Sorry to hijack the thread, I couldn't directly reply to https://news.ycombinator.com/item?id=42092845 .

The reason to not (just) use optical flow is that it isn't absolute. If you pattern your surface correctly, you can ensure that every few by few pixel region on a QR code like bitmaps surface is unique, and thus can be decoded into an absolute position. Basically a 2D absolute optical encoder fast enough to be part of a motor control loop.