Two-finger rotation gesture on the iPhone?

Question

I\'m working on an iPhone app with a lot of different gesture inputs that you can do. Currently there is single finger select / drag, two finger scroll, and two finger pinch zoo

Answer 1

Two fingers, both moving, opposit(ish) directions. What gesture conflicts with this?

Pinch/zoom I guess comes close, but whereas pinch/zoom will start off moving away from a center point (if you trace backwards from each line, your lines will be parallel and close), rotate will initially have parallel lines (tracing backwards) that will be far away from each other and those lines will constantly change slope (while retaining distance).

edit: You know--both of these could be solved with the same algorithm.

Rather than calculating lines, calculate the pixel under each finger. If the fingers move, translate the image so that the two initial pixels are still under the two fingers.

This solves all two-finger actions including scroll.

Two-finger scroll or Zoom might look a little wobbly at times since it will do other operations as well, but this is how the map app seems to work (excluding the rotate which it doesn't have).

Answer 2

I've done that before by finding the previous and current distances between the two fingers, and the angle between the previous and current lines. Then I picked some empirical thresholds for that distance delta and angle theta, and that has worked out pretty well for me.

If the distance was greater than my threshold, and the angle was less than my threshold, I scaled the image. Otherwise I rotated it. 2 finger scroll seems easy to distinguish.

BTW in case you are actually storing the values, the touches have previous point values already stored.

CGPoint previousPoint1 = [self scalePoint:[touch1 previousLocationInView:nil]];
CGPoint previousPoint2 = [self scalePoint:[touch2 previousLocationInView:nil]];
CGPoint currentPoint1 = [self scalePoint:[touch1 locationInView:nil]];
CGPoint currentPoint2 = [self scalePoint:[touch2 locationInView:nil]];