I don't think it was very common at Apple for an individual engineer to conceive, implement, and ship a feature like this. There was a general sentiment on the team of "let's do something with signatures," but we knew that very few people had scanners. We thought about touchpad input, but decided against it at the time. (That came much later, in either 10.11 or 10.12)

I was thinking, "well, almost no one has a scanner, but practically everyone using this application has a camera in their Mac." I built the initial prototype in OpenCV and then ported it to Apple's vDSP/Accelerate frameworks.

My favorite detail, which doesn't seem to be present in 10.12, is that you could just click on a horizontal line in a PDF; since I recorded the signature's offset relative to the baseline superimposed on the camera image, it would place the signature exactly on the line, with descenders nicely descending.

I've since moved on from macOS/iOS development, but I had a really positive experience at Apple. Met so many amazingly talented people there.