[x3d-public] filling in field default values

[Andreas Plesch]

For X3D importers it is necessary to have knowledge about default values
for any field. This knowledge could be built into an importer.
Alternatively, it is possible to augment an X3D document with explicit
values for all fields. Then any importer can just iterate over the given
fields and see what it can do with them.

Here is an attempt to do this using X3DUOM, with JS on an online notebook:

https://observablehq.com/@andreasplesch/x3d-defaults

It works by recursively visiting all XML scene child nodes, looking up the
X3DUOM definition of it's fields and then adding fields with default values
which are not already present. It is pretty straightforward and seems to
work well enough. It spits out a new js DOM, and new XML as text.

SF/MFNode fields are ignored. Thinking about it, it may be beneficial to
have a NULL value XML representation for such fields:

<Appearance>
  <Material USE='NULL' />
</Appearance>

would be equivalent to

<Appearance/>

(abusing the USE parameter for lack of a better idea; perhaps an XML
comment could also work).

This way an importer would have to know less about Material as a field
value of Appearance. It still would need to know that this means no diffuse
and specular, and full emissive='1 1 1'. So perhaps the defaults processing
should also already do this. I do not think that there are too many other
NULL value defaults with some actual meaning ?

Is there anything else to be aware of ?

Now that I went through this exercise, it should be pretty straightforward
to do the reverse, eg. remove all field attributes which have default
values, for optimization.

Thanks for reading, Andreas

--
Andreas Plesch
Waltham, MA 02453
---on the phone---
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