Traps are produced at color boundaries in a print page that is present in the form of run-length encoded image data, by way of distance mapping. Distances of the run lengths to an adjacent color are determined without decoding the run lengths into a pixel matrix, and the traps are produced on the basis of the distances thus determined. For this purpose, distance vectors are determined for the starting point and for the end point of a run length. The distance vectors are in each case recalculated from the comparison with the distance vectors of adjacent run lengths. The new values of the distance vectors replace the previous values of the distance vectors if the magnitudes of the distances of the new values are less than the magnitudes of the distances of the previous values.