[vtkusers] Angle between two vectors

[David Cole]

Knowledge and advice like this should be incorporated into a bonafide 
method that's available to call in vtkMath (or let me know if there's a 
better place for it).

Especially given that the most highly ranked Google results for 
"compute angle between two vectors" predominantly advise to use 
acos(dot)... There is mention of 3 techniques if you dig through enough 
links, but it certainly seems like this sort of advice would be ranked 
more highly.

I guess it comes down to how fast you need the method to be, and how 
much "error" you can tolerate. The acos(dot) technique requires fewer 
math ops, and therefore should be faster, but you might not get what 
you want from all inputs. (It's been working fine for me, and I am 
tracking an angle that goes to 0.... but apparently, YMMV.) The atan2 
technique requires more math, but gives "better results"... Arguably in 
most situations the speed difference may not even be noticeable. You'd 
have to be doing lots of angle computations inside tight loops to 
notice it probably.

So..... if I submit a gerrit patch to add this, using the atan2 
technique, should it be named "double 
vtkMath::AngleBetweenTwoVectors(double *v1, double *v2)"? Or is there 
an better name for it (to follow other method names already existing in 
vtkMath)....?


Thanks,
David C.