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<div class="moz-cite-prefix">The original purpose (and still used in
this manner) of the 'USE' attribute is to indicate that another
node should also appear in place of the node declaring 'USE'. In
fact the specification states (4.4.3 -
<a class="moz-txt-link-freetext" href="http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/concepts.html#DEFL_USESemantics">http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/concepts.html#DEFL_USESemantics</a>)
that "the same node is inserted into the scene graph a second
time, resulting in the node having multiple parents".<br>
<br>
This requirement is not allowed in DOM (see
<a class="moz-txt-link-freetext" href="https://www.w3.org/TR/dom/#concept-node-tree">https://www.w3.org/TR/dom/#concept-node-tree</a> for the standard,
<a class="moz-txt-link-freetext" href="https://www.w3.org/wiki/Traversing_the_DOM#Nodes">https://www.w3.org/wiki/Traversing_the_DOM#Nodes</a> for the
explanation). A DOM element is allowed to have at most one parent.
It is possible to create a (deep) copy of the node and insert it
into the tree. That gives a structure like:<br>
<br>
<tt>[Meant to be seen in fixed width font]</tt><br>
<br>
<font size="+1"><tt><font size="+1"><tt> B - C - D<br>
</tt></font></tt></font><font size="+1"><tt><font
size="+1"><tt><font size="+1"><tt>/ <br>
</tt></font></tt></font>A <br>
\ <br>
E -CC -DD<br>
<br>
</tt></font>Where A is the parent of this (sub-)tree, B is the
node that start one branch (e.g., Transform). C is the 'DEF'ed
node with a child of D. E is a separate child of A (e.g., a
different Transform) CC is the 'USE' version of C. Since HTML does
not allow multiple parents ('B' and 'E'), a copy of 'C' needs to
be made. This needs to be a deep copy (all children) as no node
can have more than one parent.<br>
<br>
The problem with a deep copy is that it is non-deterministic as
the element is self-referential (it refers to it's parent, which
refers to it...)<br>
<br>
It seems to me that there is a conflict in requirements between
X3D's statement on DEF/USE and the requirement to put all X3D
nodes in the DOM. There are several ways around this:<br>
1) Remove the multiple parent requirement from DEF/USE<br>
2) Remove the requirement of all nodes being in the DOM.<br>
<br>
Each has advantages and disadvantages. Which choice is made
determines how X3D operates in the HTML/DOM environment.<br>
<br>
<br>
Leonard Daly<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
</div>
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<p class="MsoNormal">Hi all,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Recently we updated the DTD/schemas to make
the “name” field of nodes like MetadataBoolean, or
FloatVertexAttribute a required field. However, we then
realised that when any of these nodes has the “USE” attribute,
the “name” field must not be set. Hence the changes needed to
be revisited.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I started to look at the JSON schema for
the MetadataBoolean node, to try to implement this stricter
validation requirement. With some online assistance I found
that I could easily make either one or the other required, but
not both. This would meet the original requirement.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">However, this raised a more general
question in my mind. When any node has the “USE” attribute
set, what other fields/attributes are permitted?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">As a test case, I concentrated on the
MetadataBoolean node. I came up with a JSON schema that might
illustrate this better. I have attached a snapshot image for
this fragment of the JSON schema.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The validation of the MetadataBoolean node
begins with a “one of” operator (shown immediately to the
right of the MetadataBoolean box. This operator requires that
one, and only one, of the two subschemas be valid. For the
first subschema (the upper of the two) I simply removed the
“@USE” property, making the “@name” field required (to meet
the original requirement). For the second subschema (the lower
of the two) I made the “@USE” property required, and only
added the “IS” property. Note that both subschemas only permit
those properties listed (i.e. additional properties are
disallowed).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In principle, there should be no difficulty
applying this validation methodology within the JSON schema to
all nodes.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Some questions:<o:p></o:p></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph"
style="margin-left:0cm;mso-list:l0 level1 lfo1">Is this
validation methodology correctly aligned with the standards?<o:p></o:p></li>
<li class="MsoListParagraph"
style="margin-left:0cm;mso-list:l0 level1 lfo1">Do we want
to apply this methodology to all nodes?<o:p></o:p></li>
<li class="MsoListParagraph"
style="margin-left:0cm;mso-list:l0 level1 lfo1">Do we want
to apply this methodology to other validation tools, e.g.
Schematron, and also consider whether the XML schema or the
DTD have sufficient expressive power too.<o:p></o:p></li>
</ol>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">All comments appreciated,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">All the best,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Roy<o:p></o:p></p>
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<pre wrap="">_______________________________________________
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<p><br>
</p>
<div class="moz-signature">-- <br>
<font class="tahoma,arial,helvetica san serif" color="#333366">
<font size="+1"><b>Leonard Daly</b></font><br>
3D Systems & Cloud Consultant<br>
LA ACM SIGGRAPH Chair<br>
President, Daly Realism - <i>Creating the Future</i>
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