to detect sheet displacements within about 0 0001 inches normally a pixel has a diameter of about 0 0033 inch as sheet 8 continues to advance in the system of fig1 the net displacement between consecutive patterns is computed and the last acquired pattern is stored in memory device 47 the stored pattern is subsequently available for cross correlation with the next acquired pattern accordingly the system of fig1 can measure sheet displacements that are longer than the field of view of the ccd sensor at this juncture it may be noted that only a small fraction of sheet 8 is 34 seen 34 at any given time by ccd sensor 19 of fig1 however as long as there is a significant amount of common information between two consecutively viewed patterns the system can provide meaningful pattern matches i e cross correlations and hence can accurately detect the sheet advancement distance it may also be appreciated that the above described pattern identification process is not affected by printing or other marks on a page such markings can in fact assist in the pattern matching process thus the system of fig1 can operate properly when pre printed forms or letterhead stationery are being printed the foregoing has described the principles preferred embodiments and modes of operation of the present invention however the invention should not be construed as limited to the particular embodiments discussed for example the invention can be used in conjunction with the printing of.