I made a starter project for optical flow (opencv.js) and visualization using p5.js: opencv-p5-optical-flow
Feel free to remix it - it may be useful for the (last few days of) coding jam of the month!
Shout out to this repository, which I used as a starting point: https://github.com/kalwalt/kalwalt-interactivity-AR/tree/master/opencvjs
