Angles calculated using atan function are -ve. How to represent it in terms of pi

I have used atan function and calculate angle between two vectors. The angle returned by this function is -ve in nature. I want to represent the angle in terms of pi; that is, the value of angle must be between 0 and 2*pi.

What to do?

Answer 1

[Jan]

I do not know the term "-ve".

You cannot determine the sign of an angle between two vectors without further information. Imagine two 3D vectors: You can measure the enclosed angle only, which is always 0 <= angle <= pi/2. If you want a sign, you need a third vector to define an orientation. If you talk about 2D vectors, the global Z-axis [0, 0, 1] is this third vector usually. Then using ATAN2 is recommended, but replies a signed angle also: -pi <= angle < pi.

To get 0 <= angle < 2*pi, you could add the wanted shift value:

index = (angle < 0);
angle(index) = 2*pi + angle(index);

See also this famous CSSM thread: Angle between two vectors

[Vinay Kumar]

-ve is negative

[Jan]

Ok. Thanks for the explanation.