Learning Mahalanobis distance for DTW based online signature verification

Signature, a form of handwritten depiction, has been and is still widely used as a proof of the writer´s identity/intent in human society. Online signatures represents the dynamic process of handwriting as a sequence of feature vectors along time. Dynamic time warping (DTW) has been popularly adopted to compare sequence data. A basic problem in using DTW for signature verification is how to estimate the difference between the feature vectors. Most previous researches made use of Euclidean distance (ED) for this problem. However, ED treats each feature equally and cannot take account of the correlations between features. To overcome this problem, this paper proposed Mahalanobis distance (MD) for signature verification. One key question is how to estimate covariance matrix in MD calculation. We formulate this problem in a learning framework and introduce two criterion for estimating the matrix. The first criteria aims at minimizing the signature difference for the same writer, while the second criteria try to maximize the signature difference between different writers while minimize the within-writer signature difference. We carried out experiments on the MCYT biometric database. The experimental results exhibit that the proposed MD based method achieved better results than ED based method.