Handling object groups in collada exporter/importer
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Description

In Blender a group is just a collection of objects unrelated to the object relationship. Apparently Group instances are made of an empty plus instances of the group objects. This blend file contains a group made of 3 simple objects and one group instance:

group_export.blend116 KBDownload

The Suzanne group in this file is made of 3 objects: "Suzanne", "Pole" and "Base"
When exporting this we can make use of Collada library nodes as follows:

'''
<library_nodes>

<node id="Group-Suzanne" type="NODE">
    <node id="Suzanne" type="NODE">
        <instance_geometry url="#Suzanne-mesh"/> 
    </node>
    <node id="Pole" type="NODE">
        <instance_geometry url="#Pole-mesh"/>
    </node>
    <node id="Stand" type="NODE">
        <instance_geometry url="#Stand-mesh"/>
    </node>
</node>

</library_nodes>

<library_visual_scenes>

<visual_scene id="Scene">
    <node id="Suzanne-Instance" type="NODE">
        <instance_node url="#Suzanne-instance"/> <!-- placed at the location of the instance -->
    </node>
</visual_scene>

</library_visual_scenes>

'''

When importing this, then basically the blend file is reconstructed like the original with one group made of the library nodes and one group instance referring to the group.

caveats

What to do when the user has selected the group instance and the objects in the group.
How to handle the situation where objects are in multiple groups.
Should we also take care of the layers where each object is placed? I think that is doable as well.

Is there anything else to consider? Opinions and suggestions are welcome :)

Thanks
Gaia