[Bug 706340] videofilter: it might make sense to implement the "transform_meta" virtual method

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=706340
  GStreamer | gst-plugins-base | git

--- Comment #5 from Mathieu Duponchelle <mathieu.duponchelle at epitech.eu> 2013-08-21 22:15:40 UTC ---
OK so after discussion on IRC, the resulting decisions and observations are :

The Metadata transform mechanism is not yet completely fixed.
A problem that can and will arise with the copying of metadata when
transforming buffers is that these metadata might not be relevant anymore.

Evident example : you put metadata on video buffers to indicate the position of
something, but a downstream element modifies the shape or size of the image.
For now, the current behaviour is that these metadata get dropped, even though
they are correctly tagged with "orientation" or "size".

The ideal situation would be in the future to let the meta update itself when
needed, with each individual filter calling the transform_func of the meta with
new state informations.

For now, it has been agreed that when a metadata has no tags, then it means it
can be copied without asking the subclass, and furthermore in videofilter, if
the metadata only has the "video" tag, it can be copied as well.

To decide that, a new API was needed in gstmeta, namely gst_meta_get_tags. The
current implementation didn't allow to get all tags.

To make sure not to modify existing behaviour, I had a look at all existing
metas and saw that one of them didn't ha

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