non-linear lerp in unity

A LERP (linear interpolation) returns you a value based on start and end key values and a fraction of the interpolation. 
E.g:

• When the fraction is 0 you are at the start
  
• When the fraction is at 1 you are at the end
  
• When the fraction is at 0.5 you are halfway
  

But if we replace the fraction with something non linear as long as its a function that returns a result in the range 0..1 we can get some more interesting results.   Trig curves like Sine are a good candidate and allow the creation of movement that has smooth acceleration and deceleration.

For my example I wanted to interpolate from key A to key B and then back to key A, so the part of the sin curve I should use goes from 0..PI.  So I take my input fraction, scale it by PI and then generate the Sin and I get a lovely smooth interpolation and back again.  If you simply wanted to go from Key A to Key B you would use 0..1/2PI.


//When something scary happens momentarily zoom the camera
float inter = (1.0f*fearCounter)/FEARCOUNTER;  //linear
inter*= 3.1415f; 
inter = Mathf.Sin(inter);
float fov = Mathf.Lerp(cameraFOV,cameraFOV/2.0f, inter);
localCamera.fieldOfView =fov;