I scanned an envelope which had a dot matrix 4-state barcode by the postal service. It did not appear on the /bilevel/ scan. So I tried very low thresholds (the point at which light gray is treated as either black or white). The threshold needed to retain the fluorescent(†) barcode is so low that black text on the same scan becomes too dirty for OCR to work.

The US postal service scans (all?) envelopes and thus has records of who is sending mail to who. (Do other countries do this?) Anyway, I wonder how we might counter the privacy intrusion. What if the return address on an envelope is printed in fluorescent orange.. would the return address be suppressed from envelope scans? IIUC, they would have to scan in grayscale or color to capture it, which would take a lot more storage space. So they are probably doing bitonal scans. Yellow would work too but it’s much harder for an eye to see. This fluorescent orange is readable enough to a human eye but apparently tricky for a machine.

Of course the return address is optional, so the best privacy is to simply not supply a return address. But if return service is wanted, supplying a return address is inherently needed.

Another thought: suppose an address is dark blue text on a light blue background, or white text on a medium blue background. The scanning software would have to be quite advanced to choose a threshold that treats the text differently than the background, no? If the return address is fluorescent orange and the destination address has a background color, envelopes could perhaps be printed in a way that stifles the mass surveillance.

(†) I cannot concretely assert that it is fluorescent; just describing what it looks like.