1. A method for determining flows, comprising steps: acquiring a video of the flows with a camera, wherein the flows are pedestrians in a scene, wherein the video includes a set of frames; extracting motion vectors from each frame in the set; constructing a data matrix from the motion vectors in the set of frames; determining a low rank Koopman operator from the data matrix; analyzing a spectrum of the low rank Koopman operator to determine a set of Koopman modes; segmenting the frames into the flows according to a clustering of the Koopman modes, wherein the flows are independent pedestrian flows, and wherein the steps are performed in a processor.