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Browsing by Author "Liu, Xiangdong"

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    Analysis and Reduction of Moire Patterns in Scanned Halftone Pictures
    Liu, Xiangdong (Virginia Tech, 1996-05-01)
    In this dissertation we provide a comprehensive theory for the formation of a moire pattern in a sampled halftone image. We explore techniques for restoring a sampled halftone image with a moire pattern and techniques for preventing a moire pattern when a halftone picture is scanned. Specifically, we study the frequency, phase, and spatial geometry of a moire pattern. We observe and explain the half period phase reversal phenomenon that a moire pattern may exhibit. As a case study, we examine the moire patterns generated by a commercial scanner. We propose three restoration methods, including a notch filtering method, a simulation method, and a relaxation method. We also describe a moire prevention method, the partial inverse Fourier transform method. Finally, we propose a research agenda for further investigation.
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    Subpixel Edge Location in Binary Images Using Dithering
    Liu, Xiangdong; Ehrich, Roger W. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1993-06-01)
    This paper concerns the problem of obtaining subpixel estimates of the locations of straight lines in digital images for purposes of machine vision. In particular, it presents a dithering method for improving the estimation accuracy on a rectangular sampling lattice. By adding uniformly distributed independent random noise it is shown that estimation bias may be removed and that the estimation variance is inversely proportional to the length of the line segment. The sensitivity to incorrect dither amplitude is calculated, and a novel approach is given for adding the dither by using grey-level image sensor and utilizing the imaging model.