Question

In an Android app I’m making, I would like to detect when a user is holding a phone in his hand, makes a gesture like he would when throwing a frissbee. I have seen a couple of apps implementing this, but I can’t find any example code or tutorial on the web.

It would be great with some thoughts on how this could be done, and ofc.

It would be even better with some example code or link to a tutorial.

Answer 1

Accelerometer provides you with a stream of 3d vectors. In case your phone is help in hand, its direction is opposite of earth gravity pull and size is the same. (this way you can determine phone orientation)

If user lets if fall, vector value will go to 0 (the process as weighlessness on space station)

If user makes some gesture without throwing it, directon will shift, and amplitude will rise, then fall and then rise again (when user stops movement). To determine how it looks like, you can do some research by recording accelerometer data and performing desireg gestures.

Keep in mind, that accelerometer is pretty noisy – you will have to do some averaging over nearby values to get meaningful results.

I think that one workable approach to match gesture would be invariant moments (like Hu moments used to image recognition) – accelerometer vector over time defines 4 dimensional space, and you will need set of scaling / rotation invariant moments. Designing such set is not easy, but comptuing is not complicated.

After you got your moments, you may use standart techniques of matching vectors to clusters. ( see “moments” and “cluster” modules from our javaocr project: http://javaocr.svn.sourceforge.net/viewvc/javaocr/trunk/plugins/ )

PS: you may get away with just speed over time, which produces 2-Dimensional space and can be analysed with javaocr on the spot.