Hierarchical Clustering Multi-Task Learning for Joint Human Action Grouping and Recognition

This paper proposes a hierarchical clustering multi-task learning (HC-MTL) method for joint human action grouping and recognition. Specifically, we formulate the objective function into the group-wise least square loss regularized by low rank and sparsity with respect to two latent variables, model parameters and grouping information, for joint optimization. To handle this non-convex optimization, we decompose it into two sub-tasks, multi-task learning and task relatedness discovery. First, we convert this non-convex objective function into the convex formulation by fixing the latent grouping information. This new objective function focuses on multi-task learning by strengthening the shared-action relationship and action-specific feature learning. Second, we leverage the learned model parameters for the task relatedness measure and clustering. In this way, HC-MTL can attain both optimal action models and group discovery by alternating iteratively. The proposed method is validated on three kinds of challenging datasets, including six realistic action datasets (Hollywood2, YouTube, UCF Sports, UCF50, HMDB51 <inline-formula> <tex-math notation="LaTeX">$\&$</tex-math><alternatives><inline-graphic xlink:href="liu-ieq1-2537337.gif"/> </alternatives></inline-formula> UCF101), two constrained datasets (KTH <inline-formula><tex-math notation="LaTeX"> $\&$</tex-math><alternatives><inline-graphic xlink:href="liu-ieq2-2537337.gif"/></alternatives></inline-formula> TJU), and two multi-view datasets (MV-TJU <inline-formula><tex-math notation="LaTeX">$\&$</tex-math><alternatives> <inline-graphic xlink:href="liu-ieq3-2537337.gif"/></alternatives></inline-formula> IXMAS). The extensive experimental results show that: 1) HC-MTL can produce competing performances to the state of the arts for action recognition and grouping; 2) HC-MTL can overcome the difficulty in heuristic action grouping simply based on human knowledge; 3) HC-MTL can avoid the possible inconsistency between the subjective action grouping depending on human knowledge and objective action grouping based on the feature subspace distributions of multiple actions. Comparison with the popular clustered multi-task learning further reveals that the discovered latent relatedness by HC-MTL aids inducing the group-wise multi-task learning and boosts the performance. To the best of our knowledge, ours is the first work that breaks the assumption that all actions are either independent for individual learning or correlated for joint modeling and proposes HC-MTL for automated, joint action grouping and modeling.