paper.clientToLocalPoint(p)

Transform client coordinates represented by point p to the paper local coordinates. This is especially useful when you have a mouse event object and want coordinates inside the paper that correspond to event clientX and clientY point.

var localPoint1 = paper.clientToLocalPoint({ x: evt.clientX, y: evt.clientY });
// alternative method signature
var localPoint2 = paper.clientToLocalPoint(evt.clientX, evt.clientY);