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 video uploaded to TikTok by user Blurblur008 has attracted hundreds of thousands of views as both sides accused the other of using invisible ink to mark cards.

A card marked with invisible markings is a card which has hidden markings. These can only be seen by special gimmick glasses or under certain lighting conditions. The most common method of this kind involves secretly scratching, denting, bending, or otherwise modifying the surface of a card in a way that only those involved will notice. Other methods include subtly tinting a part of a design, such as the left wing on a king or the head of an ace, or adding (or removing) an entire bird from 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). This is not an easy task to accomplish; cards are very thin, and their markings are only a few pixels tall. The resulting image must be able to differentiate between different cards as well as cope with possibly misaligned or twisted cards.