Pre-processing of bi-level image data

The bi-level image data supplied from an image supply means, such as an image scanner are pre-processed by the present invention, and the pre-processed image data are then supplied to a compression means, which compresses the image data under a conventional two-dimensional coding scheme, e.g. such as provided by the CCITT Recommendation T.4. The pre-process detects (3,4, 5) a changing element in a first scan line and a changing element in a second scan line. It then determines (6, 7) whether the distance between the changing element in the first scan line and the changing element in the second scan line is falling within a predetermined range. The image data are re-arranged (8, 9, 10) so as to decrease the said distance to a predetermined smaller value when the distance is falling within the said predetermined range, and otherwise are preserved unchanged. The re-arranged and the otherwise preserved image data are then supplied (11) to means for performing the compression.