Descreening Using HOG-based Adaptive Smoothing Filter

In this paper, a novel descreening method using a histogram of oriented gradients (HOG) based on an adaptive smoothing filter is proposed. Conventional descreening methods, which are used for recovering continuous-tone from scanned halftone images, are based on low-pass filtering approach. The low-pass filter has been applied to spatial domain, frequency domain, or both to remove the high frequency halftone noise. However, clipping high frequency components tends to blur images because image details also show high frequency characteristics. Moreover, due to the large variance in halftone noise, it is difficult to preserve edges by conventional adaptive smoothing methods. Therefore, to improve the descreening effect, more care has to be taken in distinguishing between edges and halftone noise. The proposed method employs HOG to distinguish edges. The amount of smoothing to be performed on the halftone image is then calculated according to the magnitude of the HOG in the edge and edge normal orientation. The proposed method was tested on various scanned halftone materials, and the results show that it removes halftone noise as effectively as the Moiré pattern while still preserving image details.