Document image sharpening using a new extension of the aperture filter

A new extension to the aperture filter is proposed to sharpen document images. The aperture filter is a nonlinear filter applied within a window having both domain and range constraints. The proposed aperture filter incorporates an adaptation to the original design by utilizing gradient directions of the input document images. Results demonstrate that the performance of the new approach is superior to that of both the aperture filter and alternative sharpening methods.