1. A method of processing image signals from a first and a second image sensor arranged as staggered linear arrays along an x-direction, 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 document, where the document is scanned by relative movement in a y-direction between the image sensors and the document, and where a portion of each respective area is recorded by the staggered first and second 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 two-dimensional sub image at a location in the overlapping area of the first image, and a second two-dimensional 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 document; correlating said first sub image with said second sub image, creating a first correlation image indicative of correlations at various displacements 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 processing the first and second correlation image to determine first and second displacements between the sub-images of the first and the second image thereby registering time-varying relative displacements in the y-direction between scanning lines of the first and the second image sensors; and wherein the first and second displacements in the y-direction are used to combine the first and the second image into a third image by re-sampling one or both of the first and second image in at least the y-direction using the determined first and second displacements.