Very nice - you should do what I figured out for a project - momentum when there is a big action, and fine control for small actions - if I scroll the wheel dramatically, give me some momentum on that zoom - if I scroll it slowly, no momentum and fine increments (like you have now)

The way I did this in javascript was to create an array of zoom in/out and the four directions - then you add or subtract from the array based on intensity of gestures over time - even keyboard action keys (+/-) can have momentum this way by adding when there is continuous keypresses - I was pleased how it turned out in the end.