Anti-counterfeiting Technology: Digital Watermarking (I)

With the rapid development of digital technology and computer network technology, the dissemination and acquisition of digital information has become unprecedentedly fast and convenient, which has promoted the development of multimedia information exchange. However, due to the characteristics of easy storage, reproduction and dissemination of digital media, a large number of digital media often encounter illegal copying and dissemination while spreading in a legal form. However, some traditional encryption technologies cannot effectively prevent counterfeiting and infringement in data transmission. In this situation, digital watermarking technology (Digital Watermarking) has emerged as an effective complement to traditional encryption technologies.

At the same time that digital media applications are becoming increasingly widespread, for a long time, most of the graphic and textual information will also exist and be transmitted in print. However, increasingly rampant counterfeit and shoddy activities have brought new challenges to the anti-counterfeiting of printed products. Under such circumstances, the use of digital watermarking technology with high technology and low production cost for anti-counterfeiting of printed materials is a new topic and direction.

Digital watermarking is a new digital media protection technology. It embeds specific information (such as copyright information, serial numbers, company logos, meaningful text, etc.) into various digital media such as images, voice, and video. To achieve copyright protection and other purposes. At the same time, the embedding of such information does not affect the use value of the main media. It is not easily perceived by the human perception system and the original embedded information can be extracted through a specific recovery method. By extracting these hidden information, the creator and buyer of the content can be confirmed and their authenticity and integrity can be detected. A common embedding of a digital watermark in an image can be thought of as superimposing a visually invisible weak signal (watermark) under a strong background (ie, the original image). Since the resolution of the human visual system (HVS) is limited as long as the amplitude of the superimposed signal is lower than the contrast threshold of the HVS, the HVS cannot sense the presence of the signal. Therefore, within the contrast threshold range of HVS, the original image is modified (watermarking), so that some hidden information can be embedded without changing the visual effect. There are several basic requirements for digital watermarking generally: 1 imperceptibility; 2 provability: 3 robustness: 4 security.

The universal digital watermarking model includes two stages: watermark embedding and watermark extraction or detection. The design of the embedding stage mainly solves two problems: 1 The generation of digital watermarks. Can be a string of pseudo-random numbers, digital identification, text and images, etc., can also be related to the author string, icon and other information generated by encryption: 2 embedded algorithm. The goal of the embedding scheme is to make the digital watermark find a good compromise between imperceptibility and robustness.

The detection phase is mainly to design a detection algorithm corresponding to the embedded process. The detection result is the original watermark (such as string or icon, etc.) or the test result based on the statistical principle to judge whether the watermark exists or not. The goal of the detection scheme is to make the probability of misjudgment and missed judgment as small as possible. In order to increase the unpredictable difficulty of removing the watermark, most watermark production schemes currently use the key only when a person who has the key is used to add the watermark and extract it.

There are many methods for digital watermarking. According to different digital watermarking methods, digital watermarks can be divided into two categories: spatial watermarking and transform domain digital watermarking. According to whether the watermark needs the original image, the digital watermarking technology can be divided into three types: private watermark technology, semi-private watermark technology and public watermark technology.

1, special requirements

Due to the necessity of screening continuous tone images in printing to obtain a discrete halftone image, the digital watermarking technique is applied to change the level and color of the continuous tone image by changing the number of dots in the unit area on the printing plate, that is, the number of dots. In the anti-counterfeiting of printed matter, it is necessary to consider the link of printing and screening. That is, the digital watermarking technology used for anti-counterfeiting of printed matter is based on halftone technology.

In addition, compared with the application in digital media, the digital watermark applied to anti-counterfeiting of printed materials meets some special requirements in addition to the above mentioned basic requirements:

1 Visual invisibility and machine reading. When using a printing or printing device, the watermark added at various resolutions can not make the watermark appear due to the resolution. At the same time, it does not affect the quality of the original print. Only through specific detection equipment and computer software can Identification.

2 resist color transformation, file format conversion In the A / D and D / A conversion process, the digital watermarking algorithm to resist gamma correction and color distortion In addition, the image in the process of circulation, due to the aging of printed materials will also produce color distortion. At the same time, it must also be able to resist the frequent transitions between image file formats (such as BMP, TIFF, JPEG, etc.) and common operations (rotation, cutting, scaling, etc.).

3 There are no special requirements for printing equipment, no change in the printing process, no increase in printing costs.

2, Principles and Algorithms

The principle of the digital watermarking technology used for print anti-counterfeiting is similar to the principle of the digital watermarking technology used for multimedia. It is only necessary to consider the screen during the watermark embedding process. The watermark extraction and detection process should be carried out on the printed matter information. Scanning, there is an analog/digital (A/D) conversion process in this process. This requires that algorithms based on halftone digital watermarking techniques must take these factors into consideration during the corresponding embedding and detection process. The embedding and detection process is shown in Figures 1 and 2.