Extend Collada Node elements to add bone tails - collada

Question

I want to write a Collada-1.4 exporter for exporting a Skeleton. I need to extend the Collada format for defining some additional information:
bone tail (relative to joint location)
bone roll (relative to longitudinal axis)
bone connect state (bone tail connected to parent joint)
The best i can think of is to use an element and add a tool-specific profile, but the documentation is not clear about how to do this exactly.
Here is what i guess is a correct working example:
<node id="Armature" name="Armature" type="NODE">
<matrix sid="transform">
1 0 0 0.1151489
0 1 0 0.01073149
0 0 1 1.730716
0 0 0 1</matrix>
<node id="A" name="A" sid="A" type="JOINT">
<matrix sid="transform">1 0 0 0 0 0 -1 0 0 1 0 0 0 0 0 1</matrix>
<node id="B" name="B" sid="B" type="JOINT">
<matrix sid="transform">
-0.7919466 -0.4411024 0.422196 0
8.9407e-8 0.6914554 0.7224193 1
-0.6105905 0.5721177 -0.5475955 0
0 0 0 1
</matrix>
<extra>
<technique profile="blender">
<!-- =============================== -->
<!-- Bone tail offset from bone head -->
<!-- =============================== -->
<float_array name="tail" sid="tail" count="3">
0.0 0.0 1.0
</float_array>
</technique>
<technique_common/>
</extra>
</node>
</node>
</node>
However i have some questions:
Is this an acceptable method to provide the bone tail information?
Can i use a technique_common element as alternative to defining the profile?
Do i have to define an empty technique_common element or can i skip that?
is it ok to use sid="tail" for every bone tail in the entire collada file?
Is there a less verbose way to define the bone tail information?
Can i use the profile in a more specific way? Like for example:
<node>
...
<extra>
<technique profile="blender">
<tail type="float_array" connect="true">0 0 1.0</tail>
<roll type="float">0</roll>
</technique>
<technique_common/>
</extra>
</node>
I am not sure if i am allowed to define new elements (tail, role, connect) as shown in the example above. Can i do this?
I am also aware that the importer must be aware of the extra data (support the blender profile) to find the bone tail info. So if a tool does not know the blender profile it won't recognize the additional bone information, but that is OK for me.

Answer 1

I can try to answer your questions.
If you read the <extra> tag from Collada specification, you can
write any XML-conform data under <technique> or
<technique_common>. So for your first question, yes it is
acceptable. If you have any schema defined for your XML elements, you
can give it with xmlns attribute in the technique.
Yes, you can choose <technique_common> element, but it is better to give the profile information in your case. I would prefer to use
<technique> and give the profile information as you have already
done.
You don't need to put <technique_common> element as a child for <extra>, if you have no information to give there.
sid should be unique among the document. So it is not OK. You can try to number the tails consecutively.
Bone tail information is more about the semantics. I cannot answer that.
I think how you did it in second example is better than the first one. But you are the one to use it for data exchange. You should think about how the importer should interpret this data. I cannot help you further without knowing the intern structure of the importer.