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application-development: index: improve content and formatting

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Reynaldo H. Verdejo Pinochet 2017-09-11 16:36:07 -07:00
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@ -4,39 +4,36 @@ title: Metadata
# Metadata # Metadata
GStreamer makes a clear distinction between two types of metadata, and GStreamer makes a clear distinction between the two types of metadata it
has support for both types. The first is stream tags, which describe the supports: Stream tags, which describe the content of a stream in a non-technical
content of a stream in a non-technical way. Examples include the author way; and Stream-info, which is a somewhat technical description of the
of a song, the title of that very same song or the album it is a part properties of a stream. Stream tags examples include the author of a song, the
of. The other type of metadata is stream-info, which is a somewhat song's title or the album it belongs to. Examples for stream-info include video
technical description of the properties of a stream. This can include size, audio samplerate, codecs used and so on.
video size, audio samplerate, codecs used and so on. Tags are handled
using the GStreamer tagging system. Stream-info can be retrieved from a Tags are handled using the GStreamer tagging system. Stream-info can be retrieved
`GstPad` by getting the current (negotiated) `GstCaps` for that pad. from a `GstPad` by getting the current (negotiated) `GstCaps` for that pad.
## Metadata reading ## Metadata reading
Stream information can most easily be read by reading it from a Stream information can most easily be obtained by reading it from a `GstPad`.
`GstPad`. This has already been discussed before in [Using capabilities Note that this requires access to all pads of which you want stream information.
for metadata](application-development/basics/pads.md#using-capabilities-for-metadata). This approach has already been discussed in [Using capabilities for metadata](application-development/basics/pads.md#using-capabilities-for-metadata).
Therefore, we will skip it here. Note that this requires access to all Therefore, we will skip it here.
pads of which you want stream information.
Tag reading is done through a bus in GStreamer, which has been discussed Tag reading is done through a bus in GStreamer. You can listen for
previously in [Bus](application-development/basics/bus.md). You can listen for `GST_MESSAGE_TAG` messages and handle them as you wish. This has been discussed
`GST_MESSAGE_TAG` messages and handle them as you wish. previously in [Bus](application-development/basics/bus.md).
Note, however, that the `GST_MESSAGE_TAG` message may be fired multiple Note, however, that the `GST_MESSAGE_TAG` message may be emitted multiple
times in the pipeline. It is the application's responsibility to put all times and it is the application's responsibility to aggregate and display the tags
those tags together and display them to the user in a nice, coherent in a coherent way. This can be done using `gst_tag_list_merge ()` but make sure
way. Usually, using `gst_tag_list_merge ()` is a good enough way of to empty the cache when loading a new song, or after every few minutes when
doing this; make sure to empty the cache when loading a new song, or listening to internet radio. Also, make sure you use `GST_TAG_MERGE_PREPEND` as
after every few minutes when listening to internet radio. Also, make merging mode, so that a new title (which came in later) has precedence over
sure you use `GST_TAG_MERGE_PREPEND` as merging mode, so that a new the old one.
title (which came in later) has a preference over the old one for
display.
The following example will extract tags from a file and print them: The following example shows how to extract tags from a file and print them:
``` c ``` c
/* compile with: /* compile with:
@ -167,16 +164,15 @@ main (int argc, char ** argv)
Tag writing is done using the Tag writing is done using the
[`GstTagSetter`](http://gstreamer.freedesktop.org/data/doc/gstreamer/stable/gstreamer/html/GstTagSetter.html) [`GstTagSetter`](http://gstreamer.freedesktop.org/data/doc/gstreamer/stable/gstreamer/html/GstTagSetter.html)
interface. All that's required is a tag-set-supporting element in your interface. All that's required is a tag-set-supporting element in your
pipeline. In order to see if any of the elements in your pipeline pipeline.
supports tag writing, you can use the function
`gst_bin_iterate_all_by_interface (pipeline,
GST_TYPE_TAG_SETTER)`. On the resulting element, usually an encoder or
muxer, you can use `gst_tag_setter_merge
()` (with a taglist) or `gst_tag_setter_add
()` (with individual tags) to set tags on it.
A nice extra feature in GStreamer tag support is that tags are preserved In order to see if any of the elements in your pipeline supports tag writing,
you can use the function `gst_bin_iterate_all_by_interface (pipeline, GST_TYPE_TAG_SETTER)`.
On the resulting element, usually an encoder or muxer, you can use
`gst_tag_setter_merge ()` with a taglist or `gst_tag_setter_add ()` with
individual tags, to set tags on it.
A nice extra feature in GStreamer's tag support is that tags are preserved
in pipelines. This means that if you transcode one file containing tags in pipelines. This means that if you transcode one file containing tags
into another media type, and that new media type supports tags too, then into another tag-supporting media type, then the tags will be handled as part of
the tags will be handled as part of the data stream and be merged into the data stream and will be so merged into the newly written media file.
the newly written media file, too.