A computing device (100, 200) comprising a display (120) and a controller (210), wherein said controller (210) is configured to detect and track an object (H) via a video stream (265) provided by a camera (160, 260), detect a movement (G1, G2) of the object (H), translate said movement (G1, G2) of the object (H) to a resulting movement (M1, M2) of a marker (136) based on a scale, detect a change in distance to the object (H), and adapt said scale accordingly.