calculate the contrast the absolute minimum value max k is preferably determined from sub maxima for an environment that is expanded 3 u00d7 3 for example from three columns sub maxima through this determination of k by means of max k the influence of the grey picture maximum value on the contrast and thus on the binarization result is slowed down bright outliers in the grey picture as well as edge transitions which occur for example with labels or see through windows on letters would otherwise result in a change of contrast that is too strong a noticeable contrast change should however be caused only by the written text each picture dot of the grey picture is stored temporarily in the pixel memory 6 until average value minimum value max k and contrast have been calculated for this dot the measured data minimum value maximum value and contrast preferably can be determined at the same time for all pictures n in that case the binary images differ only by the additional parameters of the decision diagram shown in the following fig3 in the decision diagram the actual average value is plotted on the abscissa and the actual pixel value on the ordinate for each pixel p i to be binarized three different characteristic curves are shown in fig3 wherein the binarization sensitivity increases with the increasing steepness of the linear portion the value 34 white 34 is assigned above the characteristic curve for a given pixel with a given average value and the value 34 black 34 is assigned below the.