From Rovinj to Lucerne there is a direct bus during the peak summer season, you can see operation time under the info icon below. Rest of the year it is possible to travel from Rijeka to Lucerne, you can see timetable for buses from Rovinj to Rijeka here, and here you can see bus departures from there on to Lucerne. Buses from Rovinj departure from the main bus station, location and address you can see here. Keep in mind that there is a considerable saving if you buy a return ticket