Thank you!

To be honest, I did not know how to quickly check if the matrix is 
correct, and since the resulting images looked right... :-/

I have found the problem: The quaternion used to compute the matrix uses 
single precision floats. The fix was to first create a double-precision 
quaternion from it, then re-normalize that and feed the double-precision 
quaternion into the matrix...

Thanks again!

But still, isn't the test POV performs too strict?
I understand that POV-Ray must use double precision math for most 
operations, but would, in this case, the single-precision matrix really 
break the vista buffer?
This will enforce all external applications that are used to create 
POV-scenes to use double-precision math throughout, and I don't know if 
it is available in all (script) languages...

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