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>Chapter 13. Types and Properties</H1
><P
> There is a very large set of possible types that may be used to pass data
between elements. Indeed, each new element that is defined may use a new
data format (though unless at least one other element recognises that
format, it will be most likely be useless since nothing will be able to
link with it).
</P
><P
> In order for types to be useful, and for systems like autopluggers to
work, it is neccessary that all elements agree on the type definitions,
and which properties are required for each type. The <SPAN
CLASS="application"
>GStreamer</SPAN
> framework
itself simply provides the ability to define types and parameters, but
does not fix the meaning of types and parameters, and does not enforce
standards on the creation of new types. This is a matter for a policy to
decide, not technical systems to enforce.
</P
><P
> For now, the policy is simple:
<P
></P
><UL
><LI
><P
> Do not create a new type if you could use one which already exists.
</P
></LI
><LI
><P
> If creating a new type, discuss it first with the other <SPAN
CLASS="application"
>GStreamer</SPAN
>
developers, on at least one of: IRC, mailing lists, the <SPAN
CLASS="application"
>GStreamer</SPAN
>
wiki.
</P
></LI
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> Try to ensure that the name for a new format is as unlikely to
conflict with anything else created already, and is not a more
generalised name than it should be. For example: "audio/compressed"
would be too generalised a name to represent audio data compressed
with an mp3 codec. Instead "audio/mp3" might be an appropriate name,
or "audio/compressed" could exist and have a property indicating the
type of compression used.
</P
></LI
><LI
><P
> Ensure that, when you do create a new type, you specify it clearly,
and get it added to the list of known types so that other developers
can use the type correctly when writing their elements.
</P
></LI
></UL
>
</P
><DIV
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><A
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>13.1. Building a Simple Format for Testing</A
></H1
><P
> If you need a new format that has not yet been defined in our <A
HREF="section-types-definitions.html"
>List of Defined Types</A
>, you will want to have some general
guidelines on mimetype naming, properties and such. A mimetype would
ideally be one defined by IANA; else, it should be in the form
type/x-name, where type is the sort of data this mimetype handles (audio,
video, ...) and name should be something specific for this specific type.
Audio and video mimetypes should try to support the general audio/video
properties (see the list), and can use their own properties, too. To get
an idea of what properties we think are useful, see (again) the list.
</P
><P
> Take your time to find the right set of properties for your type. There
is no reason to hurry. Also, experimenting with this is generally a good
idea. Experience learns that theoretically thought-out types are good,
but they still need practical use to assure that they serve their needs.
Make sure that your property names do not clash with similar properties
used in other types. If they match, make sure they mean the same thing;
properties with different types but the same names are
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>not</I
></SPAN
> allowed.
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