# Re: Standardizing coordinate systems, units of measure

Linas Vepstas (linas@innerdoor.austin.ibm.com)
Tue, 2 Aug 1994 20:16:00 -0500

> From: "Gavin Bell" <gavin@krypton.engr.sgi.com>
> Date: Tue, 2 Aug 1994 13:50:29 -0700
> Subject: Standardizing coordinate systems, units of measure
>
> I vote for meters being the standard of measure (that is the OI
> convention, and works well for most real-world objects). I think
> millimeters are too small, kilometers are too big, and not using the
> metric system would be stupid.

I second this motion.

> We should also decide which way is up. I like the OpenGL coordinate
> system, which has X increasing to the right, Y up, and Z coming out of
>
> Non-symmetric objects that move should be defined so that increasing
> their X position makes them look like they are moving forward. For
> example, if modelling a car, the headlights should shine down the +X
> axis and if you look out the sun roof you should be looking in the +Y
> direction.

Going with OpenGL does have some advantages ... except that most
"ordinary" (what ever that is) people think of x to the right, y
is forward, and z is up. Certainly, that's what is taught in grade
school, high school, and college ....

> Date: Tue, 2 Aug 1994 14:37:23 PDT
> From: Mike Roberts <miker@nashua.progress.COM>
> Subject: Re: Standardizing coordinate systems, units of measure
>
> I also think "inside space" co-ordinate systems and "outside space"
> co-ordinate systems are different things. What about the galactic model
> sitting, fishtank like, on my bookshelf ? On the outside it is quite small, but
> inside .. really huge ...

Mike,
That's exactly the point that Gavin Bell is making. Transmitted together
with the galaxy model is a transform matrix that says "divide all
dimensions by 365 x 24 x 3600 x 3 x 10^8", so that your galactic model
can be expressed in light-years, despite that fact that it is a mere
one meter off the floor in your living room. That's the whole point
of having "modelling matrices".

--linas