1. A device comprising: one or more computer-readable media; and one or more processors that are coupled to the computer-readable media and that are configured to cause the device to obtain frame-level feature sets of visual features that were extracted from respective frames of a video, wherein the respective frame-level feature set of a frame includes the respective visual features that were extracted from the frame, generate first-pooled-level feature sets, wherein each first-pooled-level feature set is generated by pooling the visual features from two or more frame-level feature sets, wherein each first-pooled-level feature set includes pooled features, and wherein a pooled feature in the first-pooled-level feature sets is generated by pooling two or more visual features, generate second-pooled-level feature sets, wherein each second-pooled-level feature set is generated by pooling the pooled features in two or more first-pooled-level feature sets, wherein each second-pooled-level feature set includes pooled features, and wherein a pooled feature in the second-pooled-level feature sets is generated by pooling two or more pooled features in the first-pooled-level feature sets, obtain trajectory features that were extracted from the video; fuse the trajectory features with at least some of the pooled features in the first-pooled-level feature sets and the pooled features in the second-pooled-level feature sets, thereby generating fused features; train classifiers based on the fused features; obtain a test video; and classify the test video using the trained classifiers.