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

[Klaus H. <k_h...@os...>]

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