By any chance, was Andrew Strominger involved in this at all? He gave the Andrew Chamblin Memorial Lecture in Cambridge last month and demoed something that looked similar.

I think what he showed you was likely a version of this that was coded up by Harvard graduate student Dominic Chang: https://dominic-chang.com/bhi-filter/

It works very well (and in a browser!) but is limited to a non-rotating (Schwarzschild) black hole---we really wanted to include black hole spin (the Kerr case). As we write on the github, talking with Dominic about his implementation was very useful and we are hoping to get a paper explaining both codes out before the end of the year.

Yes, Andy has been very involved in the story of the photon ring and was one of the lead authors on the original paper that started it all: https://www.science.org/doi/10.1126/sciadv.aaz1310

(And he was also my PhD advisor.)