The app can determine where each card is and if any cards are missing by scanning the deck.
Programmer at Gearbox Software Paul Nettle revealed an absolutely mind-blowing invention that allows one to scan a deck of cards and determine the position of every card in it and understand if cards are missing.
The system created by Paul is called Abra, an iOS client application running on the iPad that can scan the infra-red bar codes applied by the creator using the Raspberry Pi Zero W device with a NoIR camera. These bar codes show Abra all the information it requires and in as little as 4ms the app can decode them and show the deck's order. According to the developer, the system was created to spread awareness about card cheaters.
Raspberry Pi Zero W with a NoIR camera
Paul also commented that other Apple devices can act as a server, but they can't see those infrared marks. They, however, can see ultraviolet fluorescing ink, which can also be used to create the bar codes.
And here's the demonstration of Abra's capabilities:
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