A first shaped video signal V.sub.1 K.sub.1 and a second shaped video signal V.sub.2 V.sub.2, are combined so as to generate a third shaped video signal V.sub.3 ' given byV.sub.3 '=V.sub.1 K.sub.1 [1-K.sub.2 (1-P.sub.12)]+V.sub.2 K.sub.2 (1-K.sub.1 P.sub.12)where P.sub.12 is a priority signal. A key signal K.sub.3 given byK.sub.3 =1-(1-K.sub.1)(1-K.sub.2)is also generated. An output processor receives the shaped video signal V.sub.3 ', the key signal K.sub.3 and a matte signal M.sub.3. In a first mode of operation, the output video signal V.sub.3 " of the output processor is given byV.sub.3 "=V.sub.3 +M.sub.3 (1-K.sub.3)and the key signal K.sub.3 ' that is generated by the output processor has a constant value, and in a second mode of operation of the output processor the output video signal is given byV.sub.3 "=V.sub.3 '/K.sub.3and the key signal K.sub.3 ' is directly proportional to K.sub.3.