read only memory prom 102 the prom 102 generates a partial sum output signal corresponding to the 4 bits of image data which are applied thereto and address the prom 102 for example if 3 of the 4 bits of image data are 34 ones 34 indicating that the corresponding pixels are to be printed and one of the 4 bits is a 34 zero 34 indicating the corresponding pixel is not to be printed the partial sum is equal to three of course other translator means for example an adder circuit could be utilized to translate the incoming 4 bits of image data to arrive at a partial sum for accumulating the number of pixels to be printed on a given copy sheet however the prom is convenient and permits processing versatility in the event that the image data are presented in other data formats for example a set of 4 data bits could correspond to coded grayscale values for four by four blocks of pixels as described in the previously referenced u s patent application ser no 559 142 in that event the prom 102 can be conveniently programmed to generate the number of pixels to be printed for a given grayscale value of a four by four block of pixels i e the correct partial sum for the portion of the image represented by the 4 data bits the block diagram circuitry of fig2 is sized to accumulate the number of pixels to be printed for 4 scan strips i e 16 scan lines each partial sum generated by the prom 102 is added to any previous sum which has been accumulated for a series of 16 scan lines by means of an adder circuit 104 a 16 bit counter circuit.