[ggt-devel] Bug in matrix state code

[Patrick H. <pa...@13...>]

I think that there is a bug in the code that does optimizations for 
matrix operations based on the matrix state.  Unfortunately, I don't 
know the GMTL code (or math in general) well enough to track down the 
bug.  I can describe the symptoms, however, with the hope that someone 
can give me a suggestion on how to do further debugging.

Basically, what happens is the values of an affine matrix approach zero 
after it is inverted.  Eventually, they reach zero, and my application 
crashes.  (The application was not written by me, and the math in it is 
way beyond me.)  What is particularly vexing is that the single 
inversion appears to be the only place in the code where the matrix is 
modified, but somehow its values change constantly after that point.  I 
must be missing something--I just haven't figured out what.

In any event, if I back out the only changes to Generate.h, Matrix.h, 
and MatrixOps.h to remove the state tracking optimizations, the code 
starts working again without any problems.  I'm guessing that the 
problem is in gmtl::invertAffine() since it was added as part of the 
optimization code.  Perhaps there is something in that function that 
causes additive round-off errors?

The specific revisions in question are the following:

    Generate.h: 1.73
    Matrix.h: 1.28
    MatrixOps.h: 1.31

I would appreciate any suggestions on how to track down this problem or 
any suggested fixes (short of backing out the optimizations altogether).

  -Patrick


-- 
Patrick L. Hartling                     | Research Assistant, VRAC
pa...@13...                         | 2274 Howe Hall Room 2624
PGP: http://www.137.org/patrick/pgp.txt | T: +1.515.294.4916
http://www.137.org/patrick/             | http://www.vrac.iastate.edu/