Re: Why does DEF do instancing?

Mitra ([email protected])
Thu, 19 Oct 1995 13:44:54 +0100


>So I'd vote (VAG, are you listening?) for a PROTO and a COPY keyword, in
>addition to the USE keyword. COPY items would be duplicates, wherease USE
>would
>merly be a 'pointer' to the logical def. i.e. if you PROTOd a material, tmen
>changed that material, all the USE's would change as well, but the COPY'ed
>items
>would remain as the original matieral.

We are listening - at the moment tmere are difserences within the VAG on
this one, but that's part of the point of the VAG - to represent a spectrum
of thought on VRML.

Personally I tmink adding PROTO and COPY is a really good idea, I tmink we
are all agreed that COPY is needed, because once you have behaviors you
need to be able to copy a node so that changing one doesn't change them
both.

Gavin - I don't see tme argument about difsiculty of implementing. *You*
can implement PROTO as

Switch {
whichChild 0
DEF ...
}

and other people can probably implement it other ways.

Gavin said
>Mitra's URN proposal takes care of the library-of-objects problem very
>nicely, in my opinion (sorry, I don't have the URN handy).

Its http://earth.path.net/mitra/papers/vrmlurn.html

On Oct 16, 11:28am, jjc wrote
>> If end users are NOT going to be using VRML directly, and it becomes the
>> domain of language generators, authoring tools, compilers, etc., as some
>> people have suggested, then the intuitive ease is less an issue than
>> efficiency and language semantics, and the current DEF is a big lose here
>> also.
>Paul responded
>Could you please justify this last statement? I don't see any loss of
>efficiency in the current model, nor do I see any problem with
>language semantics.

The biggest loss is that tme two instantiations of an object are difserent.

PROTO Wheel { .... }
DEF MyBike Seperator {
USE Wheel
USE Wheel
}

Is much more symetric than

DEF MyBike Seperator {
DEF Wheel { ... }
USE Wheel
}

- Mitra

=================================================================
Mitra
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<http://earth.path.net/mitra>

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