GstMeta writers guide?

[John P Poet]

Is there a writers guide for GstMeta somewhere?  I found

https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/gstreamer-GstMeta.html

But I need more than just a description of the API.  For example
GstMetaTransformFunction() says:


Function called for each *meta* in *buffer* as a result of performing a
transformation on *transbuf* . Additional *type* specific transform data is
passed to the function as *data* .

Implementations should check the *type* of the transform and parse
additional type specific fields in *data* that should be used to update the
metadata on *transbuf* .

Okay, but...
*What* is a tranformation?
*What* would invoke a transformation?
*What* are the possible transformation types, and what do they mean?
*What* is passed in data?  It says 'type specific' but what does that mean?

Is there a "writers guide" for GStreamer anywhere that answers questions
like this?

Thanks,

John
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