RE: [Algorithms] Problem with Woo's "Fast Ray-Box Intersection"

[Akbar A. <sye...@ea...>]

something might bite you later on in that book

before you check out something in it, make sure you test it against this
http://www.acm.org/pubs/tog/GraphicsGems/Errata.GraphicsGems

here's the raybox code if anyone is currently working on it ;)
it has the fix
http://www.acm.org/tog/GraphicsGems/gems/RayBox.c

entire listing from the series in a -listed fashion
http://www.acm.org/tog/GraphicsGems/gems.html


peace.
akbar A.

"We want technology for the sake of the story, not for its own sake. When
you look back, say 10 years from now, current technology will seem quaint"
Pixars' Edwin Catmull.


-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of Klaus
Hartmann
Sent: Saturday, August 26, 2000 1:26 AM
To: gda...@li...
Subject: [Algorithms] Problem with Woo's "Fast Ray-Box Intersection"


Hi all,

I have a problem that I don't understand. I have the following line of
source in my implementation of Woo's "Fast Ray-Box Intersection":

hitpoint[i] = rayOrg[i] + maxT[whichPlane] * rayDir[i];

hitpoint, rayOrg, maxT, and rayDir are all arrays with three floating-point
values (indices 0 to 2). Now here are some values:

i = 2 (which is a valid index)
whichPlane = 0 (also a valid index)
rayOrg[i] = -2
maxT[whichPlane] = 2.82843
rayDir[i] = 0.707107

These are all very valid values, and the result should now be:
hitpoint[i] = -2 + 2.82843 * 0.707107 = 2.65201e-06

However, the debugger says, that
hitpoint[i] = -6.8457097e-008

Okay, so I thought that maybe this is a precision problem. I tried doubles,
but I got the same wrong value. Then I thought, that there might be
something wrong hitpoint[i], so I replaced that with a local double
variable, but the result is still wrong.

Why? I cannot believe that 2.65201e-06 is too small to be represented as a
double-precision floating-point value. Can someone explain this to me? Or do
you see a bug in the above C line? If you don't believe me, then I'm very
well willing to upload the code (very small), and you can step through it,
and see for yourselves.

Niki


_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list