From 2013-2014 had an engineering blog where they go into some detail of how they implemented features.

https://web.archive.org/web/20210307152819/http://making.fif...

There is also a FastCompany article about the research they did into how paint colors blend to implement the Mixer feature. But like real paint, it was too easy to quickly end up with brown. So they had to find a way to mix colors that wasn't accurate but "felt right." I think they succeeded.

https://www.fastcompany.com/3002676/magical-tech-behind-pape...

Don't forget that they were acquired by WeTransfer in 2018... which was really weird.

Weird, why?

Great work on Paper and Pencil! Both hardware and software!