variable threshold value is determined with an adaptive method the comparison of the gray value representation of the original document with this threshold value supplies the binary quantizing image if the original document exists as a colored document it is converted to begin with into a gray scale image and two color difference component images the gray scale image is used for determining the locally variable threshold value image the gray scale image which normally exists with a resolution of 150 to 600 dpi is reduced to a suitable size for this purpose a low pass filtering with subsequent sub sampling is carried out local distortions in the original scan such as noise or dither and raster effects are decreased by the reduction a local dynamics analysis is carried out next for this purpose the reduced gray scale image is subjected to a minimum filtration and a maximum filtration the difference between the maximum image and the minimum image produces a dynamics image which supplies indications regarding regions with strong edges such as text regions in the next step the dynamics image is compared with a minimum dynamic which can be specified externally and controls the sensitivity of the quantizing and of the text detection in regions the dynamics of which exceed this minimum value a quantizing threshold value is determined from half the sum of the minimum image and the maximum image in regions the dynamics of which are too low the quantizing threshold value initially is set equal to zero since the threshold values so determined are subject to strong local.