Revisiting Techniques for Lowerbounding the Dynamic Time Warping Distance
The dynamic time warping (DTW) distance has been used as a popular measure to compare similarities of numeric time series because it provides robust matching that recognizes warps in time, different sampling rate, etc. Although DTW computation can be optimized by dynamic programming, it is still expensive, so there have been many attempts proposed to speedup DTW-based similarity search by distance lowerbounding. Some approaches assume a constrained variant of DTW (i.e., fixed dimensions, warping window constraint, ground distance), while others do not. In this paper, we comprehensively revisit the problem of DTW lowerbounding, define a general form of DTW that fits all the existing variants and goes even beyond. For the constrained variants of general DTW we propose a lowerbound construction generalizing the LB_Keogh that for particular ground distances offers speedup by up to two orders of magnitude. Furthermore, we apply metric and ptolemaic lowerbounding on unconstrained variants of general DTW that beats the few existing competitors up to two orders of magnitude.
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