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28 May 2004 A directional edge-sensitive image interpolator
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One of the most common tasks in image processing is to change the resolution of a picture. In this paper we present a new nonlinear method for interpolating digital images, which is particulary effective in the rendition of edges in natural and synthetic input. The algorithm is spatial variant and applies the warped distance (WaDi) concept, generalizing the technique to a two dimensional problem, which requires a non-separable approach. It consists of three separate stages. First of all the original image is analyzed to detect its local gradient characteristics; then edge asymmetry is computed at each output pixel position according to the WaDi technique, and it is compared to a reference sigmoidal edge; the local edge asymmetry straightforwardly determines the warping factor which is applied to the bi-dimensional space of the image; eventually the actual interpolation is performed applying a conventional interpolator such as the linear or bicubic ones. The resulting interpolation method gives an output which does not present the usual blurring typical of images processed with linear interpolators and at the same time preserves the regularity of resized edges avoiding jagging artifacts. Moreover, the method adapts for zooming by a rational scaling factor. The paper is organized as follows. In the first section we introduce the problem of zooming digital images; the second section describes the state of the art; we continue describing the proposed method; then we propose a possible extension of the method to process color images; we end showing some examples of images interpolated with our method, and comparing these results with what can be obtained zooming the same input with other interpolators.
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Francesco Zotta, Paola Carrai, Giulio Ferretti, and Giovanni Ramponi "A directional edge-sensitive image interpolator", Proc. SPIE 5298, Image Processing: Algorithms and Systems III, (28 May 2004);


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