1. A document authentication method implemented in a data processing system, comprising: (a) obtaining an original grayscale image representing an original document, the original grayscale image including one or more halftone text areas or light text areas and one or more non-halftone text areas or dark text areas, wherein each halftone text area includes a plurality of halftone dots, each halftone dot being formed by a plurality of pixels, each pixel being a smallest unit of the original grayscale image and each pixel having a grayscale pixel value; (b) separating the halftone or light text areas from the non-halftone or dark text areas of the original grayscale image; (c) separately binarizing the halftone or light text areas and the non-halftone or dark text areas generated by step (b); (d) down-sampling only the binarized non-halftone or dark text areas generated by step (c) which correspond to the non-halftone or dark text areas of the original grayscale image, without down-sampling the binarized halftone or light text areas which correspond to the halftone or light text areas of the same original grayscale image, whereby a binarized original image is generated; (e) obtaining a target grayscale image representing a hardcopy target document, the target grayscale image including one or more halftone text areas and one or more non-halftone text areas, wherein each halftone text area includes a plurality of halftone dots, each halftone dot being formed by a plurality of pixels, each pixel being a smallest unit of the target grayscale image and each pixel having a grayscale pixel value; (f) separating the halftone text areas and the non-halftone text areas of the target grayscale image; (g) separately binarizing the halftone text areas and the non-halftone text areas generated by step (f); (h) down-sampling only the binarized non-halftone text areas generated by step (g) which correspond to the non-halftone text areas of the target grayscale image, without down-sampling the binarized halftone text areas which correspond to the halftone text areas of the same target grayscale image, whereby a binarized target image is generated; and (i) comparing the binarized target image with the binarized original image to determine whether the target document is an authentic copy of the original document.
1. A document authentication method implemented in a data processing system, comprising: (a) obtaining an original grayscale image representing an original document, the original grayscale image including one or more halftone text areas or light text areas and one or more non-halftone text areas or dark text areas, wherein each halftone text area includes a plurality of halftone dots, each halftone dot being formed by a plurality of pixels, each pixel being a smallest unit of the original grayscale image and each pixel having a grayscale pixel value; (b) separating the halftone or light text areas from the non-halftone or dark text areas of the original grayscale image; (c) separately binarizing the halftone or light text areas and the non-halftone or dark text areas generated by step (b); (d) down-sampling only the binarized non-halftone or dark text areas generated by step (c) which correspond to the non-halftone or dark text areas of the original grayscale image, without down-sampling the binarized halftone or light text areas which correspond to the halftone or light text areas of the same original grayscale image, whereby a binarized original image is generated; (e) obtaining a target grayscale image representing a hardcopy target document, the target grayscale image including one or more halftone text areas and one or more non-halftone text areas, wherein each halftone text area includes a plurality of halftone dots, each halftone dot being formed by a plurality of pixels, each pixel being a smallest unit of the target grayscale image and each pixel having a grayscale pixel value; (f) separating the halftone text areas and the non-halftone text areas of the target grayscale image; (g) separately binarizing the halftone text areas and the non-halftone text areas generated by step (f); (h) down-sampling only the binarized non-halftone text areas generated by step (g) which correspond to the non-halftone text areas of the target grayscale image, without down-sampling the binarized halftone text areas which correspond to the halftone text areas of the same target grayscale image, whereby a binarized target image is generated; and (i) comparing the binarized target image with the binarized original image to determine whether the target document is an authentic copy of the original document. 2. The method of claim 1 , wherein step (a) includes scanning the hardcopy original document to generate the original grayscale image, and step (e) includes scanning the hardcopy target document to generate the target grayscale image.