Order-preserving sparse coding for sequence classification

In this paper, we investigate order-preserving sparse coding for classifying multi-dimensional sequence data. Such a problem is often tackled by first decomposing the input sequence into individual frames and extracting features, then performing sparse coding or other processing for each frame based feature vector independently, and finally aggregating individual responses to classify the input sequence. However, this heuristic approach ignores the underlying temporal order of the input sequence frames, which in turn results in suboptimal discriminative capability. In this work, we introduce a temporal-order-preserving regularizer which aims to preserve the temporal order of the reconstruction coefficients. An efficient Nesterov-type smooth approximation method is developed for optimization of the new regularization criterion, with guaranteed error bounds. Extensive experiments for time series classification on a synthetic dataset, several machine learning benchmarks, and a challenging real-world RGB-D human activity dataset, show that the proposed coding scheme is discriminative and robust, and it outperforms previous art for sequence classification.