... A method is presented in which a signal, degraded by a linear shift-variant system, will undergo a warping such that the resulting warped signal will be approximately described by a warped original signal filtered by a linear shift-invariant system. ... The purpose of this distortion is to make the space-variant impulse response (which can be viewed as a space-invariant impulse response which has been warped in the original signal domain) vary as little as possible. ...