Performance Comparison of Digital Watermarking for Scanned Document in RGB and YCbCr Channels

P. Muneesawang et al. (Eds.): PCM 2009, LNCS 5879, pp. 574–589, 2009. © Springer-Verlag Berlin Heidelberg 2009

Narong Mettripun and Thumrongrat Amornraksa

Multimedia Communication Laboratory, Computer Engineering Department, Faculty of Engineering, King Mongkut’s University of Technology Thonburi

126 Pracha-uthit Rd., Bangmod, Thungkru, Bangkok, 10140, Thailand mettripun_n@hotmail.com, t_amornraksa@cpe.kmutt.ac.th

Abstract. In this paper, we propose a watermarking scheme based on pixel-wise based digital watermarking for printed and scanned images. Conceptually, the watermarked image is printed on an A4 white color paper to create a ready-to-use paper having the watermark inside. This ready-to-use paper is then used to print out text and/or images to create a watermarked document. At another end, the embedded watermark can later be extracted from the distributed print out, and used to identify the document launcher and/or distributor. We apply in this research work the watermark embedding in various color channels, namely R, G, B, Y, Cb and Cr. The performances of the proposed watermarking scheme in terms of quality of the watermarked paper and the watermarked document, and accuracy of the extracted watermark are then demonstrated and compared. The experimental results show the impressive outcome when applying our pro-posed scheme in the Cr, Cb and Y color channels, respectively.

Keywords: Digital watermarking, copyright protection, printing and scanning, color models.

1 Introduction

The extensive use of digital media in the Internet nowadays has lead to a number of related copyright problems e.g. the distribution of illegal made copy of digital media without any permission from original owner. Digital watermark technique is then con-sidered and used to prevent the violation of copyright media and to verify its original ownership. The digital watermarking is a technique used to embed copyright informa-tion, known as a watermark signal, into a digital media in such a way that the quality degradation of the watermarked media is unnoticeable by human perceptual system. Moreover, it is very difficult for anyone, except an authorized person, to remove, re-place or destroy the embedded watermark signal without significantly degrade the quality of the watermarked media. When the watermarked media is duplicated, the watermark inside will be attached to the new reproduced copy too. The attached wa-termark can later be reliably extracted from the illegally distributed copy of the repro-duced media, and used to identify the original owner, to trace the traitor, or used as evidence in the court of law. In a real-life problem, such as the forgery of official documents e.g. transcript, married licenses, etc., the digital watermarking technique

Performance Comparison of Digital Watermarking for Scanned Document 575

can be adaptively applied to the printed documents before they are distributed to the public. In this circumstance, the watermarking techniques applied should be robust against printing and scanning, so that the extracted watermark can be used to identify the distributing source, or to verify the owner’s possession. This type of problem is an important issue and very useful for a practical watermarking system.

There are a number of research works in the area of watermarking for printed and scanned images. The errors frequently encountered in the printing and scanning proc-ess can be considered as rotation, scaling and translation attacks. This is why many researchers sometimes considered the printing and scanning process as a kind of combined attack. In the past, Ruanaidh and Pun [1] proposed a watermarking method for solving the problem of rotation, scaling and translation attacks. Their method was based on log polar mapping of magnitude. Another print and scan watermarking method based on circularly symmetric watermark embedding in the 2-D DFT DFT (Discrete Fourier Transform) domain was proposed in [2]. J. Rosen and B. Javidi [3] proposed a method of hiding an image in a different halftone image using the DFT as well. P. Bas et al. [4] used affine transform and three different feature points for em-bedding the watermark. In their experiments, they performed the print and scan attack but the results were not quite impressive for all testing images, especially with the image containing a lot of high frequency components. Voloshynovkiy et al. [5] intro-duced a scheme of watermarking in the printing channel based on a self embedding. According to their proposed scheme, the information was first encoded and then em-bedded into the image part and barcode section in the original paper document. Basi-cally, they used the Gel’fand-Pinsker construction with the consideration of printing channels proprieties. In 2006, Solanki et al. [6] proposed the selective embedding scheme in the low frequencies DFT components. The scheme hided information in the image they also taking into account the Discrete Fourier Transform (DFT). It can be noticed that most of the above methods embed only a small number of information bits into the host media, and are limited to a grayscale image. In 2008, Bin Luo et al [7] proposed a document watermarking arithmetic method based on the FMT trans-formation, was robust to copying. Their method can be used to distinguish a water-mark signal between the first printing document and duplicated or tempered one. Their approach later leads to a technology progress in copyright protection of paper media including book, newspaper and magazine. In 2009, William et al. [8] develop a technique for authenticating physical documents based on naturally occurring imper-fections in paper texture. In particularly, they generated a concise fingerprint that uniquely identifies the document. Although their approach is robust against counter-feiting and harsh handling, it is hard to survive geometric attacks e.g. cropping since its own feature cannot be identified at every inch of paper sheet. Recently, N. Mettri-pun et al.[9] proposed a high capacity watermarking scheme for printed and scanned documents. They actually embedded a watermark into the blue channel of the host media. The result was printed on the A4 white color paper to create the so-called ready-to-use paper. However, their approach based on blue channel watermark em-bedding works best on the digital image, not the paper based document. Hence, their approach needs to be fully explored.

In this paper, we propose to use a digital watermarking technique based on the modifications of image pixels [10] for the scanned documents. That is, a watermark signal will be embedded into a blank A4 white color paper. This watermarked paper is

576 N. Mettripun and T. Amornraksa

then used as ordinary paper to print out text, images, or both to obtain the so-called “watermarked document”. To extract the embedded watermark, the printed water-marked paper or the watermarked document is converted back to a digital image, us-ing a scanner, and then put into the watermark extraction process. The main reason for selecting the above watermarking technique is because the capacity of the water-mark that can be embedded into a host media, i.e. A4 white color paper, is huge com-pared to the other watermarking techniques. We also investigate the suitability of color channels used to carry the watermark signal, based on RGB and YCbCr color models. The performances in terms of the quality of watermarked paper and the accu-racy of the extracted watermark between 6 different color channels are compared and discussed. Sets of experiments were carried out to demonstrate the efficiency of our proposed digital watermarking for the scanned documents, and to determine the opti-mum setting to be used for a practical watermarking system. The next section de-scribes general background of RGB and YCbCr color models, and a brief concept of digital watermarking based on the modifications of image pixels. In section 3, the proposed watermarking schemes for the scanned documents in 6 different color chan-nels are presented. In section 4, the experimental setting is given, and the results are shown and discussed. The conclusions are finally drawn in section 5.

2 Background

2.1 RGB and YCbCr Color Models

The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a lot of color. Accordingly, Red (R), Green (G) and Blue (B) are the primary colors of this model. The YCbCr model is actually a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Essentially, Y is the luminance component, while Cb and Cr are the blue-difference and red-difference chrominance components. The equation repre-senting the relationship between the RGB and the YCbCr models is shown below [11]

⎥⎥⎥

⎢⎢⎢

⎥⎥⎥

⎢⎢⎢

⎥⎥⎥

⎢⎢⎢

−−−−=

⎥⎥⎥

⎢⎢⎢

081312.0418688.05.0

5.0331264.0168736.0

144.0587.0299.0

2.2 Digital Watermarking Based on the Modification of Images Pixels

In the watermark embedding process, a watermark bit w is first converted to bipolar value, as 1 or -1. The watermark signal strength is then adjusted by a scaling factor s and the luminance of the embedding pixel L(i,j) = 0.299R(i,j) + 0.587G(i,j) + 0.114B(i,j). The luminance value is considered and used here because of the fact that changes in high luminance pixels are less perceptible to the human eye, so that more energy of watermark can be added to obtain a higher accuracy in the watermark ex-traction process. Finally, the resultant watermark is added to an image pixel P(i,j) in a chosen color channel to obtain the watermarked pixel P’(i,j). The process of water-mark embedding can be represented by the following equation.

),(),(),( ),(' jisLjiwjiPjiP += (2)

In the watermark extraction process, the embedded watermark bit w(i,j) can be ex-tracted based on two assumptions. First, any pixel value within an image is close to its surrounding neighbors, so that a pixel value at a given coordinate (i,j) can be esti-mated by the average of its nearby pixel values. Second, the summation of w around (i,j) is close to zero, so that the embedded bit at (i,j) can be estimated by the following equation.

)),('),('(8

1 ),('),('

1∑∑

−= −=

−++−=m n

jiPnjmiPjiPjiw (3)

where w’(i,j) is the estimation of the embedded watermark w around (i,j). Since w(i,j) can be either 1 and -1, the value of w’(i,j) = 0 is set as a threshold, and its sign is used to estimate the value of w(i,j). That is, if w’(i,j) is positive (or negative), w(i,j) is 1 (or -1, respectively). Notice that the magnitude of w’(i,j) reflects a confident level of es-timating w(i,j).

3 The Proposed Watermarking Scheme

To embed the watermark signal into a blank A4 white color paper, we modified the watermarking technique, as proposed in [10], and used in our watermarking scheme. Accordingly, the watermark bits w(i,j) ∈ {1,-1} to be embedded are first permuted, using XOR operation, with a pseudo-random bit-stream generated from a key-based stream cipher to improve the balance of w around (i,j). The watermark embedding is performed by modifying the image pixel P(i,j) at a given color channel, in a line scan fashion. The modifications of the image pixel P(i,j) are either additive or subtractive, depending on w(i,j), and proportional to the modification of luminance of the embed-ding pixel L(i,j). Notice that the modification of luminance L’(i,j) is obtained from a Gaussian pixel weighting mask [10]. Fig. 1 shows the block diagram of the proposed watermark embedding process.

Fig. 1. Block diagram of the proposed watermark embedding process

To extract the watermark signal from the scanned watermarked paper, the em-bedded bit at (i,j) can be estimated by the following equation

)max)_max,_('),('(8

1 ),('),('

1∑∑

−= −=

−++−=m n

nmBnjmiBjiBjiw

Fig. 2. Block diagram of the proposed watermark embedding process

where B’(m_max, n_max) is a neighboring pixel around (i,j) that most differs from B’(i,j). Fig. 2 shows the block diagram of the proposed watermark extraction process.

In the next step, we applied the above watermarking technique in our watermark-ing scheme. As shown in Fig. 3, the watermark is first embedded into a white color image, and the result will then be printed on a blank A4 white color papers to create a read-to-use watermarked paper. This paper can be used to print out text, images, etc. to obtain the watermarked document. At the receiver end, the watermarked document is converted back to the digital watermarked image to extract the watermark inside.

Fig. 3. Block diagram of the proposed watermark scheme

4 The Experimental Setting and Results

In the experiments, we created a 512×768 pixels white color image and used it as the original image. Notice that the original image size we selected was equivalent to the working area size of the A4 paper. The same 512×768 pixels black & white image containing a logo ‘CPE KMUTT’ was then used as a watermark i.e. by considering the black color pixel as -1, and white as 1. The original image and the watermark logo are shown in Fig. 4.

(a) (b)

Fig. 4. (a) The original white color image (b) The watermark logo ‘CPE KMUTT’

After embedding the watermark logo into the original image in 6 different color channels, namely, R, G, B, Y, Cb and Cr, the resulting watermarked images were printed out on the ordinary blank A4 white color papers to create the testing water-marked papers. We then used these testing papers to print out texts, photo images and/or graphic images to create the watermarked documents. To evaluate the accuracy of the extracted watermark, the watermarked document was scanned back into a per-sonal computer to reproduce the noisy version of watermarked image. The watermark contained inside was then extracted using our proposed extraction process. The ex-tracted watermark was finally compared to its original version, and the NC (Normal Correlation) between two versions of watermark was calculated. The NC can be rep-resented by the following equation.

∑ ∑∑ ∑

∑ ∑

= == =

jiwjiw

1 11 1

),('),(

),('),( (5)

At this step, we tested the quality of the printed watermarked paper or watermarked documents by a subjective test based on the human visual system i.e. human eye. The subjective test performed in our experiments was a modified version of the ITU-R Recommendation 500 Quality Rating [12] to make it suited to our experiments. All experiments were performed on the white color DoubleA A4 papers, Photoshop 7.0 with the image size of 72 PPI (Pixel Per Inch) or 512x768 pixels in bitmap (.bmp) format, and the Printer and Scanner HP F2280 All-in-One with the printer setting at the Print out Resolution of 600 DPI (Dot Per Inch) and the scanner Setting at the Out-put Resolution of 72 PPI.

4.1 Performance Comparison between Watermarked Images and Papers

In this experiment, we examined the quality of the watermarked images and the wa-termarked papers, judged by the human eye, from the computer monitor and the print

out papers, respectively. The value of s = 0.6 was used in the experiment. The results of the watermarked images and the watermarked paper in various color channels are illustrated in Fig. 5.

(a) R color channel (b) G color channel

(c) B color channel (d) Y color channel

(e) Cb color channel (f) Cr color channel

Fig. 5. The watermarked images (left) compared to the watermarked papers (right) in various color channels

From the above figure, the color represented on the watermarked images and the watermarked paper obviously different. This result verifies the difference in nature between the problems of digital watermarking in the scanned document and in the digital images. Therefore, when the watermarked image is printed on a A4 white color paper image, the result is equivalent to applying a set of attacks to the watermarked image, and when the watermarked paper is scanned back to the digital version, with-out printing anything on it, the result is equivalent to applying another set of attacks to the scanned watermarked image. A small difference in equipment setting and/or ex-perimenting can hence leads to an unpredictable outcome, and cautions must be care-fully taken when conducting the experiments on this subject. To evaluate the accuracy of the extracted watermark, we performed different watermark extraction process on the same images and watermarked papers 4 times. Each time, the NC value was com-puted, and the results obtained were averaged and given in Table 1.

Table 1. The average NC values of the watermark extracted from the watermarked images and the watermarked paper in various color channels

Input Type Average NC value

R G B Y Cb Cr

The watermarked image 0.9893 0.9893 0.9893 0.9893 0.9896 0.9896

The watermarked paper 0.7923 0.8193 0.8075 0.7850 0.8075 0.7750

Average NC dropped 0.1970 0.1700 0.1818 0.2043 0.1821 0.2146

According to the results shown in Table 1, it can be understood that the noise in-troduced from the printing and scanning process caused the reduction of NC on aver-age by 0.19. Furthermore, the accuracy of the watermark recovery in the watermarked paper can be dropped by at most 0.215, which occurred in the Cr color channel, due to such noise. At this stage, it can be presumed that the noise introduced to the scanned watermarked paper were equivalent to rotation, scaling and translation attacks.

4.2 Performance Comparison between Watermarked Papers and Documents

In the next experiments, we examined the quality of extracted watermarked from dif-ferent types of print out watermarked document. We first began to experiment on finding a threshold used to validate the extracted watermark. This threshold value was simply determined by computing the NC value between the original watermark and the one extracted from the scanned non-watermarked paper (or A4 white color paper) at various color channels. The resulting highest NC value was then taken and used as a threshold. The highest NC value in each color channel from the 4 times testing is given in Table 2.

Table 2. The NC values obtained from the scanned non-watermarked paper at various color channels

Input Type NC value

R G B Y Cb Cr

The scanned non-watermarked paper

0.6710 0.6710 0.6710 0.6710 0.6710 0.6710

As a result, a little higher NC value of 0.68 was set as the threshold and used in the remaining experiments. After a threshold was found, the testing watermarked papers obtained from the previous experiment were used to print out some predefined pat-terns consisting of texts, photo images and graphic images. The result was then scanned back to obtain the digital version, and the watermark inside between different embedding channels was extracted and compared. Fig. 6 to 9 illustrate various types of watermarked documents and their extracted watermark in various color channels, while Table 3 shows the NC values obtained from the corresponding extracted wa-termarks. Note that in this experiment and the remaining, the watermark strength was set to 0.6.

Fig. 6. The watermarked paper and its extracted watermark in various color channels

Fig. 7. Text based watermarked document and its extracted watermark in various color channels

Fig. 7. (continued)

Fig. 8. Image based watermarked document and its extracted watermark in various color channels

Fig. 8. (continued)

Fig. 9. Image & text based watermarked document and its extracted watermark in various color channels

Fig. 10. Graphic & text based watermarked document and its extracted watermark in various color channels

Table 3. The NC values obtained from various types of watermarked documents in various color channels

Type of watermarked document

NC value

R G B Y Cb Cr

Text based 0.7998 0.7400 0.7335 0.7664 0.7926 0.7431

Image based 0.8271 0.7523 0.7969 0.7446 0.7269 0.7611

Image & text based 0.8026 0.8027 0.7904 0.7790 0.7621 0.7914

Graphic & text based 0.8295 0.8558 0.8070 0.8277 0.7923 0.7844

Average 0.8147 0.7877 0.7820 0.7794 0.7684 0.7700

From the above figures it is clear that although the information printed on the test-ing watermarked paper caused a quality degradation of the embedded watermark, the word ‘CPE KMUTT’ contained in the watermark logo extracted from all color chan-nels can still be undoubtedly seen. It is also clear that the average resulting NC values obtained from all color channels were higher that the threshold, so that the existence of the embedded watermark can be reliably conformed. The average NC value ob-tained from the Cb color channel was the lowest though. Notice that the watermark signal strength set to 0.6 was high enough to survive both noises introduced from the printing and scanning process, and the print out content. Since the texts and images contained within the watermarked document can be considered as another noised in-troduced to the scanned watermarked image, we then compared performance reduc-tion in the watermark extraction process. The reduced performance will obviously decrease the accuracy of the extracted watermark. The results in term of average NC values obtained from all testing watermarked papers and all testing watermarked documents in various color channels are shown in Table 4.

Table 4. The average NC values obtained from all testing watermarked papers and all testing watermarked documents in various color channels

Input Type Average NC value

R G B Y Cb Cr

The watermarked papers 0.7923 0.8193 0.8075 0.7850 0.8075 0.7750

The watermarked documents 0.8147 0.7877 0.7820 0.7794 0.7684 0.7700

Difference in value -0.0224 0.0317 0.0255 0.0056 0.0391 0.0050

From Table 4, it can be seen that another kind of noise, based on the texts and im-ages introduced to the scanned watermarked image, can cause the reduction of the NC value on average by up to 0.039, which occurred in the Cb color channel. It should be noted that the extracted watermark however was still valid, judged by the threshold, and understandable to everyone. Also, the increase of NC from the R color channel can sometimes occur because of the energy of the watermark signal and the intro-duced noise supports each other. Nevertheless, from our observations, this incident is quite rare to occur.

4.3 Performance Comparison against Possible Errors in Scanning Process

To evaluate the robustness of the embedded watermark in different embedding chan-nels, in this experiment, we directly applied four possible errors that might occur in the scanning process, i.e. by introducing Gaussian distributed noise, salt and pepper noise, image cropping and image rotation, to the scanned document before extracting the watermark inside. Particularly, the first introduced error was the zero mean addi-tive Gaussian distributed noise with variances (σ2) ranging from 0.001 to 0.6. The second one was the salt and pepper noise with densities ranging from 0.01 to 0.6. The third one was the center image cropping at various percentages ranging from 10 to 90%, and the last one the image rotation at various degrees ranging from -4° to 4°. Notice that, to extract the embedded watermark from the cropped scanned document, the missing parts in the watermarked image were replaced by the black color pixels.

Also, the signal strength of 0.6 was used among comparing channels. The plots of the average NC values obtained from the experiments are shown in Fig. 11-14. According to the results obtained, it is obvious that the watermark embedded in the blue color channel was more robust against errors than the other two channels. However, it should be noticed that the valid extracted watermark can still be obtained from all three different color channels.

0.001 0.05 0.1 0.2 0.3 0.4 0.5 0.6Variance of Gaussian Noise

Blue Channel

Cb Channel

Cr Channel

Threshold

0.030.05

0.2 0.4

Salt and Pepper Noise Density

Blue Channel

Cb Channel

Cr Channel

Threashold

Fig. 11. Zero-mean Gaussian noise Fig. 12. Salt and pepper noise

10 20 30 40 50 60 70 80 90

Cropping Percentage

Blue channel

Cb Channel

Cr channel

Threashold

-0.3 0.5 1.3 2.0 2.8 3.5

Rotational Degree

Blue Channel

Cb Channel

Cr Channel

Threshold

Fig. 13. Image cropping Fig. 14. Image rotation

4.4 Performance Comparison between Different Embedding Channels for Easy-to-Read Aspect

In the final experiment, we performed the test on the subjective point of view to evaluate the quality of the watermarked documents in term of the ease of reading. The subjective test was based on the ITU-R recommendation 500 [12], with some modifi-cations to fit our objective. Eleven people were selected to be observers; all were or-dinary people (6 males and 5 females) having good health and had no known vision problems. In the experiment, each type of watermarked document was presented to an observer. Afterwards, the observer gave a numeric score to each test as a result. The conformability of reading or seeing the contents in the watermarked document was rated by numbers ranging from 1-10, as the most comfortable to the least comfortable

to read/see. Finally, the scores from all observers were accumulated and represented by an average value. Fig. 15 shows the comparison results in reading/seeing conform-ability obtained from the watermarked document in different embedding channels.

4.093.45

8.008.59

R G B Y Cb Cr

Channel to be embedded

Fig. 15. The comparison results in reading/seeing conformability obtained from the water-marked document in different embedding channels

From the figure, it is obvious that, on average, watermarking in the YCbCr color model was more comfortable to read/see than that in the RGB color model. Also, the watermarking in the Cr color channel obtained the highest score, while the watermark-ing in the Cb and B color channels obtained a closed score. In summary, it can be said that our proposed watermarking scheme was most suited to be implemented in the Cr color channel, and the next suited color channels would be Cb and B, respectively.

5 Conclusions

We have presented in this paper the implementation of the digital watermarking scheme for printed and scanned images. The watermarking technique based on the modification of image pixels was applied in our proposed watermarking scheme. The performance of the watermark embedding and extracting in various color channels of the print out watermarked paper were examined and compared including the quality of both watermarked paper and watermarked document, and the accuracy of the ex-tracted watermark. The experimental result showed that the Cr, Cb and Y color chan-nels are the best color channels for the watermark application of printed and scanned images. For the future work, we are currently investigating techniques to minimize the error term from the printing and scanning process tin order to improve the accu-racy of the extracted watermark.

Acknowledgments

This research work was partially supported by the Commission on Higher Education, which granted to Mr. Narong Mettripun with the reference ‘CHE-PhD-SW-NEWU’.

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Performance Comparison of Digital Watermarking for Scanned Document in RGB and YCbCr Channels

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