The purpose of this paper is to propose a procedure that automatically extracts color halftones from a document image and then converts them into continuous-tone images. An extraction method of color halftones is proposed, which is based on analysis of the Fourier spectrum of the color halftones. The characteristics of color halftone patterns in the Fourier spectrum are used to determine halftone regions in the document image. Particularly, this method is designed not to extract the background-halftone region that consists of text and simple halftones. This method is also applicable to extract arbitrarily shaped halftone regions. The extracted halftone regions are then converted into continuous-tone images by using the color inverse halftoning that is proposed in our previous work. A new color-channel separation method for the color inverse halftoning is proposed in this paper. Additionally, an enhancing method for inverse halftoned images is also presented in order to reduce various distortions in printing or scanning process. The proposed methods can be effectively used in the field of digital library, World Wide Web (WWW), and content-based image retrieval.
A new color inverse halftoning method that converts a scanned color image, halftoned by the clustered-dot ordered dither, into a more natural continuous-tone image, is proposed by analyzing the Fourier spectrum of color image. A color channel separated from the color image makes three kinds of peaks in its Fourier spectrum: a channel peak, an interference peak, and a moire peak. The channel peak is formed by the repeated pattern of the channel halftone cells in the color channel, whereas the interference peak is made by the other channel halftone cells, due to imperfect separation of the color channel. The moire peak is the secondary peak formed by the interaction of the channel peak with the interference peak. A new smoothing mask for each color channel was designed to effectively remove not only the channel peak but also the interference peak, thereby making a smoother continuous-tone image. A block based moire extraction algorithm has also been developed to remove the moire peak adaptively, which can smooth the moire region in the color channel without blurring the other regions. Experiments show that the proposed method outperforms all the published results from other color inverse halftoning methods.
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