You may also be interested in: https://en.wikipedia.org/wiki/EURion_constellation
And the lesser known, newer, invisible, digital watermark based version of it. Unfortunately very little information is available about that one. http://www.cl.cam.ac.uk/users/sjm217/projects/currency/
alt Hacker News
And the lesser known, newer, invisible, digital watermark based version of it. Unfortunately very little information is available about that one. http://www.cl.cam.ac.uk/users/sjm217/projects/currency/