JP4772151B2 - Digital watermark synchronization recovery device and digital watermark synchronization recovery program - Google Patents

Description

  The present invention relates to a digital watermark synchronization recovery apparatus and a digital watermark synchronization recovery program that detect the positions of a plurality of reference points embedded in a document and identify the embedded position of the digital watermark.

Conventionally, for example, there is a technique for improving the security of a printed material by superimposing a light pattern on a document such as a printed material and concealing information in a microscopic shape of the pattern.
For example, the following Patent Document 1 discloses a method of forming a pattern with dots and concealing binary information depending on the presence or absence of a dot at a specific position.
Patent Document 2 below discloses a technique in which blocks having a plurality of types of waveforms called information units are arranged and printed on a document, and information is embedded depending on which type of block exists at a specific position. ing. A pattern in which this information is embedded is often called a digital watermark.

In these methods, information is embedded by associating the position of a document with a specific dot (or block), so that the position information at the time of embedding must be recovered in order to detect the embedded information.
That is, at the time of embedding and detection, it is necessary to perform embedding processing and detection processing using the same coordinate axis. This is called digital watermark synchronization recovery.

Many methods have been proposed as methods for performing synchronization recovery of digital watermarks, but a typical method is a method of embedding a synchronization recovery pattern simultaneously with a digital watermark.
Patent Document 1 below discloses a method in which a rectangular information block is constituted by a set of dots, and reference patterns for synchronization recovery are arranged at four corners (or a part thereof) of the information block.
Since this information block is repeatedly arranged on the document, the reference pattern is also periodically arranged similarly to the information block.
A conventional digital watermark synchronization recovery device uses a pattern matching technique to extract a reference pattern that is periodically arranged, and determines the coordinates based on the reference pattern to perform digital watermark synchronization recovery. I am doing so.

JP-A-11-112787 (paragraph numbers [0012] to [0013], FIG. 13) JP 2003-209676 A (paragraph numbers [0010] to [0012], FIG. 1)

  Since the conventional digital watermark synchronization recovery device is configured as described above, if a part of a document is cut, it is possible to identify which part is cut out even if some reference patterns are extracted. There is a problem that the digital watermark synchronization recovery cannot be performed. In addition, it is necessary to periodically arrange the reference pattern in the same manner as the information block. However, since the human eye is sensitive to regularity, the periodic reference pattern is easily noticeable, and the confidentiality of the digital watermark deteriorates. There were some issues.

  The present invention has been made in order to solve the above-described problems. Even when a part of a document is cut out and a part of reference points is lost, the digital watermark can be synchronized and recovered. It is an object of the present invention to obtain a digital watermark synchronization recovery apparatus and digital watermark synchronization recovery program that can arrange dots regularly and perform digital watermark synchronization recovery without degrading confidentiality.

  A digital watermark synchronization recovery device according to the present invention includes a reference point position calculating unit that detects a plurality of reference points embedded in a document and calculates the positions of the plurality of reference points with reference to provisional coordinate axes. For each of the plurality of reference points detected by the reference point position calculating means, the direction coordinate calculating means for calculating the direction of the line segment connected to other reference points and the coordinates of the midpoint of the line segment, and the direction coordinate calculating means Compare the calculated line segment direction and midpoint coordinates with the line segment direction and midpoint coordinates indicated by the registration information acquired by the registration information acquisition means, and the geometry generated in the communication path of the document. A geometric parameter calculating means for calculating a geometric parameter indicative of a change in the reference point, and the reference point position correcting means uses the geometric parameter calculated by the geometric parameter calculating means to determine a temporary coordinate axis. Base By converting a point in the coordinate axis at the time of embedding, in which so as to correct the position of the plurality of reference points calculated by the reference point position calculating means.

  According to the present invention, the reference point position calculating means for detecting the plurality of reference points embedded in the document and calculating the positions of the plurality of reference points with reference to the provisional coordinate axes, and the reference point position calculating means For each of the plurality of reference points detected by the above, the direction coordinate calculation means for calculating the direction of the line segment connected to other reference points and the coordinates of the middle point of the line segment, and the line segment calculated by the direction coordinate calculation means Geometry that indicates the geometric change that occurs in the communication path by comparing the direction and midpoint coordinates with the direction and midpoint coordinates of the line segment indicated by the registration information acquired by the registration information acquisition means A geometric parameter calculation means for calculating a physical parameter, and the reference point position correction means uses the geometric parameter calculated by the geometric parameter calculation means as a temporary coordinate axis when the reference point is embedded. In In other words, since the positions of the plurality of reference points calculated by the reference point position calculating means are corrected, even if a part of the document is cut out and some of the reference points are lost, the digital watermark In addition to performing synchronization recovery, there is an effect that synchronization of the digital watermark can be performed without causing a deterioration in confidentiality by regularly arranging reference points.

1 is a configuration diagram showing a digital watermark synchronization recovery apparatus according to Embodiment 1 of the present invention; FIG. It is a flowchart which shows the processing content of the synchronous recovery apparatus of the digital watermark by Embodiment 1 of this invention. It is explanatory drawing which shows the direction of the line segment which connects between two reference points, and the coordinate of the midpoint of a line segment. It is explanatory drawing which shows arrangement | positioning of a reference pattern. It is explanatory drawing which shows the reference point arrange | positioned at random. It is explanatory drawing which shows the reference point arrange | positioned regularly.

Hereinafter, in order to describe the present invention in more detail, the best mode for carrying out the present invention will be described with reference to the accompanying drawings.
Embodiment 1 FIG.
FIG. 1 is a block diagram showing a digital watermark synchronization recovery apparatus according to Embodiment 1 of the present invention. In FIG. 1, a database 1 is used to specify a digital watermark embedding position in a document (eg, document, digitized image). For each of a plurality of reference points embedded for use, the memory stores registration information indicating the direction of a line segment connected to another reference point and the coordinates of the midpoint of the line segment.
When the document is a document, the reference point is embedded as a digital watermark. However, when the document is a digitized image, the reference point may not be embedded as a digital watermark. A feature point may be extracted by a filter or the like, and the feature point may be used as a reference point.

The registration information acquisition unit 2 includes an input interface for the database 1 and performs a process of acquiring registration information stored in the database 1. Note that the registration information acquisition unit 2 constitutes a registration information acquisition unit.
Here, an example is shown in which the registration information acquisition unit 2 acquires registration information from the database 1, but this is only an example. For example, the communication path from the digital watermark embedding device 3 that embeds a digital watermark or a reference point into a document is shown. You may make it acquire via.

The document receiving unit 4 is composed of, for example, a data receiver, and performs processing for receiving the document from the digital watermark embedding apparatus 3 via a communication path.
The reference point position calculation unit 5 is composed of, for example, an image detector that scans an image on a document and an arithmetic unit such as an MPU. For example, a plurality of reference points embedded in the document by performing a pattern matching method. And a process of calculating the positions of a plurality of reference points on the basis of the temporary coordinate axes. The reference point position calculation unit 5 constitutes reference point position calculation means.

The direction coordinate calculation unit 6 is configured by an arithmetic unit such as an MPU, for example, and for each of a plurality of reference points detected by the reference point position calculation unit 5, the direction of the line segment connected to other reference points and the line segment A process for calculating the coordinates of the midpoint is performed. In addition, the direction coordinate calculation part 6 comprises the direction coordinate calculation means.
The geometric parameter calculation unit 7 is composed of an arithmetic unit such as an MPU, for example. The direction and midpoint coordinates of the line segment calculated by the direction coordinate calculation unit 6 and the registration acquired by the registration information acquisition unit 2 are used. A process of calculating a geometric parameter (θ, s, u, v) indicating a geometric change occurring in the communication path of the document by comparing the direction of the line segment indicated by the information and the coordinates of the midpoint. carry out. The geometric parameter calculation unit 7 constitutes a geometric parameter calculation unit.

  The reference point position correction unit 8 is composed of an arithmetic unit such as an MPU, for example, and uses the geometric parameters (θ, s, u, v) calculated by the geometric parameter calculation unit 7 to tentatively. A process of correcting the positions of a plurality of reference points calculated by the reference point position calculation unit 5 is performed by converting the coordinate axes into the coordinate axes at the time of embedding the reference points. The reference point position correction unit 8 constitutes reference point position correction means.

In the example of FIG. 1, a registration information acquisition unit 2, a document reception unit 4, a reference point position calculation unit 5, a direction coordinate calculation unit 6, a geometric parameter calculation unit 7, and a reference, which are components of the digital watermark synchronization recovery apparatus. Although each of the point position correction units 8 is configured by dedicated hardware, when the digital watermark synchronization recovery device is configured by a computer, the registration information acquisition unit 2, document reception unit 4, reference A digital watermark synchronization recovery program describing the processing contents of the point position calculation unit 5, the direction coordinate calculation unit 6, the geometric parameter calculation unit 7 and the reference point position correction unit 8 is stored in the memory of the computer, The CPU of the computer may execute a digital watermark synchronization recovery program stored in the memory.
FIG. 2 is a flowchart showing the processing contents of the digital watermark synchronization recovery apparatus according to Embodiment 1 of the present invention.

Next, the operation will be described.
The digital watermark synchronization recovery apparatus according to the first embodiment recovers synchronization using generalized Hough transform.
Hough transform is a technique for detecting a specific figure from an image, and is basically a pattern matching technique. However, since it recovers synchronization by voting, it has a higher processing efficiency than the brute force method. There is.
The Hough transform was originally proposed as a method for detecting a straight line, but can also be used for detecting a figure formulated with a small number of parameters such as a circle.
The generalized Hough transform is an extension of the Hough transform so that a figure having an arbitrary shape can be detected by registering the coordinates of each point of the contour and the direction of the contour at each point.

In this first embodiment, in order to use this technique for digital watermark synchronization recovery, a plurality of reference points whose arrangement is known are embedded in the document in advance with the digital watermark, and all lines connecting the two reference points are used. For the minute, the coordinates of the midpoint and the direction of the line segment are registered.
That is, when the digital watermark embedding device 3 determines the arrangement of N reference points P _k (k = 0,..., N−1) with respect to the document (the arrangement of the reference points will be described in the second embodiment). For example, a reference pattern as disclosed in Patent Document 2 may be arranged (see FIG. 4), and any two reference points from among the N reference points P _k. P _i, coordinates of the midpoint M _ij of the line segment P _i P _j connecting _{P j (x ij, y ij} ), polar coordinates of the middle point _{M ij (r ij, α ij} ) as, the line segment P _i An angle ω _ij between the normal vector of P _j and the x axis is obtained (see FIG. 3).
x _ij = r _ij cosα _ij
y _ij = r _ij sin α _ij (1)

When the digital watermark embedding device 3 obtains the angle ω _ij between the normal vector of the line segment P _i P _j and the x axis, the vector v _ij in the following equation (2) is determined as the direction of the line segment P _i P _j . Stored in the database 1 as registered information indicating the coordinates of the midpoint of the line segment P _i P _j .
v _ij = (ω _ij , r _ij , α _ij ) (2)
The digital watermark embedding device 3 obtains registration information for all combinations of the reference points P _i and P _j and stores them in the database 1.

The document receiving unit 4 receives the document in which the digital watermark is embedded from the digital watermark embedding device 3 via the communication path (step ST1).
It is assumed that the document undergoes a geometric change in the communication path after the document is transmitted from the digital watermark embedding apparatus 3 and before the document is received by the document receiving unit 4 via the communication path.
In the first embodiment, a geometric parameter indicating a geometric change is represented by (θ, s, u, v). Here, θ is a rotation, s is a scaling, and (u, v) is a parameter indicating translation.

When the document reception unit 4 receives the document, the reference point position calculation unit 5 performs a pattern matching method, for example, and detects a plurality of reference points embedded in the document.
Further, when detecting a plurality of reference points, the reference point position calculation unit 5 calculates the positions of the plurality of reference points with reference to the provisional coordinate axes (step ST2).
Note that the coordinate system for calculating the position of the reference point may be provisionally determined. In general, in the Hough transform, the performance is improved when the origin is set at the center of the document.
Here, it is assumed that M reference points Q _k (k = 0,..., M−1) are detected by the reference point position calculation unit 5.
There may be a certain amount of error in the detection of the reference point Q _k. Such an error is eliminated in the stage of calculating geometric parameters in step ST5 below.

When the reference point position calculation unit 5 detects M reference points Q _k (k = 0,..., M−1), the direction coordinate calculation unit 6 selects any two of the M reference points Q _k. The coordinates (x _ij , y _ij ) of the midpoint of the line segment Q _i Q _j connecting the two reference points Q _i , Q _j and the angle ψ _ij formed by the line segment Q _i Q _j and the x axis are calculated (step) ST3).
The direction coordinate calculation unit 6 calculates the angle ψ _ij between the coordinates of the midpoint (x _ij , y _ij ) and the combination of all the reference points Q _i and Q _j .

The registration information acquisition unit 2 acquires registration information stored in the database 1 (step ST4).
When the registration information acquisition unit 2 acquires the registration information, the geometric parameter calculation unit 7 registers the coordinates (x _ij , y _ij ) of the midpoint calculated by the direction coordinate calculation unit 6 and the angle ψ _ij formed, and the registration thereof. By comparing the coordinates (x _ij , y _ij ) of the midpoint indicated by the information and the angle ω _ij formed, the geometric parameters (θ, s, u, v) is calculated (step ST5).

Hereinafter, the calculation process of the geometric parameters (θ, s, u, v) in the geometric parameter calculation unit 7 will be specifically described.
First, the geometric parameter calculation unit 7 prepares a four-dimensional buffer h (θ, s, u, v) used for voting in the Hough transform. The four-dimensional buffer h is prepared for the combination of geometric parameters (θ, s, u, v).
However, the values of the four-dimensional buffer h (θ, s, u, v) are all initialized to “0”.

Next, the geometric parameter calculation unit 7 searches for the angle ω _kl formed by the registration information in which the following inequality is established, with θ _{min as} the minimum value of the rotation detection range and θ _max as the maximum value.
θ _min <θ = ψ _ij −ω _kl <θ _max (3)
If an angle ω _kl formed by the expression (3) is included in the registration information, the geometric parameter calculation unit 7 uses the polar coordinates (r _kl , α _kl ) corresponding to the formed angle ω _kl as follows: (U, v) is calculated.
u = x _ij −s · r _kl cos (α _kl + θ)
v = y _ij −s · r _kl sin (α _kl + θ) (4)

After calculating (u, v), the geometric parameter calculation unit 7 discretely changes “s” in Equation (4) within the scaling detection range (minimum value s _min , maximum value s _max ). (For example, s is changed in steps of 0.01), and the value of the buffer h (θ, s, u, v) corresponding to the geometric parameter (θ, s, u, v) after the change is set to “ Increment by 1 ”.

The geometric parameter calculation unit 7 calculates (u, v) from all the angles ω _kl formed by the expression (3) and changes the scaling s to the geometric parameters (θ, s, When the process of incrementing the value of the buffer h (θ, s, u, v) corresponding to u, v) by “1” is completed, the values of all the buffers h (θ, s, u, v) are compared. The maximum value buffer h (θ, s, u, v) is identified, and the geometric parameter (θ, s, u, v) corresponding to the maximum value buffer h (θ, s, u, v) is specified. Is determined to be a geometric parameter (θ, s, u, v) indicating a geometric change occurring in the communication path of the document.

When the geometric parameter calculation unit 7 determines the geometric parameter (θ, s, u, v), the reference point position correction unit 8 uses the geometric parameter (θ, s, u, v). The temporary coordinate axis determined by the reference point position calculation unit 5 is converted into a coordinate axis at the time of embedding the reference point (coordinate axis when the reference point is embedded by the digital watermark embedding device 3), and the reference point position calculation unit 5 The calculated positions of the M reference points Q _k are corrected (step ST6).
For example, if θ = 5 °, correction is performed by rotating the position of the reference point Q _k by −5 °. If s = −8%, correction is performed to enlarge the document by 8%.
Further, if u = 3, correction is performed to translate the position of the reference point Q _k by 3 in the negative direction of the x axis. If v = −4, correction is performed to translate the position of the reference point Q _k by 4 in the positive direction of the y-axis.
In the latter-stage digital watermark detection apparatus (not shown), when detecting the digital watermark embedded in the document, the position of the digital watermark is determined by referring to the position of the reference point Q _k corrected by the reference point position correction unit 8. Identify.

  As is apparent from the above, according to the first embodiment, a plurality of reference points embedded in a document are detected, and a reference point for calculating the positions of the plurality of reference points with reference to provisional coordinate axes is used. The position calculation unit 5 and a direction coordinate calculation unit 6 that calculates the direction of a line segment connected to another reference point and the coordinates of the midpoint of the line segment for each of a plurality of reference points detected by the reference point position calculation unit 5 And the direction and midpoint coordinates of the line segment calculated by the direction coordinate calculation unit 6 and the direction and midpoint coordinates of the line segment indicated by the registration information acquired by the registration information acquisition unit 2 are compared. Is provided with a geometric parameter calculation unit 7 for calculating a geometric parameter (θ, s, u, v) indicating a geometric change occurring in the communication channel, and the reference point position correction unit 8 is a geometric Geometric parameters (θ, s, u, v) are used to convert the temporary coordinate axes to the coordinate axes at the time of embedding the reference points, and to correct the positions of the plurality of reference points calculated by the reference point position calculation unit 5. Even if a part of the document is cut out and some reference points are lost, digital watermark synchronization can be recovered, and the reference points are regularly arranged without causing deterioration of confidentiality. The digital watermark synchronization recovery can be achieved.

  That is, since the digital watermark synchronization is recovered using the generalized Hough transform based on the direction of the line segment connecting the reference points and the midpoint of the line segment, the reference points are not arranged regularly. The digital watermark synchronization recovery can be achieved. Further, since the calculation of the geometric parameters (θ, s, u, v) is performed based on the coincidence of the main reference points, when some reference points are lost or there are erroneous reference points. Even when it exists, there is an effect that a correct geometric parameter (θ, s, u, v) can be stably obtained.

In the first embodiment, the geometric parameter calculation unit 7 calculates the geometric parameters (θ, s, u, v) corresponding to the maximum value buffer h (θ, s, u, v), The document determines what is determined as the geometric parameter (θ, s, u, v) indicating the geometric change occurring in the communication path. The reference point Q _k detected by the reference point position calculation unit 5 is shown. If there is an error in the position, the voting is dispersed around the true value, and the detection accuracy may be lowered.

This problem can be alleviated by smoothing the four-dimensional buffer h (θ, s, u, v).
That is, a geometric parameter (θ ₀ , s ₀ , u ₀ , v ₀ ) that maximizes the following equation (5) is obtained.

ただし、Ｎは加算の範囲に含まれる投票の総数である。Further, an average of parameters in the vicinity of ± 1 of the geometric parameters (θ ₀ , s ₀ , u ₀ , v ₀ ) is taken as a final geometric parameter.
For example, θ is obtained as in the following formula (6).

Here, N is the total number of votes included in the range of addition.

Embodiment 2. FIG.
In the first embodiment, the case where a plurality of reference points are embedded in the document has been described. However, the digital watermark embedding apparatus 3 determines the arrangement of the plurality of reference points with respect to the document using, for example, a random number or the like. Thus, a plurality of reference points may be arranged at random with respect to the document.

Here, FIG. 5 is an explanatory view showing reference points arranged at random.
In the figure, x indicates the position of the reference point, a plurality of reference points are randomly arranged in the plane, and the arrangement of a part of the reference points is less likely to overlap with other parts. .
For this reason, when a part of the document is found, it can be estimated with high probability from only the arrangement of the reference points which part of the original document the part corresponds to.

For example, when only the central region surrounded by the dotted line in the figure is cut out, there is a low possibility that the arrangement of the reference points inside the region overlaps with other parts.
On the other hand, as shown in FIG. 6, when the reference points are regularly arranged, the arrangement of some of the reference points overlaps with the other parts, and the cut fragment is the original document. It cannot be determined from the reference point which part corresponds to.
As is apparent from the above, according to the second embodiment, since a plurality of reference points are randomly arranged on the document, even when a part of the document is cut out, the digital watermark is synchronized and recovered. There is an effect that can be performed.

  As described above, the digital watermark synchronization recovery apparatus according to the present invention can perform digital watermark synchronization recovery even if a part of a document is cut out and a part of reference points is lost. In order to recover digital watermark synchronization without degrading confidentiality, a plurality of reference points embedded in the document are detected and a plurality of reference points are used as a reference. The reference point position calculating means for calculating the position of the reference point, and the direction of the line segment connected to other reference points and the coordinates of the midpoint of the line segment for each of the plurality of reference points detected by the reference point position calculating means Direction coordinate calculation means for calculating the direction of the line segment calculated by the direction coordinate calculation means and the coordinates of the midpoint, and the direction of the line segment indicated by the registration information acquired by the registration information acquisition means and the coordinates of the midpoint Compare the documents A geometric parameter calculating means for calculating a geometric parameter indicating a geometric change occurring in the road, and the reference point position correcting means uses the geometric parameter calculated by the geometric parameter calculating means. Thus, the provisional coordinate axis is converted to the coordinate axis at the time of embedding the reference point, and the positions of the plurality of reference points calculated by the reference point position calculating means are corrected. The present invention is suitable for use in a digital watermark synchronization recovery device that detects the position of a reference point and identifies the digital watermark embedding position.

Claims (3)

  Registration that acquires registration information indicating the direction of a line segment connected to another reference point and the coordinates of the midpoint of the line segment, for each of a plurality of reference points embedded for specifying the embedded position of the digital watermark in the document An information acquisition means; a reference point position calculating means for detecting a plurality of reference points embedded in the document and calculating the positions of the plurality of reference points with reference to provisional coordinate axes; and the reference point position For each of the plurality of reference points detected by the calculation means, the direction coordinate calculation means for calculating the direction of the line segment connected to other reference points and the coordinates of the midpoint of the line segment, and the direction coordinate calculation means The direction of the line segment and the coordinates of the middle point are compared with the direction of the line segment and the coordinates of the middle point indicated by the registration information acquired by the registration information acquisition means, and the geometry generated by the document on the communication path is compared. Showing changes Using the geometric parameter calculating means for calculating the geometric parameter and the geometric parameter calculated by the geometric parameter calculating means, the temporary coordinate axis is converted into the coordinate axis at the time of embedding the reference point, A digital watermark synchronization recovery apparatus comprising: a reference point position correcting unit that corrects the positions of a plurality of reference points calculated by the reference point position calculating unit.   The registration information acquisition unit acquires registration information indicating a direction of a line segment and a coordinate of a midpoint of the line segment with respect to a plurality of reference points randomly arranged with respect to the document. Digital watermark synchronization recovery device.   Registration that acquires registration information indicating the direction of a line segment connected to another reference point and the coordinates of the midpoint of the line segment, for each of a plurality of reference points embedded for specifying the embedded position of the digital watermark in the document An information acquisition processing procedure, a reference point position calculation processing procedure for detecting a plurality of reference points embedded in the document, and calculating positions of the plurality of reference points with reference to provisional coordinate axes, and the reference For each of a plurality of reference points detected by the point position calculation processing procedure, the direction coordinate calculation processing procedure for calculating the direction of the line segment connected to another reference point and the coordinates of the midpoint of the line segment, and the direction coordinate calculation The direction of the line segment calculated by the processing procedure and the coordinates of the midpoint are compared with the direction of the line segment indicated by the registration information acquired by the registration information acquisition processing procedure and the coordinates of the midpoint, and the document is Caused by Using the geometric parameter calculation processing procedure for calculating the geometric parameter indicating the geometric change and the geometric parameter calculated by the geometric parameter calculation processing procedure, the provisional coordinate axis is used as a reference. A digital watermark synchronization recovery program for causing a computer to execute a reference point position correction processing procedure for correcting the positions of a plurality of reference points calculated by the reference point position calculation processing procedure by converting into coordinate axes at the time of point embedding .

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