KEYWORDS: Digital watermarking, Tolerancing, Geographic information systems, Distance measurement, Computer simulations, Data acquisition, Photographic analysis, Photography, Astatine, Information security
A blind watermarking system for geographic vector-data is proposed which is robust against the following attacks:
polyline simplifications, cropping and shifting of data-points as well as small perturbations caused by random
noise. The watermark information is embedded by changing the x/y-coordinates of the data-points within the
tolerance range of the data. For that purpose the map is divided into horizontal and vertical stripes whose widths
depend on a secret key. Each strip possesses a 0-line and a 1-line. Depending on the watermark information
all points within a stripe are shifted on either one of the two lines. As a consequence of the variable strip-size
structure no extra bits for synchronization are necessary. The application of the proposed scheme is not restricted
to the case of vector geo-data, other kinds of vector-graphics data can be treated in the same way.
KEYWORDS: Distortion, Digital watermarking, Geographic information systems, Visualization, Data hiding, Data acquisition, Computer security, Computer graphics, Photographic analysis, Photography
We propose a reversible embedding watermarking algorithm for georeferenced 2D-vectordata which provides a promising solution to GIS (geographic information system) data hiding and authentication applications with a high requirement of fidelity or bit-by-bit exactness with the original point coordinates.
The proposed scheme uses an 8-point integer DCT-transform to exploit the high correlation among neighboring coordinates in the same polygon. There exist two kinds of distributions of DCT-coefficients where the typical one shows energy concentration in the low frequency range while in the other case the highest frequency coefficient holds the maximum DCT-coefficient. In the first step of our scheme a distinction between these two cases has to be done. For this discrimination the DCT-coefficient of the highest frequency is compared with the coefficients of the lower frequency range. Only for the typical cases the information is embedded. This is accomplished with a bit-shift procedure where the DCT-coefficients of certain frequencies are shifted by one or two bits. The watermarking information is embedded in the resulting gaps. The frequencies are lying in the range not being used for the discrimination task of step one. Depending on a key there is an alternating sequence of shifts by one ore two bits.
This paper deals with the issue of watermarking 2D-vector data (point coordinates) which are used in Geographical Information Systems (GIS). The tolerance range of the coordinates is used to embed the information. As an embedding constraint no violation of the tolerance range at each point may occur. In the first step of our approach we lay a rectangular grid over the map, where the origin, the extension, and the subdivision of this grid is only known by the embedder. Based on a secret key we choose two disjunctive sets A and B of grid elements namely “patches.” Now these selected elements are further divided into smaller subareas or “subpatches.” In subpatches belonging to set A the distances of points to a reference line within each subpatch are decreased while the distances of subpatches of set B are unchanged. In the retrieving procedure we consider all subpatches belonging to the sets A and B and calculate for each of the two sets their sample variance. The decision whether the map is watermarked with a certain key or not is based on this feature and is made with the help of a hypothesis test. The proposed method is robust against attacks changing the coordinates within the tolerance range and against polygon simplification procedures.
This paper deals with the issue of watermarking 2D-vector data which are used in Geographical Information Systems (GIS). The watermark is embedded in the tolerance range of the coordinates, where one bit of the watermarking information is represented by one PN-sequence, whose elements consist of the two values +tolerance and - tolerance. To robustly embed one bit of the watermarking information the length of the PN-sequence has to be much greater than the square maximum coordinate value leading to non-acceptable sequence lengths due to high coordinate values. To achieve a PN-sequence length that is suitable to the size of the data domain we do not consider the whole coordinate value but only those decimal digit positions of the coordinate value, where changes are significant but not violating the tolerance requirements. Due to this restriction on a smaller range of values, overflow and underflow has to be considered during the embedding process. Within the retrieval process we first extract this fraction of the coordinate value before correlating it with the PN- sequence. The proposed method is robust against attackers changing the coordinates within the tolerance range.
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