A Linear Filter Design Technique for Equalizing Document Scanners

摘要

In this paper, we propose a new technique to automatically restore the sharpness of blurred documents by-equalizing the frequency response of given scanners using linear filters. To measure the blur characteristics of a scanning device, we measure its both horizontal and vertical Spatial Frequency Response (SFR). Starting from the measured SFR of the scanning device, we design an equalizing filter so that the combined SFR of the equalizing filter and the scanner resembles a perfect SFR. The desired 2D frequency response of the filter is computed using linear interpolation of the horizontal and vertical responses derived from the corresponding SFRs of the scanner. The filter design technique is two steps. First, a linear system of equations is constructed using the unknown filter coefficients and the desired filter 2D frequency response. The linear least squares method is used to solve the linear system of equations. The second step of the filter design uses a nonlinear optimization technique to refine the results of the first step. Our experimental results show that this automated process can be applied to different document scanning devices to equalize their spatial frequency response resulting in consistent output sharpness levels.