Generic easing function with tweakable power and midpoint.

easeInOut.h

/** Ease in/out function with settable power and midpoint.
 Use p parameter to adjust speed at endpoints (cubic: p=3, quartic: p=4)
 Midpoint is point at which speed is fastest and uneased. 
 
 @param t time or independent variable in range [0,1]. Input should be scaled to this range from whatever source range is expected. 
 @param p power, e.g. quadratic=2, cubic=3. Non-integral powers can be used to fine-tune the curve.
 @param mid midpoint of curve, where it is "most linear".
 
 @return output value in range [0,1]. Can be multiplied to scale to desired range. 
 */
inline float easeInOut(float t, float p = 3.f, float mid = 0.5f)
{
    if (t < mid) {
        return powf(t/mid, p)*mid;
    } else {
        return 1.f-powf((1.f-t)/(1-mid), p)*(1-mid);
    }
}