[fruit2020]

How does one detect the finder patterns when they are not squares?

[daveslash]

Not sure. But I'm guessing that the finder patters represent a pattern that is not possible in the data encoding. That is, according the the encoding scheme you couldn't have data that, just by chance looked like a finder pattern.

Also: A more fun and technical sounding term for "finder pattern" is "Fiducial" https://en.wikipedia.org/wiki/Fiducial_marker