A zero result detector for detecting a zero result in the sum of a first operand A, a second operand B and a carry bit Cin operates by calculating {overscore } and {overscore }1 and then comparing one of these with B }; Cin1, {overscore }1) in dependence upon Cin. When output of a document is instructed, it is determined whether an output history associated with the document exists in the recorded operation history. To achieve sub-pixel resolution, the virtual mask, after a first partial exposure, is offset by less than a pixel-width and a second partial exposure is made. The output of the control circuit is connected to the bases of the differential pair of the variable gain amplifier, whereupon the ratio of the output current to the input current of the pair becomes dependent on the control voltage.