[LAD] Implementing software knob behaviour

Hi all, I'm currently trying to write a software MIDI/OSC control panel in Kivy/Python [1]. I want to implement a sensible knob behaviour but I have trouble getting this right. The general idea is: - Click (starting somewhere in the area of the knob control) and drag upwards/downwards (y-axis) increases / decreases the value. - Dragging left/right (x-axis) decreases step size of increments / decrements. - Each knob has a min/max value. My questions are: - What should I use as the reference point for finding the distance of the movement? The centre of the knob or where the first touch event occurred? - Should I apply the distance to the value directly or use the distance as a relative value (i.e. increment / decrement to the value when the touch started)? If the former, how do I prevent value jumps? - Or, to put it another way: how do you guys actually implement knob behaviour? Chris [1] http://kivy.org/