Non-Linear Temporal Subspace Representations for Activity Recognition
This work addresses the challenge of capturing non-linear temporal evolution in action recognition, which is important for computer vision applications, but it appears incremental as it builds on existing pooling and subspace methods.
The authors tackled the problem of representing temporal dynamics in human action recognition by proposing kernelized rank pooling, a novel method that encodes sequences as parameters of a kernelized low-rank feature subspace, achieving state-of-the-art results on multiple datasets.
Representations that can compactly and effectively capture the temporal evolution of semantic content are important to computer vision and machine learning algorithms that operate on multi-variate time-series data. We investigate such representations motivated by the task of human action recognition. Here each data instance is encoded by a multivariate feature (such as via a deep CNN) where action dynamics are characterized by their variations in time. As these features are often non-linear, we propose a novel pooling method, kernelized rank pooling, that represents a given sequence compactly as the pre-image of the parameters of a hyperplane in a reproducing kernel Hilbert space, projections of data onto which captures their temporal order. We develop this idea further and show that such a pooling scheme can be cast as an order-constrained kernelized PCA objective. We then propose to use the parameters of a kernelized low-rank feature subspace as the representation of the sequences. We cast our formulation as an optimization problem on generalized Grassmann manifolds and then solve it efficiently using Riemannian optimization techniques. We present experiments on several action recognition datasets using diverse feature modalities and demonstrate state-of-the-art results.