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Subject: Scaling the universe
Date: Tue, 18 Jul 95 06:36:00 PDT
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     The following issue is being discussed regarding
     the Reality Labs Rendermorphics system.  I'm curious
     as to whether or not any LW user may have come
     accross a similar problem?  I've allready thought of
     an alternative solution (which only visually solves the
     problem).  Any advice?


"I'm curious about the mathmatical limitations of Reality Labs. Since
all   inputs are doubles, your range is accurate only to 15 digits.

How is the following math handled:

I'm the center of our solar system, and I'm using 1 to represent 1 m.

This means that my effective range that I can track an object is
999,999,999,999,999 meters, which is 2/3 of 1 au (not even the
distance   from the sun to the earth).

Say, then, I really don't care about tracking the accuracy of an
object   to within 1 meter beyond 2/3 of an au (BTW: au=Astromomical
Unit, the   distance from the Earth to the Sun, 150 billion km).

So, I put these objects way out there... (just to make up a number)
5.5x10^100 (10 to the 100th power). Way out there.

So, I stick this planet out there.

There's two tricky parts to the problem: 1. Gee. I want to add this
moon.   I don't really care that this object is really out there
really far. If I   want to add a moon, and have it obit the planet,
will the moon be tracked   properly given a basic eliptical path
around the planet.

I guess there's only 2 tricky parts, 'cause question 2 is the cause of
  question 1. How can you begin to move twards your planet which is
really   far way? Since you're only accurate to 15 digits, if I began
to move even   300 km a second, 5.5*10^100 - 300km = 5.5*10^100
because the significance   of the digits is lost, resulting in no
motion at all. Is it at all   possible, and if not, is question 1
possible since it's true position is   based on an object which has
lost 90% of it's significant digits. Ok, if   it can be tracked (the
planets position from the center is truly   ignored).. how would it be
drawn? When rendering the object, it's   position is based on another
with 90% of it's significance gone, so 90%   of the moons significance
is gone too, and the moon and planet are in the   same place.. you
never see the moon.

Recommendations? I'm working on the math problems before I even try to
  code up the smallest example to even test a theory. Right now,
theory   states it's not possible at all, so I'm grabbing at straws,
seeking info,   and trying new theories.
--
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