> I wonder if there is not a bug in the formula when r is k.a1 or k.a2 (but
> maybe this is already tested, and the correction is obviously to replace
> the null vector with the cross product of r and the other new axis)
> 
> Alas, I do not have the code at hand right now...

Well, if anyone has interest about that patch, I see two bugs and more
in the code of megapov 0.7 :

 - first, minor impact: the length of the second vector is lost
   the oriented area light is a square (or circle if "circular" is used)
   (Looks like a copy-paste of code was made too quickly)   

 - second, crash expected: the result of the cross-product is not checked
   against the null-vector and there is a macro that will normalize 
   the length of the new vector; if the new vector is null, there
   will be a division by 0... With float values, the chance are low
   that this will happen, but they still exist.

 - more: I do not understand why the code wastes some time to
   first invert (scale -1) the ray direction, do its calculation and
   then invert again the ray direction.

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