[jcalx]

Apple's easing function (like others) is parameterized by physical characteristics (e.g. spring force, damping) that are easier to model from first principles, but there are other parameters (overshoot distance, anticipation size, animation time) that are more useful for animation. A closed-form parametrization with the latter might be tricky to derive, but some kind of iterative solver (plug in desired animation parameters, get values for the physical parameters) shouldn't be too difficult?