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

[Pierre T. <p.t...@wa...>]

Errr..............

Doubles notoriously are slower than floats. Moreover if you use D3D you
really shouldn't be messing around with doubles. From the DX7 docs:

"Applications that require double-precision accuracy can include the
DDSCL_FPUPRESERVE flag when calling the IDirectDraw7::SetCooperativeLevel
method to set the DirectDraw cooperative level. When the DDSCL_FPUPRESERVE
flag is used, the system still uses single-precision mode, but it saves the
state of the FPU prior to changing the precision mode, then restores the
mode when it returns control to the application. (Calling the method again
using the DDSCL_FPUSETUP flag returns Direct3D to its default behavior.)
Obviously, this check, save, then restore process takes time and can
noticeably affect performance. This flag should only be used when the
application must have double-precision accuracy and you do not want to
include code to manually set and restore the FPU mode as the application
requires."

And all in all, using doubles is just a way to hide a problem, not a way to
fix it. No magic bullet there, it really depends on the piece of code
involved I'm afraid.

Pierre


----- Original Message -----
From: Jason Zisk <zi...@n-...>
To: <gda...@li...>
Sent: Thursday, August 31, 2000 7:14 PM
Subject: [Algorithms] floats, doubles, epsilons, joy


> 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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