How to detect a circle motion with UIGestureRecognizer

Question

I want to be able to detect someone's finger drawing a circular motion on the screen - as if they were drawing an 'O'. Is this possible with UIGestureRecognizer?

Answer 1

I think the answer to this depends on your definition of circular motion and how you intend to use it. For example, do you want to know how many degrees along a circle the users finger has travelled? Or, do you only care about a circle being completed? What is the degree of accuracy you require? Do you want to allow for the motion to be interrupted or does this have to be more of a touch-down > draw-circle > touch-up (in other words, single motion)?

One approach would be to define a bunch of rectangular zones along the circumference and detect if the user is touching these in sequence. This can provide you with direction and a coarse indication of angle.

Another approach is to store the points between touch down and touch up and do some filtering and curve fitting to figure out what shape is approximated by the points. First low-pass-filter using a basic FIR filter and then look at the dx and dy from point to point. A circle (as a series of arcs) will have to fall within a certain range of slope changes from point to point, otherwise you have some other shape.

Yet another approach is to use a Neural Network to take the points and tell you what the shape looks like.