Question

I wanted to calculate the angle between two vectors but I have seen these inverse trig operations such as acos and atan uses lots of cpu cycles. Is there a way where I can get this calculation done without using these functions? Also, does these really hit you when you in your optimization?

Answer 1

There are no such functions in the STL; those are in the math library.

Also, are you sure it’s important to be efficient here? Have you profiled to see if there’s function calls like this in the hot spots? Do you know that the performance is bad when using these functions? You should always answer these questions before diving into such microoptimizations.

In order to give advice, what are you going to do with it? How accurate does it have to be?

If you need the actual angle to a high precision, you probably can’t do better. If you need it for some comparison, you can use absolute values and the dot product to get the cosine of the angle. If you don’t need precision, you can do that and use an acos lookup table. If you’re using it as input for another calculation, you might be able to use a little geometry or maybe a trig identity to avoid having to find an arccosine or arctangent.

In any case, once you’ve done what optimization you’re going to do, do before and after timing runs to see if you’ve made any significant difference.