Unity3D C# Negative Lerp?

Question

I am wanting to lerp the variable that controls my animations speed so it transitions fluidly in the blend tree.

 void Idle()
     {
         _agent.SetDestination(transform.position);
             //Something like the following line
         _speed = Mathf.Lerp(0.0f, _speed, -0.01f);
         _animationController._anim.SetFloat("Speed", _speed);
     }

I'v read that you can't lerp negatives, so how do I do this?

Answer 1

The t value in a lerp function between a and b typically needs to be in the 0-1 range. When t is 0, the function returns a and when t is 1, the function returns b. All the values of t in between will return a value between a and b as a linear function. This is a simplified one-dimensional lerp:

return a + (b - a) * t;

In this context, negative values of t don't make sense. For example, if your speed variable is 4.0f, the result of lerp(0, 4, -0.01f) would be:

0 + (4 - 0) * -0.01

which returns -0.04.

The simplest way to solve this issue is to simply flip a and b instead of trying to use a negative t.

_speed = Mathf.Lerp(_speed, 0.0f, 0.01f);

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Another suggestion would be to use Mathf.SmoothStep and store the original speed instead of applying the function on the constantly changing _speed to get a really smooth transition.