Dynamic Time Warping Under Translation: Approximation Guided by Space-Filling Curves

TL;DR

This paper develops algorithms for optimizing the Dynamic Time Warping distance between curves under translation, providing exact solutions for certain norms and efficient approximation algorithms for Euclidean norms in 2D.

Contribution

It introduces the first polynomial-time exact algorithm for DTW under translation for the $L_1$ norm and efficient approximation algorithms for Euclidean norms in 2D, utilizing space-filling curves.

Findings

Exact polynomial-time algorithm for $L_1$ norm in fixed dimensions.

$(1+ ext{epsilon})$-approximation algorithm for Euclidean norm in 2D.

Subcubic time algorithm approaching quadratic time for DTW under translation.

Abstract

The Dynamic Time Warping (DTW) distance is a popular measure of similarity for a variety of sequence data. For comparing polygonal curves $\pi, \sigma$ in $\mathbb{R}^d$, it provides a robust, outlier-insensitive alternative to the Fr\'echet distance. However, like the Fr\'echet distance, the DTW distance is not invariant under translations. Can we efficiently optimize the DTW distance of $\pi$ and $\sigma$ under arbitrary translations, to compare the curves' shape irrespective of their absolute location? There are surprisingly few works in this direction, which may be due to its computational intricacy: For the Euclidean norm, this problem contains as a special case the geometric median problem, which provably admits no exact algebraic algorithm (that is, no algorithm using only addition, multiplication, and $k$-th roots). We thus investigate exact algorithms for non-Euclidean norms as well as approximation algorithms for the Euclidean norm: - For the $L_1$ norm in $\mathbb{R}^d$, we provide an $\mathcal{O}(n^{2(d+1)})$-time algorithm, i.e., an exact polynomial-time algorithm for constant $d$. Here and below, $n$ bounds the curves' complexities. - For the Euclidean norm in $\mathbb{R}^2$, we show that a simple problem-specific insight leads to a $(1+\varepsilon)$-approximation in time $\mathcal{O}(n^3/\varepsilon^2)$. We then show how to obtain a subcubic $\widetilde{\mathcal{O}}(n^{2.5}/\varepsilon^2)$ time algorithm with significant new ideas; this time comes close to the well-known quadratic time barrier for computing DTW for fixed translations. Technically, the algorithm is obtained by speeding up repeated DTW distance estimations using a dynamic data structure for maintaining shortest paths in weighted planar digraphs. Crucially, we show how to traverse a candidate set of translations using space-filling curves in a way that incurs only few updates to the data structure.