1. A method for using a depth image to extract user behavior, comprising: receiving a depth image that specifies that a plurality of pixels correspond to a user; fitting a curve to a portion of the plurality of pixels corresponding to the user to thereby produce a fitted curve comprising a plurality of straight line segments; identifying a first straight line extending between endpoints of the fitted curve; identifying a second straight line extending orthogonally from the first straight line to a point of the fitted curve that is farthest away from the first straight line; and determining, based on the first and second straight lines, information indicative of a curvature of the user's body.