Re: LANG: VRML Transforms (fwd)

Stephen Chenney (schenney@CS.Berkeley.EDU)
Fri, 6 Oct 1995 10:32:43 -0700 (PDT)


> >>>>> "C" == Colin Dooley <cdooley@ibm.net> writes:
> C> The only way to do multiple transformations which makes any sense is:
> C> 1) Scale
> C> 2) Rotate
> C> 3) Translate
> C> Anything else will probably cause confusion at some point.

The order specified above is NOT the only way that makes sense. What makes
sense at any given time depends on what you are trying to achieve. In fact,
enforcing an ordering like that above makes it more difficult to achieve
certain arrangments of objects. In any case, that is not the issue.

> The order in which transformations appear in the VRML file must be
> respected.

Yes, agreed. But there are two directions in which to apply the sequence.
The question is which direction is right.

> In my eyes, this is no special Inventor issue, just basics of Computer
> Graphics Transformations.

The "basics" of transformations make no assumptions about the order of
application. The idea that transformations be applied from the top of the
stack to the bottom is merely one possible method for doing it. It is
in no way fundamental to the use of transformations in general. Hence my
original point: The order MUST be stated in the spec.

The other thing missing from the spec is which way a matrix node is
applied. Is it multiplied on the left or on the right? Hence, does the
translation part go on the bottom row or the right column? The spec says
nothing about this, forcing people to waste their time finding out. And
don't tell me that "everyone does it that way" because not everybody does,
and even if they did it is no excuse for omiting it.

As a final note I would add that it would only take the addition of five
or so lines of text to fix these loopholes, so the issue of "spec bloat"
does not arise.

Steve.


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