Image processing unit (3), and method (300), for detecting event data during a game round of a roulette wheel (1) via a camera (2), thereby enabling detection of a roulette wheel bias. The roulette wheel (1) comprises a ball track (11A), upon which a roulette ball (14) is brought into rotation around the roulette wheel (1), and a sloping lower ball track (11B) comprising ball deflectors (13, 15), encircling a rotor (7). The image processing unit (3) is configured for: receiving a captured image of the roulette ball (14) from the camera (2); detecting that the roulette ball (14), under influence of gravity, reaches an imaginary circle (17) encircling the ball deflectors (13, 15) of the lower ball track (11B); and determining a position where it is detected that the roulette ball (14) reaches the imaginary circle (17); and storing the determined position in a database (6).