yes, you are right.
One more thing, optical flow can reflect motion patterns to some extents, but it is different from motion descriptor like dense trajectory. There exist some implementations to extract optical flow, such as fast-flow, brox-flow and TVL1. This part I didn’t study to much. Here are some references:
[1]. FlowNet 2.0: Evolution of Optical Flow Estimation with Deep Networks [Flow-Net v2] CVPR’17
[2]. Flownet learning optical flow with convolutional networks [Flow-Net v1] ICCV’15
[3]. Fast optical flow using dense inverse search [fast-flow] ECCV’16
[4]. TV-L1 Optical Flow Estimation, Image Processing’13
[5]. High accuracy optical flow estimation based on a theory for warping. [brox-flow] ECCV’04