Futatabi is a free software solution for doing instant replay, e.g. for sports production. It supports multiple cameras, high-quality realtime slow motion on the GPU through optical flow, and seamless integration with Nageru, my live video mixer. We'll talk a bit about how interpolation through optical flow works, challenges in transporting the streams back and forth, and demonstrate a real-world sports production done last year using Nageru and Futatabi.