Re: Re(2): VR-YART and VRML

Linas Vepstas (
Tue, 1 Nov 1994 18:31:23 -0600

> Date: Thu, 27 Oct 94 09:05:02 +0100
> From: Ekkehard.Beier@Prakinf.TU-Ilmenau.DE (Ekkehard Beier)
> To: yart@metallica
> Subject: Re(2): VR-YART and VRML
> Hi,
> >> Fourth --
> >> You need to understand that Inventor and WebOOGL file formats are very
> >> different in philosophy from yart. Both are really "scene description
> >> langauges", and not proceedural or object-oriented languages like yart,
> >> and so many of the problems these languages have, and many of the
> >> solutions, are quite different than Tcl - type languages.
> Why is YARTs Tcl API not usable as scene description language?
> Please enlight me.

I'm sorry, I used the wrong words. I meant to say, "both are
scene declaration languages"; yes, you can use Yart/Tcl for a scene

Note that Tcl is a procedural, not a declarative language. Some
(including myself) had voiced concern that this could cause
editability, performance, functional, extendability, and security problems.
However, I am trying to understand a bit more, there are some aspects
of Tcl that might be very nice, esp. for behaviours.

> In my eyes such scene language could easyly specificated by:
> - using the Tcl API of the graphics kernel
> - using the class system
> - limiting the use of Tcl to set, for, if, while, ... (i.e. no
> usage of exec, file related commands etc.)


> For objects that need data files for their creation (OFF polygons,
> images, ...) there have to be specified additional mechanisms
> (load path, etc.).

Yes. There have been long discussions of this on the vrml mailing list.

> For YART extensions (scientifc class lib) there could be defined
> related scene description extensions.
> >> I do beleive, however, that animated, interacting, multi-presence,
> >> multi-user, distributed vrml integrated with www is possible with either
> >> inventor/weboogl style languages, or with yart/tcl style languages.
> >> The questions are:
> On very important feature of the YART solution is, that you can
> do incremental scene changes via ASCII commands (here Tcl).
> This reduces the network costs drastically.

Yes. Important point.

> E.g.: When a new user logs into the scene or someone changes anything
> in the scene there will be sent Tcl commands to all mirror
> scenes. These are typically less than 1KByte. As I know Inventor
> and other systems can (initially) read a complete scene descrip-
> tion and thats it.
> >> And when you do this, be sure to announce to the vrml group.
> Do you know its email address?

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> Bye,
> Ekki
> ****************************************************************
> * Ekkehard 'Ekki' Beier *
> * email: *
> * phone: ++49-3677-691243 fax: ++49-3677-691201 *
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> * Technical University of Ilmenau *
> * Department of Computer Graphics *
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