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, copying and dissemination of digital media, a large number of digital media often encounter illegal copying and dissemination while being distributed in legal form. However, some traditional encryption technologies cannot effectively prevent forgery and infringement during data transmission. In this situation, as an effective supplement to traditional encryption technologies, digital watermarking technology (Digital Watermarking) came into being. While the application of digital media is becoming more and more widespread, for a long period of time, most of the graphic information will also exist and spread in print. However, the increasingly rampant counterfeit and shoddy behaviors have brought new challenges to the anti-counterfeiting of printed materials. In this case, the use of digital watermarking technology with high technical content and low production cost for the 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, voices, videos, etc. To achieve the purpose of copyright protection. At the same time, the embedding of such information does not affect the use value of the main media, is not easily noticed by the human perception system, and through a specific recovery method, the original embedded information can be extracted. By extracting these hidden information, the creators and purchasers of the content can be confirmed and their authenticity and integrity can be checked. The common embedding of digital watermarks in images can be regarded as superimposing a visually invisible weak signal (watermark) against a strong background (ie the original image). Because the resolution of the human visual system (HumanVisualSystem, HVS) is limited to a certain extent, as long as the amplitude of the superimposed signal is lower than the contrast threshold of the HVS, the HVS cannot feel the existence of the signal. Therefore, within the range of HVS's contrast threshold, by making certain changes to the original image (adding a watermark), it is possible to embed some hidden information without changing the visual effect. Generally, there are several basic requirements for digital watermarking: â‘ imperceptibility; â‘¡ provability: â‘¢ robustness: â‘£ security. The general digital watermark model includes two stages: watermark embedding and watermark extraction or detection. The design of the embedding stage mainly solves two problems: â‘ Generation of digital watermark. It can be a series of pseudo-random numbers, digital signs, text, images, etc., or it can be generated by encrypting strings and icons related to the author: â‘¡ Embedded algorithm. The goal of the embedding scheme is to make the digital watermark find a better compromise between imperceptibility and robustness. The detection stage is mainly to design a detection algorithm corresponding to the embedding process. The result of the detection is the original watermark (such as a character string or icon) or a test result based on statistical principles to determine the presence or absence of the watermark. The goal of the detection scheme is to minimize the probability of misjudgment and missed judgment. In order to increase the unpredictable difficulty of removing watermarks, most current watermark production schemes use a key when adding and extracting. Only the person who has the key can read the watermark. There are many methods of digital watermarking technology. According to different loading methods of digital watermarking, digital watermarking can be divided into two categories: spatial domain digital watermarking and transform domain digital watermarking. According to whether the watermark needs the original image or not, the digital watermarking technology can be divided into three categories: private watermarking technology, semi-private watermarking technology and public watermarking technology. 1. Special requirements Since the continuous tone image must be screened in printing to obtain a discrete halftone image, the level of the continuous tone image and the color change are expressed by changing the amount of ink per unit area on the printing plate, that is, the number of dots, so digital watermarking technology is applied For the anti-counterfeiting of printed matter, it is necessary to consider the process of screening in printing, 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, in addition to the basic requirements mentioned above, the digital watermark used in the anti-counterfeiting of printed matter must also meet some special requirements: â‘ Visual invisibility and machine readable. When using printing or printing equipment, the watermark added at various resolutions cannot make the watermark appear because of the resolution. At the same time, it does not affect the quality of the original printed matter. It can only be passed through specific testing equipment and computer software. Identify. â‘¡ Resistance to color conversion and file format conversion In the process of A / D and D / A conversion, the digital watermarking algorithm needs to resist gamma correction and color distortion. In addition, during the circulation of the image, color distortion will also occur due to the aging of the printed matter. At the same time, it should be able to resist the conversion and common operations (rotation, cut, zoom, etc.) that are often encountered between image file formats (such as BMP, TIFF, JPEG, etc.). â‘¢ There are no special requirements for printing equipment, without changing the printing process and increasing the printing cost. 2. Principles and algorithms The principle of the digital watermarking technology used for anti-counterfeiting of printed matter is roughly the same as that of the digital watermarking technique used for multimedia, except that the watermarking process must take into account the screening, and the extraction and detection of the watermark should be based on the graphic information of the printed matter Scanning, in this process there is an analog-to-digital (A / D) conversion process. This requires that algorithms based on halftone digital watermarking technology must consider these factors in the corresponding embedding and detection process. The embedding and detection process is shown in Figures 1 and 2. Our
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