1. 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 wherein the comparison function comprises a vector element comparison for each dimension of the vector space summed over the dimensions of the vector space; training a linear classifier on the set of projected training vectors to generate a trained linear classifier operative in the comparison space; and transforming the trained linear classifier operative in the comparison space into a trained nonlinear classifier operative in the vector space; wherein the projecting, the training, and the transforming are performed by a digital processing device.