9. An apparatus for removing a mark in a document image, the apparatus comprising: an extracting device configured to extract connected components from a binary image corresponding to the document image; a clustering device configured to cluster the connected components based on grayscale features of the connected components to obtain one clustering center; a searching device configured to search, within numerical ranges of a clustering radius R from the clustering center and a grayscale threshold T, for a combination (R, T) which causes an evaluation value based on the grayscale features of the connected components to be higher than a first evaluation threshold; and a fine removing device configured to remove the mark in the document image based on the grayscale threshold in the combination; wherein the grayscale features of the connected components comprise: a minimum one of grayscale values of pixels in the document image, which correspond to all black pixels in each of connected components; wherein the fine removing device is further configured to remove the connected components, the grayscale features of which are greater than the grayscale threshold, as the mark in the document image.
9. An apparatus for removing a mark in a document image, the apparatus comprising: an extracting device configured to extract connected components from a binary image corresponding to the document image; a clustering device configured to cluster the connected components based on grayscale features of the connected components to obtain one clustering center; a searching device configured to search, within numerical ranges of a clustering radius R from the clustering center and a grayscale threshold T, for a combination (R, T) which causes an evaluation value based on the grayscale features of the connected components to be higher than a first evaluation threshold; and a fine removing device configured to remove the mark in the document image based on the grayscale threshold in the combination; wherein the grayscale features of the connected components comprise: a minimum one of grayscale values of pixels in the document image, which correspond to all black pixels in each of connected components; wherein the fine removing device is further configured to remove the connected components, the grayscale features of which are greater than the grayscale threshold, as the mark in the document image. 11. The apparatus according to claim 9 , wherein the clustering device is further configured: to count a number of connected components within a range of a given clustering radius when each connected component serves as a clustering center, in a feature space of the grayscale features; and to determine a connected component corresponding to a maximum one of counted numbers as the clustering center.