Re: [Algorithms] floats, doubles, epsilons, joy

[Jason Z. <zi...@n-...>]

A bit of buffer, good idea.  So the general algorithm would be: after I find
the distance to collision make sure its always at least X (maybe 0.01 like
Steve suggested) units.  If its larger than X units, move X-0.01 units
instead of the full X.  This sounds like it will make the system much more
robust, thanks again.

- Jason Zisk
- nFusion Interactive LLC


----- Original Message -----
From: "Gil Gribb" <gg...@ma...>
To: <gda...@li...>
Sent: Thursday, August 31, 2000 3:36 PM
Subject: Re: [Algorithms] floats, doubles, epsilons, joy


> I wouldn't use doubles, they just obscure the problem but don't eliminate
> it. If an algorithm is unstable with floats, it is usually unstable with
> doubles too.
>
> So, how to solve the problem? The only solution is carefully analize the
> situation and do things robustly. That isn't very helpful, but it is true,
> you gotta really think about the situation to solve the problem.
>
> I solve your collision problem like this typically. I don't attempt to
rest
> directly on the floor, I attempt to hover .1 inches or so off the floor.
> Therefore, even with floating point inaccuracy, I never find I am IN the
> floor at the start of the next round. This does use a tolerence, but it is
> subtly different than your approach. Your approach attempts to correct the
> problem after is occurs and my approach is proactive, insuring the problem
> does not occur. (This all assumes finding yourself in the floor is the
> problem).
>
> Even if you get this problem licked, if you are doing anything
interesting,
> like say having things push the player, you will run into half a dozen
other
> more subtle problems. You gotta think through each one to figure out how
to
> solve the problem with inexact arithmatic. It is really really hard work.
>
> -Gil
>
>
> > I've been working on some collision stuff lately and I keep running into
> > problems with floating point inaccuracies.  Specifically if the player
is
> > standing on top of a triangle, the distance should be zero, but with
float
> > it was very close to zero but not quite.  So I converted all my code to
> use
> > double and its much better but I'm still not getting exactly zero.  With
> > float they were large enough to cause unwanted movement, with double
they
> > are tiny but I can easily imagine the player finding a way to make these
> > values be higher than my epsilon amount (0.00001).  Just so everyone
> knows,
> > I'm doing ellipsoid vs. triangle collision with an algorithm I found on
> > Flipcode (http://www.flipcode.com/tutorials/tut_coll-ellipsoids.shtml).
> >
> > This is the first time I've run into this, I'm wondering if there are
some
> > common ways to get around these types of problems?
> >
> > Also I was wondering if there was a down side to using doubles rather
than
> > floats.  This code doesn't use any ram so size doesn't matter, but is
> doing
> > floating point math with doubles much slower or have other unwanted side
> > effects?
> >
> > Thanks for any suggestions,
> >
> > - Jason Zisk
> > - nFusion Interactive LLC
> >
> >
> >
> >
> >
> >
> >
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