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Here's one more thing that is used to track counterfeit currency and other purposes: secret tracking dots on printers. https://www.youtube.com/watch?v=sit6zUQK

• gherkin0 11y ago
  Couldn't that be defeated by a preprocessing step that inserts yellow dots at every grid position on the printout? If the extra dots were aligned correctly, I don't see how someone could recover the encoded information. Someone could probably create a printer driver that does that automatically. It probably would need some calibration to work, though.
  • schoen 11y ago
    I was briefly trying to do that as part of our project about this but I didn't figure out the proper pixel size and offset, nor whether you can use a fixed offset relative to the edge of the page or whether the printer tries to defeat this by slightly randomizing the offsets. I hope someone will figure it out. There's an additional challenge that some more recent printers often use a different watermarking technique whose details have never been documented.