata store with the entries from the system tag and the update the entries with the data in normal tags.

Below is an initial list of tag systems: ID3V1 - GST_TAG_SYSTEM_ID3V1 ID3V2 - GST_TAG_SYSTEM_ID3V2 RIFF_INFO - GST_TAG_SYSTEM_RIFF_INFO XMP - GST_TAG_SYSTEM_XMP

We would basically need this for each container format.

See also https://bugzilla.gnome.org/show_bug.cgi?id=345352

Lost metadata

A case slighly different from the previous is that when an application sets a GstTagList on a pipeline. Right elements consuming tags do not report which tags have been consumed. Especially when using elements that make metadata persistent, we have no means of knowing which of the tags made it into the target stream and which were not serialized. Ideally the application would like to know which kind of metadata is accepted by a pipleine to reflect that in the UI.

Although it is in practise so that elements implementing GstTagSetter are the ones that serialize, this does not have to be so. Otherwise we could add a means to that interface, where elements add the tags they have serialized. The application could build one list from all the tag messages and then query all the serialized tags from tag-setters. The delta tells what has not been serialized.

A different approach would be to query the list of supported tags in advance. This could be a query (GST_QUERY_TAG_SUPPORT). The query result could be a list of elements and their tags. As a convenience we could flatten the list of tags for the top-level element (if the query was sent to a bin) and add that.

Tags are per Element

In many cases we want tags per stream. Even metadata standards like mp4/3gp metadata supports that. Right now GST_MESSAGE_SRC(tags) is the element. We tried changing that to the pad, but that broke applications. Also we miss the symmetric functionality in GstTagSetter. This interface is usually implemented by elements.

Open bugs

https://bugzilla.gnome.org/buglist.cgi?query_format=advanced;short_desc=tag;bug_status=UNCONFIRMED;bug_status=NEW;bug_status=ASSIGNED;bug_status=REOPENED;bug_status=NEEDINFO;short_desc_type=allwordssubstr;product=GStreamer

Add GST_TAG_MERGE_REMOVE https://bugzilla.gnome.org/show_bug.cgi?id=560302