CN102368328B - Digital watermarking method applied to counterfeit prevention for print documents - Google Patents

Abstract

The invention discloses a digital watermarking method applied to counterfeit prevention for print documents, wherein a multilayer watermark is used for preventing counterfeit of the print documents so as to protect the print documents. The method comprises two aspects of constructing the multi-layer watermark and constructing an anti-scanning/copying watermark layer, wherein the multi-layer watermark is constructed by firstly utilizing factors which mainly comprise contrast ratio and chromatic aberration and influence human vision on identifying objects, providing a concept of watermark contrast ratio by combining with a used scene in the method, further judging factors influencing an watermark extraction effect according to the watermark contrast ratio, finally comprehensively analyzing results and designing parameters; and the anti-scanning/copying watermark layer which is weak to scanning/copying is designed by firstly analyzing a scanning process and summing the features of scanning, therefore the purpose of preventing counterfeit due to scanning/copying can be achieved. The final multi-layer watermark is finallyformed by combining the multi-layer watermark with the anti-scanning/copying watermark layer.

Description

The digital watermark method that a kind of printed document is false proof

Technical field

The present invention relates to multimedia digital watermark structure, embedding and detection method, particularly the false proof digital watermark method of a kind of printed document.

Background technology

At present; the printed documents such as various certificates, bill, trade mark have been brought into play irreplaceable vital role in people's life; yet the fast development along with the constantly universal and technique of internet of digital technology; forge; the problems such as piracy grow in intensity; how to protect these and our closely bound up product, i.e. copyright protection, information security, data authentication and access control etc. becomes problem in the urgent need to address now.

Printed document anti-counterfeiting technology commonly used has that paper false proof, specialty inks are false proof, adhesive sticker micro text antiforge, bill extraordinary anti-counterfeit printing, safety line, encryption technology, laser hologram transfer techniques, locating thermoprint, telephone code is false proof, the atomic nucleus double card is false proof, anti-counterfeiting mark and hand engraving intaglio printing etc.But there is weakness separately in these technology, such as, the material that the false proof needs of paper false proof and specialty inks are special or manufacture craft.And traditional anti-counterfeiting technology is easily grasped by the fake producer, thereby loses its antiforge function.On the other hand, traditional printed document anti-counterfeiting technology and equipment and the material of use are closely related, and can't accomplish versatility, and range of application is narrow.

The appearance of digital watermarking provides a kind of new solution for printed document is false proof.Compare with traditional approach, digital watermark technology has that hiding is good, cost is low, good confidentiality, be difficult to the advantages such as forgery, has higher practical value.Be applied to the false proof digital watermarking of printed document and mainly comprise two kinds of anti printing and scanning watermark and vision watermarks, wherein, the vision watermark is a kind of visual watermark technology that grew up in recent years, has the characteristics such as efficient, applied range.The United States Patent (USP) of Adi Shamir application has used a kind of visual watermark for print image, adopted and substituted an image to realize false proof with the monochrome pixels of fine structures, this method is more responsive to scanning device, and along with the development of scanning device, its effect also worse and worse.Another United States Patent (USP) of William H.Mowry application has used a kind of method of controlling the rear size of pixel printing to realize false proof, because the meeting that large pixel remains is more, and the error that small pixel has after printing due to printer disappears, this kind method is generally for specific printer, specific resolution is carried out, and the lifting of effect depends on the machine that micro is printed.The multilayer optical watermark based on dot matrix that Huang and Wu proposed in 2007, the method is on the basis of principles of modulation and demodulation, the use phase modulating method is realized, then Lu YiFeng has proposed in 2008 based on the optical watermark of phenomenon how not, compare with the optical watermark based on dot matrix, how phenomenon is not to be interfered by black and white strip to form, so larger noise is arranged itself, the watermark that realizes multilayer is more difficult, and capacity is less.

Summary of the invention

The technical problem to be solved in the present invention is: overcome the deficiencies in the prior art, provide a kind of printed document false proof digital watermark method, at first the method passes through the factor of analyzing influence watermark extracting effect, the standard that has proposed the watermark contrast is weighed the effect of multilayer watermark, then on based on this basis, the parameter of structure multilayer watermark is analyzed, and made a concrete analysis of the relation of cycle and live width, then analyze the angle problem of multilayer watermark, constructed multilayer watermark according to analysis result.Last this paper has proposed the anti-counterfei waterprint layer after the characteristics of analysis scan instrument work, in conjunction with multilayer watermark, finally obtain the scanning anti-counterfei waterprint of multilayer.

The technical solution adopted for the present invention to solve the technical problems: the digital watermark method that a kind of printed document is false proof comprises watermark Analysis of Contrast, anti-scanning/duplicating watermark layer, watermark embedding and watermark extracting four major parts.The watermark Analysis of Contrast is to utilize contrast and these two human eyes of colour-difference to distinguish that the object key factor comes the quantisation watermarking extraction effect with the watermark contrast; Anti-scanning/duplicating watermark layer is by scanning process being analyzed, being constructed the anti-watermark layer that scans/duplicate according to scan characteristic; It is on the basis of watermark Analysis of Contrast that watermark embeds, and selects suitable embedding cycle and angle according to analysis result, and in conjunction with preventing scanning/duplicating the watermark of watermark layer structure Multi-layer anti-counterfeit;

Analytic process to the watermark contrast is as follows:

A1. the analyzing influence human vision is recognized the factor of object;

A2., the watermark contrast is proposed;

A3. the watermark extracting effect is carried out mathematical analysis;

A4. the based on analysis result is selected suitable embedding cycle and angle;

The building process of anti-scanning/duplicating watermark layer is as follows:

B1. the impact of analysis scan process on image

B2. build anti-scanning/duplicating watermark layer according to scan characteristic

The telescopiny of watermark is as follows:

C1. according to embedding cycle and the angle basis of formation layer selected;

C2. select certain one deck as anti-scanning/duplicating watermark layer;

C3. watermark is embedded in basal layer;

C4. build multilayer watermark;

C5. generate respective decoder;

The leaching process of watermark is as follows:

D1. use corresponding demoder to carry out watermark extracting;

The advantage that the present invention compared with prior art has is:

(1) a kind of standard---watermark contrast of new measurement watermark extracting effect has been proposed, the extraction effect that this standard can more efficiently quantisation watermarking.

(2) the embedding cycle that affects the watermark extracting effect is carried out mathematical analysis, and used a large amount of repeated experiments that analysis result is verified, watermark extracting effect that experimental result has shown the lifting larger according to the embedding cycle of analysis result selection.

(3) the embedding angle that affects the watermark extracting effect is carried out mathematical analysis, and used a large amount of repeated experiments that analysis result is verified, experimental result shows that the embedding angle of selecting according to analysis result has prevented that effectively same demoder from extracting several watermarks and a plurality of demoder can only extract the abnormal conditions of same watermark.

(4) scanning process is analyzed, and constructed anti-scanning/duplicating watermark layer according to scan characteristic, this layer watermark is more fragile to scanning/duplicating, can disappear in scanning or after duplicating, and can effectively protect printed document.

Description of drawings

Fig. 1 is the false proof digital watermark method overall flow figure of a kind of printed document of the present invention;

Fig. 2 is the watermark contrast schematic diagram in the present invention;

Fig. 3 is that in the present invention, D=1 is along with the cycle increases, and the watermark contrast is at the decline schematic diagram;

Fig. 4 is that D in the present invention＞1m=2 optimal period is the 2k+1 schematic diagram;

Fig. 5 is that D in the present invention＞1m=3 optimal period is the 3k+1 schematic diagram;

Fig. 6 changes schematic diagram before and after image scanning in the present invention;

Fig. 7 is original watermark schematic diagram in the present invention;

Fig. 8 is watermark schematic diagram after in the present invention, numeral is replaced;

Fig. 9 is basal layer schematic diagram in the present invention;

Figure 10 is image schematic diagram after embed watermark in the present invention;

Figure 11 is demoder schematic diagram in the present invention;

Figure 12 is the watermark extracting schematic diagram that does not add anti-scanning/duplicating watermark layer in the present invention;

Figure 13 is the watermark extracting schematic diagram of anti-scanning/duplicating watermark layer in the present invention.

Embodiment

The present invention is further described in detail below in conjunction with the drawings and specific embodiments.

The overall flow figure of the digital watermark method that a kind of printed document of the present invention is false proof as shown in Figure 1.Overall flow comprises watermark Analysis of Contrast, anti-scanning/duplicating watermark layer, watermark embedding and watermark extracting four major parts.

One, watermark Analysis of Contrast

The study route of this part comprises four step: a1 steps, affects the factor of human vision identification object; The a2 step, the concept of proposition watermark contrast; In the a3 step, analyze the watermark extracting effect; In the a4 step, select suitable embedding cycle and angle.Specific as follows:

Step a1: the factor that affects human vision identification object

Because the extraction effect of watermark is apparent in view for human eye, but can't weigh on computing machine, so before carrying out mathematical analysis, need to seek the standard of a suitable measurement watermark extracting effect.According to the experimental result of Hrishikesh, mark PSNR and cycle that tradition is weighed the watermark extracting effect do not have rule substantially, so PSNR has been not suitable for carrying out mathematical analysis in this case, so need a kind of new standard of weighing the watermark extracting effect.According to the analysis to human eye of Peter G.J.Barten and Oracle related data, the factor that impact is differentiated object mainly contains two: the contrast of background and prospect, and the color of background and prospect.

Step a2: watermark contrast

Because the watermarking images of this paper is bianry image, so do not relate to the problem of color, the best means of therefore weighing the watermark extracting effect are exactly to weigh the contrast that extracts watermark.Based on above analysis, this paper has proposed a kind of measurement new standard of watermark extracting effect in such cases---watermark contrast, and it defines as shown in formula (1):

contrast＝(A-B)/M×N (1)

Wherein A represents the number of black color dots, and B represents the number of white point, and the size of M * N presentation video is M * N pixel, and its span is [1,1] ,-1 expression pure white, 1 expression ater.

The watermark contrast has been arranged, just can weigh the extraction effect of a width watermark picture, according to formula (1), this paper is divided into two parts to the watermark contrast: the outer part of watermark part and watermark.So whole watermark contrast can be calculated with formula (2).

contrast＝abs(((A ₁-B ₁)/M ₁×N ₁)-((A ₂-B ₂)/M ₂×N ₂)) (2)

A wherein ₁The stain of expression watermark part, B ₁The white point of expression watermark part, M ₁* N ₁The size of expression watermark part; A ₂The stain of expression watermark periphery, B ₂The white point of expression watermark periphery, M ₂* N ₂The size of expression watermark periphery.The value of contrast is worth larger expression contrast larger between [0,2], namely effect is better, as shown in Figure 2.

Step a3: analyze the watermark extracting effect

A. use the watermark contrast that the embedding cycle is analyzed

The value of live width D can exert an influence to the extraction effect of watermark in the process of embed watermark, establishes the 1/m that the live width D that uses in construction process accounts for cycle T, and because D is positive integer, has so

So the conclusion of formula (3) is arranged,

N＜m wherein, k=1,2,3...

$D=\left\{\begin{array}{cc}k& T=m\×k\\ k+1& T=m\×k+n,1\≤n\≤m\end{array}\right.---\left(3\right)$

When T changed to m * k+1 from m * k, live width D had been increased to k+1 from k, so the increase of live width at this moment is more violent with respect to the increase in cycle.Due to the increase of live width ratio, the reservation degree of watermark integral body is better, so the watermark contrast also has corresponding increase, therefore watermark this moment contrast is best.

Fig. 3 is we and the curve map of watermark contrast in calculate cycle when D=1, and this figure shows when D=1, and along with the increase of T, the watermark contrast is monotone decreasing; Fig. 4 works as m=2, the curve map of D＞1 o'clock cycle and watermark contrast, and this chart open fire seal contrast is got peak value when getting 2 * k+1; Fig. 5 works as m=3, the curve map of D＞1 o'clock cycle and watermark contrast, and this chart open fire seal contrast is got peak value when getting 3 * k+1.

According to top analysis and calculation result, T _kDuring=m * k+1, the extraction effect of watermark is reasonable, and contrast (T is arranged ₁)＞contrast (T ₂)＞...＞contrast (T _k).

In addition, as 2≤T _kDuring＜m, might exist effect to compare T _kAlthough the cycle that=m * k+1 is good is because T _kDuring=m * k+1, the increase of live width has promoted extraction effect, yet extraction effect is the decline that is increased in along with T, so 2≤T _k＜m need to consider separately as particular point.

B. use the watermark contrast to analyze embedding angle

The embedding angle does not exert an influence to the extraction of single-layer watermark.For multilayer watermark, if select inappropriate words, and when the cycle of two-layer watermark is identical, i.e. T _x1=T _x2Perhaps T _y1=T _y2The time, two kinds of special circumstances can appear after carrying out the watermark stack:

A. use a plurality of demoders can only extract same watermark, perhaps can't successfully extract at all.

Can see simultaneously the situation of a plurality of watermarks when b. using a demoder.

The generation of above two kinds of situations is all because embedding angle chooses improperly, causes rotating too little causing of distance between two points of rear mapping.

Be located at that in rectangular coordinate system, any point in original watermark layer is that P (x, y) rotates respectively θ ₁And θ ₂After obtain corresponding point in two-layer watermark:

P ₁(xcosθ ₁+ysinθ ₁，ycosθ ₁-xsinθ ₁) (4)

P ₂(xcosθ ₂+ysinθ ₂，ycosθ ₂-xsinθ ₂) (5)

The distance that calculates between this two-layer watermark according to the Euclidean distance formula is:

$\mathrm{dis}=2\sqrt{x^2+{\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="HDA0000092166570000021.png,HDA0000092166570000031.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}\sin \left(\frac{|{\mathrm{\&theta;}}_1-{\mathrm{\&theta;}}_2|}{2}\right)---\left(6\right)$

Can find out, in formula (6)

Can't change the differential seat angle item that only has the back that can control | θ ₁-θ ₂|, and have | θ ₁-θ ₂|≤90 °, can control suitable distance so calculate suitable angle difference, and then avoid the generation of the two kinds of special circumstances in front.

Through a large amount of experiments repeatedly, angle difference is got | θ ₁-θ ₂| can prevent the generation of two kinds of fortuitous events in the time of 〉=60 °, and the larger effect of value is better, and because θ is angle with the longitudinal axis, thereby angle difference is 90 ° to the maximum, namely 90 ° of effects of value are best.

Step a4: select suitable embedding cycle and angle

According to the analysis in step 3 and experimental result, just like drawing a conclusion: the cycle is got T _kDuring=m * k+1 and | θ ₁-θ ₂| in the time of 〉=60 °, the watermark extracting effect is relatively good.In this patent experiment, the concrete parameter of selecting is m=3, so when the cycle is selected T=3k+1, the watermark extracting effect is relatively good, and owing to there being particular point, so the final parameter of selecting is T _x1=2, T _y1=2, θ ₁=60 °, T _x2=4, T _y2=4, θ ₂=120 °.

Two, anti-scanning/duplicating watermark layer

In order to build the anti-counterfei waterprint layer, which impact is the process of analysis scan produced to image is very necessary.The course of work of scanner is as follows: at first the hard copies such as the image of papery, text are placed on the glass plate of scanner, light source throws a branch of pure light on reverberator, CCD (Charged Coupled Device, charge-coupled image sensor) inductor is converted into electronic signal with the optical signalling of sensing, the electronic signal that optics obtains is fed to ADC (Analog to Digital Converter, analogue-to-digital converters) system, ADC is with the electronic signal generating digital image sent here and be sent to main frame.

The optics of scanner has determined the resolution of scanner, so optical MTF (Modulation Transfer Function, modulation transfer function) has been played the part of important role in scanning process.The optical modulation transition function MTF of scanner has roughly following character:

$\mathrm{MTF}\left(f\right)={\left|\sin c\left(\frac{f}{\mathrm{dscan}}\right)\right|}^n---\left(7\right)$

Wherein, dscan refers to the resolution (pixel/millimeter) of scanner, and MTF (f) is modulation transfer function, and f is spatial frequency.The optical modulation transition function can come modeling with a gaussian filtering, is equal to original image has been carried out a Gassian low-pass filter attack.To a certain degree fuzzy can appear in the periphery of original image after overscanning, as shown in Figure 6:

According to this characteristic, this paper has built an anti-counterfei waterprint layer based on numeric structure, watermarking images is divided into 3 * 5 piece, then according to the number of black color dots in image, then replaces with corresponding digital block, finally obtains watermark picture to be embedded.

In the note watermark block, the number of black color dots is p, 1≤p≤15, and corresponding relation is (8):

$\mathrm{numblock}=\left\{\begin{array}{cc}1& p\&le;4\\ \mathrm{2,4,7}& 5\&le;p\&le;9\\ \mathrm{3,5,6,9}& 10\&le;p\&le;12\\ 8& 13\&le;p\&le;15\end{array}\right.---\left(8\right)$

Represent when numblock=1, select digital block 1; When 5≤p≤9, random selection 2,4,7 digital blocks.The digital block table is as follows:

$\left[\begin{array}{ccc}1,& 0,& 1\\ 1,& 0,& 1\\ 1,& 0,& 1\\ 1,& 0,& 1\\ 1,& 1,& 1\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 1,& 1,& 0\\ 0,& 0,& 0\\ 0,& 1,& 1\\ 0,& 0,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 1,& 1,& 0\\ 0,& 0,& 0\\ 1,& 1,& 0\\ 0,& 0,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 1,& 0\\ 0,& 1,& 0\\ 0,& 0,& 0\\ 1,& 1,& 0\\ 1,& 1,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 0,& 1,& 1\\ 0,& 0,& 0\\ 1,& 1,& 0\\ 0,& 0,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 0,& 1,& 1\\ 0,& 0,& 0\\ 0,& 1,& 0\\ 0,& 0,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 1,& 1,& 0\\ 1,& 1,& 0\\ 1,& 1,& 0\\ 1,& 1,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 0,& 1,& 0\\ 0,& 0,& 0\\ 0,& 1,& 0\\ 0,& 0,& 0\end{array}\right]\left[\begin{array}{ccc}0,& 0,& 0\\ 0,& 1,& 0\\ 0,& 0,& 0\\ 1,& 1,& 0\\ 0,& 0,& 0\end{array}\right]$

Being Fig. 7 before replacing, is Fig. 8 after replacing, and after obtaining the watermarking images after numeral is replaced, it is carried out watermark as one in watermarking images embed, and so just can construct the anti-counterfei waterprint layer.According to the diffusion property of scanning, this watermark layer can be after scanning the pixel value with periphery close, and then can't differentiate, reach the purpose of None-identified scanning and printing spare, just can't extract watermark scanning/duplicate is rear like this.

Three, watermark embeds

The embedding step of watermark is as follows:

C1. according to embedding cycle and the angle basis of formation layer selected;

C2. select certain one deck as anti-scanning/duplicating watermark layer;

C3. watermark is embedded in basal layer;

C4. build multilayer watermark;

C5. generate respective decoder;

Basal layer is as shown in Fig. 9-a, and Fig. 9-b is the basal layer after amplifying, wherein T _x, T _yRespectively the embedding cycle of the direction of x axle, y axle, D _x, D _yBe respectively x axle, the axial live width of y, after embed watermark, as shown in figure 10, demoder as shown in figure 11.

Four, watermark extracting

Watermark extracting is to use respective decoder to extract watermark, Figure 12 is four layers of watermark extracting design sketch that do not add anti-scanning/duplicating watermark layer, figure a is the extraction effect of embed watermark information " watermark ", figure b is the extraction effect of embed watermark information " Beijing Institute of Aeronautics ", figure c is the extraction effect of embed watermark information " original paper ", figure d is the extraction effect of embed watermark information " false proof ", Figure 13 revises the original watermark layer to be anti-scanning/duplicating watermark layer, then scan respectively and duplicate, using demoder to cover the schematic diagram that watermark layer carries out watermark extracting.

In sum, the present invention is by the factor of analyzing influence watermark extracting effect, the standard that has proposed the watermark contrast is weighed the effect of multilayer watermark, then on based on this basis, the parameter of structure multilayer watermark is analyzed, and made a concrete analysis of the relation of cycle and live width, then analyze the angle problem of multilayer watermark, constructed multilayer watermark according to analysis result.Last this paper has proposed the anti-counterfei waterprint layer after the characteristics of analysis scan instrument work, in conjunction with multilayer watermark, finally obtain the scanning anti-counterfei waterprint of multilayer.Experimental result shows, mathematical analysis provide the more rational cycle and angle selective: the cycle that reasonably embeds can reach and embed preferably effect, reasonably embeds the generation that angle can effectively be avoided two kinds of fortuitous events.So use analysis result can more obviously promote the extraction effect of watermark.

The part that the present invention does not elaborate belongs to techniques well known.

Claims (3)

1. digital watermark method that printed document is false proof is characterized in that: the method overall flow comprises watermark Analysis of Contrast, anti-scanning/duplicate watermark layer, watermark embedding and watermark extracting four major parts; The watermark Analysis of Contrast is to utilize contrast and these two human eyes of colour-difference to distinguish that the object key factor comes the quantisation watermarking extraction effect with the watermark contrast; Anti-scanning/duplicating watermark layer is by scanning process being analyzed, being constructed the anti-watermark layer that scans/duplicate according to scan characteristic; It is on the basis of watermark Analysis of Contrast that watermark embeds, and selects suitable embedding cycle and angle according to analysis result, and in conjunction with preventing scanning/duplicating the watermark of watermark layer structure Multi-layer anti-counterfeit; Be watermark extraction process at last; Specific as follows:

The analytic process of watermark contrast is as follows:

A1. the analyzing influence human vision is recognized the factor of object;

A2., the watermark contrast is proposed;

A3. the watermark extracting effect is carried out mathematical analysis;

A4. the based on analysis result is selected suitable embedding cycle and angle;

The building process of anti-scanning/duplicating watermark layer is as follows:

B1. the impact of analysis scan process on image

B2. build anti-scanning/duplicating watermark layer according to scan characteristic

The telescopiny of watermark is as follows:

C1. according to embedding cycle and the angle basis of formation layer selected;

C2. select certain one deck as anti-scanning/duplicating watermark layer;

C3. watermark is embedded in basal layer;

C4. build multilayer watermark;

C5. generate respective decoder;

The leaching process of watermark is as follows:

D1. use corresponding demoder to carry out watermark extracting;

Wherein, described watermark contrast is the standard of quantisation watermarking extraction effect, it is defined as: contrast=(A-B)/M * N wherein A represents the number of black color dots, B represents the number of white point, the size of M * N presentation video is M * N pixel, and watermark contrast contrast span is [1,1],-1 expression pure white, 1 expression ater.

2. the false proof digital watermark method of a kind of printed document according to claim 1, is characterized in that: when building multilayer watermark, added anti-scanning/duplicating watermark layer.

3. the false proof digital watermark method of a kind of printed document according to claim 2, is characterized in that: because scanning process is equivalent to low-pass filtering one time, so build the watermark that anti-scanning/duplicating watermark layer uses a kind of numeric structure.

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