It's their lab notes, so it's exploring a general idea, but they're also referencing previous software they've built (like crosscut)

Judging from previous Ink&Switch work, the target devices are likely tablets or below.

There's also various levels of "real-time" - pushing something in Illustrator might require less responsiveness to feel fluid than handwriting feedback.

Isn't this kinda how joints and other constraints (and IK) work in physics engines?

I'm sure it's not trivial, but there's a large literature and existing code that helps us in figuring out how to do this in real time.

Ink and switch is a research lab. As far as I'm aware, they're not a building any specific products, just exploring what is possible for human computer interactions

I had the same thought… that’s a lot of text and very few illustrations for an article about graphics.

I was confused too. There's no mention of current implementations. It's like half an encyclopedia article missing the good half. For example, TLDraw has it's own inking algorithm of pressure+velocity+path -> generated outlines comprised of bezier curves. InkCanvas in Microsoft implementations afaik is closed-source. Is there a standard library for ink for the pencil in macos? Or does everyone roll-their-own on the mac side?