[x3d-public] [x3dom-users] DEF to ID mapping

[Andreas Plesch]

Leonard,

thanks for the background. This is very helpful. Let me see how I
understand the background.

On Thu, Mar 24, 2016 at 11:37 AM, Leonard Daly <web3d at realism.com> wrote:

> Andreas,
>
> When the XML encoding spec was written (about the turn of the century
> now...) we anticipated the use of 'id' in nodes. It was never used as a
> field name for that reason. [Note: I will use ID instead of 'id' below.]
>

Ok. it was anticipated that an additional id attribute was used in xml
(xhtml) nodes defining a x3d scene ? So there should be a way to make such
a scene spec. conforming ?

http://www.web3d.org/documents/specifications/19776-1/V3.3/Part01/definitions.html

ID is the name of a data type.

http://www.web3d.org/documents/specifications/19776-1/V3.3/Part01/concepts.html#DEFAndUSEAttributeSyntax

DEF is an attribute of type ID.


> It is good practice not to reuse names that must be unique within a
> namescope/namespace, even external to that scope. So while there are no
> explicit provisions preventing it, DEF names and ID values should not
> appear within the same scene; however, I don't think it can be stated that
> it is never the case.
>

Well, currently, conforming x3d scenes cannot have an ID value, right ?
They can just have DEF values of type ID.


>
> DEF functions a little different than ID. DEF is name-scope limited. Each
> X3D occurrence of a file defines its own name scope. Also each PROTO or
> CreateX3dFromString/Url is its name scope. If an X3D file contains
> DEF='foo' and it is Inlined multiple times, each occurrence of 'foo' is
> different from any other occurrence of 'foo'. This is also a historical.
> Until X3DOM, X3D always ran in it's own environment. In X3DOM, it is
> running in the DOM environment. X3DOM takes care to prefix DEF names to
> prevent collisions.
>

Inlining the same file multiple times is a good case to consider. x3d has
IMPORT/EXPORT to avoid collisions, and x3dom has namespacename.


>
> For ID, what is needed is globally (within the HTML page) unique values,
> including any Inlined, generated, or inserted nodes. I don't think it is
> possible to require that across all X3D code. Even if there was some sort
> of international registry, it would still fail if an X3D file was Inlined
> more than once. X3DOM avoids the name conflict issue by requiring the
> developer to create a prefix.
>

It is necessary for ID to be unique on the web page (globally). Both x3d
and x3dom have mechanisms to make this easy for scene or web page authors.
In theorym x3dom could implement IMPORT/EXPORT instead of using
namespacename.
It may a good idea to adopt namespacename in the x3d spec. as well.


> I think it would be good to only have a single node label. If you have
> different values for DEF and ID, there is always the confusion as to which
> one goes (or does) what. If they are the same value, why have two labels.
>
>
Completely agree. It does not make sense to have two labels for the same
function.


> Perhaps DEF can be deprecated and ID be used going forward for all
> encodings. If both are specified, then ID overrides DEF.
>
>
Yes, this is the (big) question. If not for backward compatibility, I think
simply stop using DEF and instead using ID would help a lot.
Perhaps, it is time take this fork in the road. The problem is it would
make all x3d scenes which use DEF invalid. Replacing all occurrences of DEF
with ID is not difficult, howevver.

Another interim solution may be to define an implicit ID attribute in the
spec. which always has the value of the DEF field.

To make this discussion more concrete, here is a proposal:

http://www.web3d.org/documents/specifications/19776-1/V3.3/Part01/concepts.html#NodeAndFieldStatementSyntax

could be amended by:

A node instance can be given a label using the attribute DEF followed by an
equals sign and the quoted name of the node, as provided by the DEF
statement. In addition, for DOM compatibility reasons, an additional
attribute with the name ID is expressed implicitly which has the same
value. This ID attribute is not part of the x3d scene graph but it is part
of the fully parsed xml data structure and allows access to the element by
the DOM API.

Feel free to criticize this proposal. Hopefully, the process can be
constructive.

-Andreas
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