AUTHENTICATION OF PAPER DOCUMENTS WITH HELP FROM DIGITAL MEANS

The invention relates to a method for facilitating forgery detection of a received paper document (12) using a mobile device (13). The method comprises the following steps: a) receiving (1) an original reference image (10), b) receiving (2) an image (15) of the received paper document (12) captured using a camera in the mobile device (13), c) receiving (3) a first threshold δ and a second threshold τ, d) determining (4) a homography between the captured image (15) and the original reference image (10), e) aligning (4) the captured image (15) to the original reference image (10) based on the determined homography, the aligning providing an aligned captured image (17), f) determining (5) a difference map (16) based on the aligned captured image and the original reference image, g) determining (5) a first difference region in the difference map using the first threshold δ and the second threshold τ, and determining (5) a first region in the aligned captured image and a first region in the original reference image corresponding to the determined first difference region, h) determining (6) a further region in the aligned captured image based on the first region in the aligned captured image, and determining (6) a further region in the original reference image based on the first region in the original reference image, i) determining (7) a further homography between the further region in the aligned captured image and the further region in the original reference image, j) aligning (7) the further region in the aligned captured image to the further region in the original reference image based on the determined further homography, the aligning providing an aligned further region of the aligned captured image, k) determining (8) a further difference map based on the aligned further region and the further region in the original reference image, I) determining (8) a second difference region in the further difference map using the first threshold δ and the second threshold τ, and determining (8) a second region in the aligned further region and a second region in the original reference image corresponding to the determined second difference region, and m) providing an output region, based on the second region, in the aligned further region and an output region, based on the second region, in the original reference image as output (9). The invention also relates to a computer programme product and to a mobile device configured to carry out the computer programme product