[gst-devel] sending extra data along with buffers

[Jan Schmidt]

<quote who="Stefan Kost">

> hi,
>
> I am looking for a way to send some private data along with a buffer. 
> During data capture I get some metadata about capture settings and I would 
> like to store/analyze them later along with the data stream. I tried to add 
> a payload item to caps and put a GstBuffer there. But this seems to be 
> painful and I am afraid that it will cause renegotiation as the content 
> changes for each buffer. Has anyone been doing something similar?
>
> alternative (better approach) seems to be using a buffer subclass, right?
>
> Attached is my current test. Its a bit hackish and fails. One thing I was 
> wondering is that fakesrc send buffer with caps=NULL. A bug?
>

>From time to time, I've thought about adding something like this directly
to GstBuffer, for carrying information whose presence been negotiated in
the caps, but whose content should be allowed to change without
renegotiating.

Examples:
  * Pan and scan (video subregion) position information
  * interlacing markers (top field, bottom field)
  * theora granulepos, instead of slapping it in the 'offset' field
  * MPEG Decoding timestamps, which we currently never send along.

My thought was to add a GstStructure to GstBuffer which is allowed to be
NULL.

Wim has expressed concern that it would cause performance issues, and it
would certainly require extending various bits of GStreamer to support
properly (buffer serialisation/deserialisation in GDP, for example). Also,
the semantics of how to handle the 'extradata' when transforming a buffer
need to be clearly defined, ie the extradata has to be clearly specified
by the caps attached to the buffer, and there needs to be a way for it to
be handled by elements that don't understand the extradata they've been
given. It can't just be a dumping ground in which to place arbitrary data,
or I don't think it can be made to work.

J.