13. The method as set forth in claim 12 , wherein the classifying does not include projecting the input vector to the comparison space.
12. A method comprising: projecting a set of training vectors in a vector space to a comparison space defined by a set of reference vectors using a comparison function to generate a corresponding set of projected training vectors in the comparison space; training a linear classifier on the set of projected training vectors to generate a trained linear classifier operative in the comparison space; transforming the trained linear classifier operative in the comparison space into a trained nonlinear classifier operative in the vector space; and classifying an input vector in the vector space using the trained nonlinear classifier operative in the vector space; wherein the projecting, the training, the transforming, and the classifying are performed by a digital processing device.