Ho-Keun Lee

Embedding positional-independent hidden data into hardcopy

We propose an approach for embedding of imperceptible hidden data into printed text document with ability to extract this data during scanning. Our approach is based on intrusion of cluster of small black dots (patterns) with predefined positional-independent structure into the document. Such intrusion will not visibly deteriorate document itself. Proposed multiple duplications of embedded information and statistical approach during hidden message extraction provide method with high immunity against document modification or noise.

Generation of PDF with vector symbols from scanned document

The paper is devoted to the algorithm for generation of PDF with vector symbols from scanned documents. The complex multi-stage technique includes segmentation of the document to text/drawing areas and background, conversion of symbols to lines and Bezier curves, storing compressed background and foreground. In the paper we concentrate on symbol conversion that comprises segmentation of symbol bodies with resolution enhancement, contour tracing and approximation. Presented method outperforms competitive solutions and secures the best compression rate/quality ratio. Scaling of initial document to other sizes as well as several printing/scanning-to-PDF iterations expose advantages of proposed way for handling with document images. Numerical vectorization quality metric was elaborated. The outcomes of OCR software and user opinion survey confirm high quality of proposed method.

High-performance automatic cropping and deskew of multiple objects on scanned images

The paper is devoted to a novel high-performance algorithm for automatic segmentation and skew correction of several objects on a scanned image. The complex multi-stage technique includes preprocessing, initial segmentation, classification of connected regions, merging of fragmented regions by heuristic procedure, bounding boxes detection and deskew of rectangular objects. Our method is highly effective owing to unification most of operations in one pass. Algorithm provides users with additional functionality and comfort. The method is evaluated by suggested quantitative quality criteria.

Descreening of scanned images

Screen or halftone pattern appears on the majority of images printed on electrophotographic and ink-jet printers as well as offset machines. When such halftoned image is scanned, a noisy effect called a Moiré pattern often appears on the image. There are plenty of methods proposed for descreening of images. Common way is adaptive smoothing of scanned images. However the descreening techniques face the following dilemma: deep screen reduction and restoration of contone images leads to blurring of sharp edges of text and other graphics primitives, on the other hand insufficient smoothing keeps screen in halftoned areas. We propose novel descreening algorithm that is primarily intended for preservation of sharpness and contrast of text edges and for restoration contone images from halftone ones accurately. Proposed technique for descreening of scanned images comprises five steps. The first step is decrease of edge transition slope length via local tone mapping with ordering; it is carried out before adaptive smoothing, and it allows better preservation of edges. Adaptive low-pass filter applies simplified idea of Non-Local Means filter for area classification; similarity is calculated between central block of window and different adjacent block that is selected randomly. If similarity is high then current pixel relates to flat region, otherwise pixel relates to edge region. For prevention of edges blurring, flat regions are smoothed stronger than edge regions. By random selection of blocks we avoid the computational overhead related to excessive directional edge detection. Final three stages include additional decrease of edge transition slope length using local tone mapping, increase of local contrast via modified unsharp mask filter, that uses bilateral filter with special edge-stop function for modest smoothing of edges, and global contrast stretching. These stages are intended to compensate decreasing of sharpness and contrast due to low-pass filtering, it allows to enhance visual quality of scanned image. For parameters adjusting for different scanning resolutions and comparison with existing techniques test target and criteria were proposed. Also the quality of proposed approach is evaluated by surveying observers opinions. According to obtained outcomes the proposed algorithm demonstrates good descreening capabilities.