GstMeta writers guide?

[jpoet]

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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[Sebastian Dröge-3]

On Fri, 2016-11-11 at 07:56 +0000, John P Poet wrote:

>
> Is there a writers guide for GstMeta somewhere?  I found
>
> https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/h
> tml/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?

There isn't. Please file a bug with those questions so we can add
something to the documentation, or maybe you can also write a patch to
add it there.


a) Transformation: example would be if you have a meta that marks a
specific, absolute position in a video frame. Rescaling or cropping the
frame would require the position in the meta to also change. That's a
transformation and for that the meta's transform function is called
with the appropriate transform parameter.

b) Who does that: the element that performs the transformation should

c) Possible transformation types: copy, the things in gstvideometa.c/h
and they can also be externally defined.

d) Parameter passed in there: depends on the transformation really. It
should be documented with the transformation.


(Note that all this transformation stuff does not matter to your VANC
meta AFAIU)

--
Sebastian Dröge, Centricular Ltd · http://www.centricular.com
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