Detection of hidden marks on Hwatu cards used in illegal gambling.

Hwatu is a popular card game widely played in both Korea and Japan. This study examined hwatu cards used in the fraudulent gambling in Korea. Fraudsters used the hwatu cards, with hidden marks printed on the back using special ink to identify the cards so that they could deceive unknowing game players. These marks are invisible to the naked eye under normal conditions and are only visible when wearing special lenses. When suspicious hwatu cards are sent to the forensic laboratory, detection of the hidden mark conducted by the microscope, video spectral comparator (VSC), and radiography, which are laborious. In this study, we developed visualization of the hidden mark on the hwatu cards by utilizing a set of the algorisms including color splitting, histogram normalization, FFT denoising, These algorithms were applied to mobile applications affording convenience and accessibility to help prevent fraud or to help law enforcement conduct an immediate investigation. The proposed method was confirmed to be simple and effective in detecting hidden marks on hwatu cards without the need for costly equipment.