1. A method of processing image signals from a first and a second image sensor, both of a document scanner unit, where said first image sensor provides a first image, and said second image sensor provides a second image, said first and second images being two-dimensional images wherein said first and second image are recorded at respective areas of a scanned medium, where a portion of the respective areas is recorded also by the other of the respective image sensors, whereby image data from overlapping and non-overlapping areas are present in the first and the second image; the method comprising: selecting a first sub image at a location in the overlapping area of the first image, and a second sub image at a location in the overlapping area of the second image, wherein the first and the second sub image both comprise image data from the same area of the scanned medium; correlating said first image with said second image, creating a first correlation image indicative of correlations at various displacement between said first and second sub images; wherein the method further comprises repeating the above steps with sub images taken from different locations than the first and the second sub image, thus creating a second correlation image, and determining a displacement between the first and the second image resulting from a physical displacement between the first and the second image sensor, by processing the first and the second correlation image; and wherein a plurality of displacements between the first and the second image is determined by processing a plurality of correlation images.