Re: Standardizing coordinate systems, units of measure

D. Owen Rowley (
Wed, 3 Aug 1994 10:59:34 -0700

> From: "Gavin Bell" <>
> On Aug 2, 2:37pm, Mike Roberts wrote:
> > instance, I am thinking of the space which contains an object
> defining the
> > scale units of the objects contained within it.

> Sure, but I think it is important for it to define the scale relative
> to an absolute, agreed-upon scale.
> The goal is to be able to take your teapot, put it on my table next to
> their cup and saucer, and have everything be the right size without any
> user intervention.

I'm glad to see that my argument for this issue took hold, I kinda thought
I was getting resistance at the BOF.

> > example, should I choose to implement a model of the local galactic
> > cluster, I might find meters a touch on the small side

> Easy to support-- just put a Scale that transforms from meters into
> light-years at the top of your scene.

So why can't we abstarct the units at the higher level if we are prepared
to allow abstarction at lower levels ?

> So, imagine a user visits your galactic "page" (we do need jargon for
> this...). How does the VRML browser know that navigation gestures
> (e.g. move the mouse forward to walk into the world) should move in
> light-year-size steps?

> I tend to think of this as the "size of observer" problem-- if I'm
> looking at a galaxy, I'd better have legs that are light years long to
> move me around fast enough to see anything interesting. So, I'd be
> inclined to encode the size of the observer into the camera's
> information (side note: if using Inventor, I'd wedge this into the
> focalDistance field of the camera, reasoning that when walking around I
> tend to focus farther ahead than an ant).

In CDK we found that we had to utilise different viewere movemeont modalitys
for different situations. target viewing can be implemented which moves an
adaptive distance each step depending on the distance of the target.
platform viewing moves a set distance each step, etc.

> Again, having a default coordinate system of real-world coordinates
> makes it easy for the common case where the virtual observer in the
> virtual world is the same size as real people in the real world
> (molecule and galaxy visualizations are exceptions, of course).

of course.

objects are objects ..... after all.


LUX ./. owen