CN102110281A - Method for embedding and extracting watermarks by adopting difference expansion aiming at shape point map layer data - Google Patents
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Abstract
The invention discloses a digital watermark method for embedding and extracting watermarks by adopting a difference expansion technology aiming at shape point map layer data. The watermark embedding method comprises the following processes of: analyzing all points in shape carrier data to determine a projection axis; sequencing all the points according to the projection axis to form a continuous point sequence with unique value; sequentially pairing every two data of the point sequence, judging whether the watermarks are suitable to be embedded into the points of the group by adopting the difference expansion technology by using the corresponding data precision requirements of geographic information system (GIS) vector data as judgment conditions, if so, modifying the coordinate difference of the related points to embed watermark information, otherwise, embedding the watermark information by using a least significant bit (LSB) displacement method; and simultaneously recording the embedded information in the process. According to the method, the difference expansion technology can be applied in reversible watermark embedment and extraction of the GIS point map layer data and applied in interpolation identification of the GIS vector point map layer data, copyright protection, GIS vector point map layer data-based concealed communication and the like.
Description
Technical Field
The invention belongs to the field of geographic information copyright protection, and particularly relates to a digital watermark method for embedding and extracting watermarks by adopting a difference value expansion technology aiming at shape point diagram layer data; the invention relates to a reversible method, which can recover original data by combining a secret key in the process of extracting a watermark.
Background
At present, the digital watermarking technology is becoming a main technical means for solving the safety problem of Geographic Information System (GIS) vector data products. Scholars at home and abroad propose various digital watermarking algorithms aiming at GIS vector data. As the embedding of watermark data inevitably brings disturbance to the original data, the quality of GIS vector data is reduced to a certain extent, and therefore, a reversible watermark technology is introduced. The reversible watermarking technology, namely lossless data hiding, refers to a watermarking algorithm capable of completely recovering original carrier data. The main methods of the current reversible watermarking technology include utilizing difference value expansion, lossless compression, reversible modulo addition and histogram change to achieve reversibility of an algorithm, and reversible algorithm research aiming at GIS vector data is only concerned by a small number of students.
The basic idea of the difference expansion technology is to provide extra storage space for watermark data without losing original information by performing a shift operation on the coordinate difference of adjacent points in carrier data that satisfy the embedding precision condition. Among them, shore cheng et al (the graphic journal of Chinese image, Vol.12, No.2, 206 and page 211, 2007) propose a lossless data hiding algorithm applied to vector maps based on the idea of difference expansion. The essential characteristic is that the characteristic that coordinate data of adjacent vertexes of the map have strong correlation is utilized, so that the algorithm is only suitable for two data types, namely a polygonal curve and a polygon in GIS vector data, and cannot process point data, which undoubtedly brings certain obstacles to the propagation and popularization of the technology.
Disclosure of Invention
The invention aims to: aiming at the problem that the existing difference value expansion technology cannot be applied to reversible watermark embedding and extraction of GIS point diagram layer data, a watermark embedding and extracting method aiming at shape point diagram layer data is provided, so that the difference value expansion technology can be applied to a digital watermark algorithm of each data type of GIS vector data.
In order to achieve the purpose, the technical scheme adopted by the invention mainly comprises the following processes:
(1) and (3) watermark embedding process:
the method comprises the following steps: reading GIS point map layer data, configuring parameters of an embedded algorithm, mainly comprising precision error tolerance, coordinate expansion multiple and key file storage path, and then checking the legality of input data and configuration parameters;
step two: analyzing the point data, and determining the projection axis adopted by the embedding process according to the following formula:
Y＝Ymax-(Ymax-Ymin)×1/2
wherein Y represents the transverse projection axis, Ymax is the maximum ordinate in the data points, and Ymin is the minimum ordinate in the data points; sorting according to the coordinate values of the data points on the projection axis from small to large, and selecting the point which is closest to the projection axis in the vertical direction before projection as the point for embedding the watermark for the overlapped points with the same coordinate; thereby, a point sequence with unique and continuous coordinates in the projection axis direction is formed;
step three: embedding the reversible watermark by using a difference expansion technology by taking the point sequence as a watermark embedding carrier, specifically adopting a mode of doubling the difference, namely, the coordinate difference in a binary form is shifted by one bit to the left;
(a) firstly, pairing the data in the second step in pairs in sequence, and matching the original decimal coordinate x0iBecomes an integer xiThe formula is as follows:
wherein,
expressing rounding-down, wherein K is the coordinate expansion multiple in the step one;
(b) then calculating the difference d and the integer average value m of the coordinate values of every two matching points;
(c) and taking the data precision requirement corresponding to the GIS vector data as a judgment condition, substituting d into the following formula to judge whether the group of points are suitable for embedding the watermark by adopting a difference value expansion mode:
-2×R+1≤d≤2×R-2
where R is the precision error margin, xfrontAnd xbackIs the point pair xiAnd xi+1Coordinates of the front and rear points; if d does not satisfy any of the three items, in order to increase the embedding capacity, embedding the watermark by replacing the least significant bit; otherwise, embedding the watermark by adopting a difference expansion mode, and recording related information in the embedding process so as to recover the original data;
step four: storing the point data after embedding the watermark, and storing the related information generated in the embedding process as a key file;
(2) watermark extraction and original data recovery process:
the method comprises the following steps: reading GIS point map layer data of the watermark to be extracted, reading and embedding relevant parameters from a key file, and checking the legality of the parameters;
step two: determining a projection axis adopted in the watermark extraction process according to the read parameters, sequencing the projection axis according to the coordinate values of the points on the projection axis from small to large, and selecting the point which is closest to the projection axis in the vertical direction before projection as the point for extracting the watermark for the overlapped points with the same coordinate; therefore, a point sequence with unique and continuous coordinates in the projection axis direction is formed, and the point sequence is a carrier for extracting the watermark;
step three: by using the method in the third step of the watermark embedding process, pairwise pairing and grouping are carried out on the point sequences, and the difference value and the integer average value of coordinate values of every two paired points are respectively calculated after the coordinates are enlarged; then sequentially extracting the lowest bit of the coordinate difference value of each point pair to obtain an embedded watermark; and respectively calculating an original difference value for each point pair: finally, calculating the recovered coordinate data according to the coordinate difference value and the integer mean value;
step four: and storing the restored original data and outputting the extracted watermark.
Wherein the second step in the embedding process selects the intermediate value of the carrier data as the projection axis, mainly to reduce the influence of the embedded watermark on the data quality. In addition, the embedding, extraction, and the like are exactly the same as the above method for the ordinate.
The invention provides a method for embedding reversible watermarks in shape point diagram layer data by adopting a difference expansion technology according to the strict requirement of GIS vector data on data precision, and original point data can be restored without damage while the watermarks are extracted. The method is also suitable for GIS vector point data in other non-shape formats. The application prospect of the method comprises tampering identification and copyright protection of the GIS vector dot diagram layer data, hidden communication based on the GIS vector dot diagram layer data and the like.
Drawings
Fig. 1 is a flow chart of watermark embedding in the method of the present invention.
Fig. 2 is a flow chart of watermark extraction and recovery data in the method of the present invention.
Fig. 3 is the administrative village point diagram layer vector data of the Jiangsu province selected by the embodiment of the invention.
Fig. 4 is data of fig. 3 after partial enlargement.
Detailed Description
The following is a more detailed description of the embodiments with reference to the drawings.
The present embodiment selects a typical shape point data, and further describes the present invention in detail with respect to the whole process of data reading, processing, watermark embedding, result saving, watermark extraction, and data recovery. In this embodiment, the administrative point-level vector data (as shown in fig. 3) of Jiangsu province is selected as the experimental data, and the number of data records is 17521. The watermark marking content is text information of Nanjing university, and the corresponding binary watermark sequence W is 11000100110011111 … … 100111 with the length of 96.
1. And expanding a watermark embedding method aiming at the difference value of shape point diagram layer data.
Fig. 1 is an embedding flowchart of a difference expansion watermark embedding method for shape point diagram layer data according to the present invention. The whole process is divided into the following parts:
the method comprises the following steps: and reading and processing data.
1) Reading corresponding dot diagram data by using a reading function of the MapWinGIS open-source plug-in, organizing the data into a List < Point > structure, and recording the List < Point > structure as S0;
2) inputting copyright mark watermark information W, setting precision error tolerance R as 500m and coordinate expansion multiple K as 10000;
3) the data processing includes converting W into a watermark W2 to be embedded inside the algorithm, checking the validity of all input data and configuration parameters.
Step two: the transverse projection axis is calculated and determined.
Traversing the points in S0, getting the maximum ordinate Ymax 35.116 and the minimum ordinate Ymin 30.769, and then according to the formula:
Y＝Ymax-(Ymax-Ymin)×1/2 (1)
the projection axis Y is determined 32.9425.
Step three: and selecting a point sequence embedded with the watermark.
Sorting the points in the S0 from small to large according to the abscissa to obtain a result S1; checking S1, calculating the longitudinal distance from the point with the same horizontal coordinate to the transverse projection axis, reserving the point with the minimum distance, and removing the rest points to obtain a result S2, namely the point sequence to be embedded with the watermark, wherein the length is recorded as N. If it is
If the length of the watermark W2 is less than the length of the watermark W2, the point sequence cannot be embedded with a complete watermark once, and the process is ended; otherwise, the next step is entered. If the length N is 17489, the operation is continued
Greater than the length 96 of the watermark W2, the next step is performed.
Step four: and embedding the watermark into the point sequence by using a difference value expansion technology.
Firstly, the data S2 in step three are paired two by two in turn, and the paired first point pair x is used01、x02For example, the original decimal coordinate is changed into an integer, and the formula is as follows:
wherein,
meaning rounding down, K is the coordinate expansion factor entered in step one.
Then calculating the difference d and the integer average value m of the coordinate values of every two matching points:
d＝x1-x2 (4)
and (3) judging whether the group of points are suitable for embedding the watermark by adopting a difference expansion mode according to the following formula:
-2×R+1≤d≤2×R-2 (6)
where R is the precision error margin, xfrontAnd xbackIs the point pair x01、x02Before and after point coordinates of (a). The formula (6) is to ensure embedding of the watermarkThe caused data disturbance is smaller than the precision error tolerance of the map data, and equations (7) and (8) constrain the watermark-containing coordinate data to be between the front and rear coordinates of the data, so that the dot sequence order is prevented from being disordered due to the change of the data. If d does not satisfy any of the three items, embedding the watermark by replacing the least significant bit; otherwise, the watermark is embedded by adopting a difference value expansion mode. In the process, a binary flag sequence is generatedWherein f isiA position of 0 represents a point pair that is not suitable for differential extension embedding, and fiA position of 1 corresponds to a point pair suitable for differential extension embedding. This sequence of tokens is stored as part of the key.
Embedding a watermark in each point pair by adopting differential value expansion according to each bit mark (marked as f) in a mark sequence flag, wherein the formula is as follows:
f＝1，d′＝2×d+w (9)
obtaining the water-containing point pair x by calculating according to inverse transformation formulas of the formulas (4) and (5)01、x02The formula is as follows:
in the process of expanding the difference value to embed the watermark, the point pair with the flag of 0 stores the watermark information by directly replacing the Least Significant Bit (LSB). To ensure the reversibility of the algorithm, these replaced original LSB data must be saved for use in restoring the original data, denoted as LSB. As with flag, will be stored as part of the key.
Step five: and storing the data and the key file after the watermark is embedded. The specific process is as follows:
1) and storing the embedding parameters (such as coordinate expansion multiple) input in the embedding process, projection axis data obtained by calculation, a binary flag sequence flag and least significant bit lsb information of the original data as a key file in a certain form.
2) Integrating the watermark information-embedded point sequence S2 into the overall data S0;
3) the data S0 is saved to the hard disk.
2. And expanding the watermark extraction and data recovery method aiming at the difference value of shape point diagram layer data.
Fig. 2 is a flowchart of the operation of the method of extracting a watermark and recovering data. The method comprises the following specific steps:
the method comprises the following steps: and reading the point data of the watermark to be extracted.
Reading the corresponding dot diagram data by using a reading function of the MapWinGIS open-source plug-in, and organizing the data into a List < Point > structure, which is recorded as S0.
Step two: and reading and analyzing the key file.
In the step, a coordinate expansion multiple K, a projection axis Y, a binary identification sequence flag and an original least significant bit data sequence lsb are obtained.
Step three: and determining a point sequence for extracting the watermark.
Sorting the points in the S0 from small to large according to the coordinate data by taking the projection axis as a reference direction to obtain a result S1; checking S1, calculating the vertical distance from the point with the same coordinate to the projection axis, reserving the point with the minimum distance, and removing the rest points to obtain a result S2, namely the point sequence of the watermark to be extracted, wherein the length is recorded as N.
Step four: and extracting the watermark and recovering the data by using a difference value expansion method.
Pairwise pairing and grouping are carried out on the point sequences by the same method in the embedding process, the difference value d ' and the mean value m (formulas (4) and (5)) are respectively calculated after the coordinates of each point pair are enlarged (formulas (2) and (3)), and then the lowest bit position of the difference value d ' of each point pair is sequentially extracted to obtain the embedded watermark w '. Then, the original difference is calculated according to the following formulas (13) and (14) for each point pair respectively:
where f is the flag in the corresponding flag and l is the corresponding data in lsb.
Then, the coordinate data after the recovery is calculated according to the equations (11) and (12).
Step five: outputting the watermark and storing the recovered data.
1) Analyzing the watermark w' according to a rule to obtain an original watermark w (11000100110011111 … … 100111); the original copyright information is 'Nanjing university'.
2) Integrating the point sequence after recovery S2 into the total data S0;
3) the restored data S0 is saved to the hard disk.
In the extraction process of the watermark, not only can watermark information be obtained according to the embedded information and the original LSB data which are stored when the watermark is embedded, but also the original carrier data can be recovered without damage. The denser the points are, the stronger the correlation of coordinate values is, and the smaller the error introduced.
In addition, the embedding, extraction, and the like are exactly the same as the above method for the ordinate.
3. And (6) testing and analyzing.
The invention aims to solve the problem that the differential value expansion reversible watermarking technology cannot be applied to GIS vector point data, so that the test is mainly developed based on the data reversible recovery aspect. And through the step of extracting the watermark and recovering the data, the recovered data is obtained. The restored data and the original data are analyzed and checked by using the topology analysis of the ArcGIS software, and the result shows that the data is completely correct, so that the method has the capability of completely restoring the original data. In addition, data disturbance introduced by the algorithm is commonly borne by two points, so that errors borne by each point are greatly reduced, and the usability of the data is not influenced.
In conclusion, the method successfully applies the difference value expansion reversible watermarking technology to shape point diagram layer data, and meanwhile, the method is also applicable to GIS vector point data in other non-shape formats.
Claims (1)
1. A difference value expansion watermark embedding and extracting method for shape point diagram layer data mainly comprises the following processes:
(1) and (3) watermark embedding process:
the method comprises the following steps: reading GIS point map layer data, configuring parameters of an embedded algorithm, mainly comprising precision error tolerance, coordinate expansion multiple and key file storage path, and then checking the legality of input data and configuration parameters;
step two: analyzing the point data, and determining the projection axis adopted by the embedding process according to the following formula:
Y＝Ymax-(Ymax-Ymin)×1/2
wherein Y represents the transverse projection axis, Ymax is the maximum ordinate in the data points, and Ymin is the minimum ordinate in the data points; sorting according to the coordinate values of the data points on the projection axis from small to large, and selecting the point which is closest to the projection axis in the vertical direction before projection as the point for embedding the watermark for the overlapped points with the same coordinate; thereby, a point sequence with unique and continuous coordinates in the projection axis direction is formed;
step three: embedding the reversible watermark by using a difference expansion technology by taking the point sequence as a watermark embedding carrier, specifically adopting a mode of doubling the difference, namely, the coordinate difference in a binary form is shifted by one bit to the left; (a) firstly, pairing the data in the second step in pairs in sequence, and matching the original decimal coordinate x0iBecomes an integer xiThe formula is as follows:
wherein,
expressing downward rounding, wherein K is the coordinate expansion multiple in the step one, and N is the number of data points to be embedded with the watermark;
(b) then calculating the difference d and the integer average value m of the coordinate values of every two matching points;
(c) and taking the data precision requirement corresponding to the GIS vector data as a judgment condition, substituting d into the following formula to judge whether the group of points are suitable for embedding the watermark by adopting a difference value expansion mode:
-2×R+1≤d≤2×R-2
where R is the precision error margin, xfrontAnd xbackIs the point pair xiAnd xi+1Coordinates of the front and rear points; if d does not satisfy any of the three items, in order to increase the embedding capacity, embedding the watermark by replacing the least significant bit; otherwise, embedding the watermark by adopting a difference expansion mode; simultaneously recording related information in the embedding process so as to recover the original data;
step four: storing the point data after embedding the watermark, and storing the related information generated in the embedding process as a key file;
(2) watermark extraction and original data recovery process:
the method comprises the following steps: reading GIS point map layer data of the watermark to be extracted, reading and embedding relevant parameters from a key file, and checking the legality of the parameters;
step two: determining a projection axis adopted in the watermark extraction process according to the read parameters, sequencing the projection axis according to the coordinate values of the points on the projection axis from small to large, and selecting the point which is closest to the projection axis in the vertical direction before projection as the point for extracting the watermark for the overlapped points with the same coordinate; therefore, a point sequence with unique and continuous coordinates in the projection axis direction is formed, and the point sequence is a carrier for extracting the watermark;
step three: by using the method in the third step of the watermark embedding process, pairwise pairing and grouping are carried out on the point sequences, and the difference value and the integer average value of coordinate values of every two paired points are respectively calculated after the coordinates are enlarged; then sequentially extracting the lowest bit of the coordinate difference value of each point pair to obtain an embedded watermark; and respectively calculating an original difference value for each point pair: finally, calculating the recovered coordinate data according to the coordinate difference value and the integer mean value;
step four: and storing the restored original data and outputting the extracted watermark.
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| CN113284036A (en) * | 2021-06-15 | 2021-08-20 | 苏州科技大学 | Vector map reversible watermarking algorithm based on virtual grid interval mapping |
| CN113556440A (en) * | 2021-06-17 | 2021-10-26 | 中国人民解放军战略支援部队航天工程大学 | Line map lossless data hiding and restoring method for dividing embedded area |
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