On the infinite clipping of handwritten signatures

The infinite clipping-a two-level quantization of a given signal based on its polarity-is applied to velocity signals from online handwritten signatures (i.e., pen movement recorded as a time series). In terms of signature authenticity verification, a counter-intuitive result is observed, similar to those reported by Licklider and Pollack, in the 1940's, concerning speech intelligibility. As in their work, the strong signal distortions caused by the infinite clipping have little effect on the conveyed information. Motivated by these observed results, we further investigate a related point of view, according to which the infinite clipping is just the two-level quantization of the first detail output from the Haar wavelet transform. As a consequence, we study the relevance of each subsequent wavelet analysis output, as compared to the energy concentration inside each corresponding signal subspace. Experimentally, we conclude that velocity subspace retains less than 1% of the total signal energy, whereas it carries most of the information necessary to attain authenticity verification performance similar to those of state-of-the-art systems.