The fate of old notes is a critical issue for central banks that oversee their disposal. These notes cannot be thrown away like ordinary rubbish. Instead, central banks have developed standards to ensure the disposal is safe and secure.

This often involves a type of shredding that slices each note into hundreds of tiny pieces, each typically smaller than about thirty square millimeters. The shredded paper, or increasingly plastic, can then be disposed of, or recycled, off site.

Some central banks have even taken to selling shredded banknotes as souvenirs. And this raises the question of whether it is possible to reassemble the notes and reclaim them at face value.

Clearly manual re-assembly is a challenging task. But now Chung Kong, a PhD student at Hong Kong University, has shown that computer vision can help reassemble shredded notes at least partially.

Kong became intrigued after noticing that the Hong Kong Monetary Authority visitor center sells souvenir paperweights filled with shredded notes. Each paperweight, it says, contains the remains of 138 notes each worth HDK 1000 (equivalent to just over $100).

Kong imagined that, when reassembled, the notes ought to be worth HDK138,000. A tempting amount. And since computer vision systems can already solve jigsaw puzzles, he began the task of testing whether such a system might help reassemble a complete note.

Kong's approach was straightforward. He first scanned complete versions of the HDK1000 notes to create . He then broke open the paperweights, scanned each shredded piece, and cropped it to a rectangular form. Finally, he used a machine vision system to match the pattern on the shredded piece to a location on the ground truth image.

This process was hugely successful. It appears to be straightforward for a computer to find these locations.

However, there is another crucial step that Kong does not deal with in his paper. The shredded pieces are irregular shapes but the matching is done with parts of the patterns cropped into rectangles.

The problem is that the pieces may not have all come from the same note. So even if the computer vision system were to find matches for an entire note, there is no guarantee that those irregular pieces would fit together to form a complete note.

To read the complete article, see:
When One Man Used Computer Vision To Reassemble Shredded Bank Notes(https://www.discovermagazine.com/technology/when-one-man-used-computer-vision-to-reassemble-shredded-bank-notes)

To read an earlier E-Sylum article, see:
TIME-LAPSE VIDEO: SHREDDED BANKNOTE RECONSTRUCTION(https://www.coinbooks.org/v21/esylum_v21n04a36.html)