Pinch vs. Rotate gestures: can they be made size/resolution-independent?

I'm trying to come up with a way to detect pinch in/out, and rotation gestures. If you delay the detection of a gesture, and allow some data to be collected for a few frames, I think this would be easy. But in Apple's Preview App, for example, the detection seems to be instantaneous to me.

As far as I can tell, in Preview, the only differences between a pinch and a rotate gesture lie with detecting distances between the positions of the two fingers at the beginning of the gesture. Is that really how the pros do it? ;-) If so, I can't think of a way to make the gestures independent of screen size and resolution the data that we get from the touch positions is in pixels, not units of length. It's no big deal now, but it would be nice if people could still play my games, say, after I die and can't update the code for new devices.

My current idea is to set gesture differentiation differences based on iPhoneSettings.generation, and allow users to set their own preferred threshold in a tucked-away menu. Got any better ideas? :-)

I would suggest normalizing the touch positions. For example (using mouse input to make this a bit more general):

private var screenWidth : float;
private var screenHeight : float;
private var ratio : float;

function Start () {
    screenWidth = Screen.width;
    screenHeight = Screen.height;
    ratio = screenWidth / screenHeight;

function Update () {
    var normalizedMousePos = Vector2(Input.mousePosition.x / screenWidth * ratio,
                                     Input.mousePosition.y / screenHeight);

That way the input coordinates are the same regardless of screen size.

I have figured out the simplest solution and this is what Iphone SDK's Gesture recognizers do. pls go through this link.