[x3d-public] review of various X3D statement implementations for X3DOM

[Michalis Kamburelis]

2017-12-20 15:51 GMT+01:00 Andreas Plesch <andreasplesch at gmail.com>:
> (DEF/USE in x3dom: There are a several x3dom issues about picking a
> USE Shape. Since x3dom literally places the referenced DEF Shape in
> the scene graph, the picking actually reports the node of the
> referenced DEF Shape when the USE Shape is picked. This is because
> x3dom actually does not keep a distinct representation of the USE node
> at all. Except for the fact that the DEF node now has two parents.
> This may be too literal an interpretation of the DEF/USE concept, and
> I wonder if in the spec. there is a section which explains how the
> node which has a USE reference is still a distinct node. I suspect
> that keeping such a distinct node around is how most x3d browsers work
> ?

This DEF / USE (in x3dom) treatment is required by the X3D
specification, and it (should) be the same in all VRML / X3D browsers.
I know it is the same in view3dscene and Castle Game Engine. When we
encounter a "USE Xxx" then internally we *do not* make a copy of the
"Xxx" node, instead we make an additional link to the "Xxx" node that
in effect has 2 parents. This implementation is most natural and
practically required by the X3D specification:

http://www.web3d.org/documents/specifications/19775-1/V3.2/Part01/concepts.html#DEFL_USESemantics

"""
The USE statement does not create a copy of the node. Instead, the
same node is inserted into the scene graph a second time, resulting in
the node having multiple parents (see 4.3.5 Transformation hierarchy,
for restrictions on self-referential nodes).
"""

And that's a very good concept, in my opinion: it allows to use DEF /
USE mechanism for various optimizations. Since you have only one copy
of the node, you should also be able to have only one copy of various
internal things (like OpenGL VBO data) associated e.g. with Shape that
is USEd multiple times. This is really powerful, you can construct a
large scene by USEing the same shapes multiple times, and it will
still render fast and not eat too much memory:)

In contrast, expanding the PROTOtypes makes a copy on the contents.

Regards,
Michalis