1. A method for unwrapping phase shift data, comprising: receiving phase shift data measured by a detector; including the phase-shift data as parameters into a non-convex or non-concave objective function without a regularization term; finding critical points of said objective function; only among said critical points, finding a target critical point of a regularized objective function, said regularized objective function being formed from said objective function and a regularization term; and outputting said target critical point as an estimate for unwrapped phase shift data.