The invention relates to a method and a system for purifying process water, particularly for continuously purifying process water in the paper industry. These two input devices can be used substantially simultaneously to provide a command to the device. The controller sets a processing area within the image captured by the image sensor based on the position of the object determined by the position sensor and a predetermined size for the object to be recognized. The buddy list system attempts to match co-users currently logged into the system with the entries on the user’s buddy list. In response to the number of missed touches reaching a predetermined threshold, both the size of the viewable area and the clickable area of at least one of the input areas, and possibly all areas of a similar size or type, are increased so that the input areas will be easier to touch.