Looks amazing, thank you for doing this. I use Synthesia for a few years now and it was also quite helpful teaching piano to my daughter. She takes piano lessons but Synthesia is how she learns the songs she likes by her own. If there will be an open-source variant we can be sure that we can revisit and relearn our favourite songs even in many years, when commercial Synthesia might have long been gone[1]

Also, I think Synthesia made a business out of producing Youtube videos of the Synthesia visualization. If there was a free way to produce this kind of videos many Youtubers would be grateful.

EDIT: Seems I'm wrong with the above paragraph. I thought to remember that you had to pay for the rights to use the visualization on Youtube, but that is not the case. The conditions[3] look quite fair to me.

One small remark regarding your visualization: From the screenshot it looks as if the keyboard shows the black keys centered around the gap between the white keys. This is not how a real keyboard looks like. Better explained than I ever could in a thread[2] from a few weeks ago.

[1] Another software we use is PianoJacq. For learning and practicing it is even better suited than Synthesia.

[2] https://news.ycombinator.com/item?id=29943081

[3] https://www.synthesiagame.com/Video