M$^3$Video: Masked Motion Modeling for Self-Supervised Video Representation Learning

We study self-supervised video representation learning that seeks to learn video features from unlabeled videos, which is widely used for video analysis as labeling videos is labor-intensive. Current methods often mask some video regions and then train a model to reconstruct spatial information in these regions (e.g., original pixels). However, the model is easy to reconstruct this information by considering content in a single frame. As a result, it may neglect to learn the interactions between frames, which are critical for video analysis. In this paper, we present a new self-supervised learning task, called Masked Motion Modeling (M$^3$Video), for learning representation by enforcing the model to predict the motion of moving objects in the masked regions. To generate motion targets for this task, we track the objects using optical flow. The motion targets consist of position transitions and shape changes of the tracked objects, thus the model has to consider multiple frames comprehensively. Besides, to help the model capture fine-grained motion details, we enforce the model to predict trajectory motion targets in high temporal resolution based on a video in low temporal resolution. After pre-training using our M$^3$Video task, the model is able to anticipate fine-grained motion details even taking a sparsely sampled video as input. We conduct extensive experiments on four benchmark datasets. Remarkably, when doing pre-training with 400 epochs, we improve the accuracy from 67.6\% to 69.2\% and from 78.8\% to 79.7\% on Something-Something V2 and Kinetics-400 datasets, respectively.