Can You See Through Playing Cards?

A dispute over cheating has broken out during a game played by a host with a group of players he invited to play at his house. The 4.5-minute TikTok video uploaded by Blurblur008 attracted hundreds and thousands of views, as both sides accused each other of using invisible markers to mark cards.

A card marked with invisible ink is a card that has hidden markings that can only be read by special gimmick sunglasses or under certain lighting conditions. This method involves secretly scratching or denting a card’s surface in a way only those involved can see. Other methods involve marking cards by subtly tinting parts of a design feature, such as the head for an ace or the left wing for a king, or by adding (or cutting out) an entire bird to the card back design.

The kernel module, written in C code, handles the reading and decoding of the cards. The app only uses this information to display them on screen along with the number of players and other information like whether the haptic feedback is connected, as well as the current result and the type (H6, D8 in this case). The cards are very thin and the markings are only few pixels tall. This is a difficult task. The resulting image needs to be able to distinguish between different cards and also cope with potentially misaligned or bent cards, so the recognition algorithm is not trivial.