Anti-counterfeiting new technology: digital watermark (on)
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With the rapid development of digital technology and computer network technology, the dissemination and acquisition of digital information has become unprecedentedly fast and convenient, and promoted the development of multimedia information exchange. However, due to the ease of storage, reproduction and dissemination of digital media, a large number of digital media are often illegally copied and disseminated while being spread in a legal form. However, some traditional encryption technologies can not effectively prevent counterfeiting and infringement in data transmission. In this situation, as an effective complement of traditional encryption technology, digital watermarking technology (Digital Watermarking) came into being.
While digital media applications are becoming more widespread, for a long time, most of the graphic information will exist and spread in print. However, the increasingly rampant counterfeit behavior has brought new challenges to the anti-counterfeiting of printed matter. In this case, the use of digital watermarking technology with high technology content and low production cost for anti-counterfeiting of printed matter is a new topic and direction.
Digital watermarking is a new digital media protection technology that embeds specific information (such as copyright information, serial number, company logo, 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, and it is not easy to be perceived by the human perception system and the original embedded information can be extracted through a specific recovery method. By extracting these hidden information, you can confirm the authenticity and integrity of the creator and buyer of the content. A common way to embed a digital watermark in an image can be seen as superimposing a visually invisible weak signal (watermark) under a strong background (ie, the original image). Since the resolution of the human visual system (HumanVisual 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 feel the presence of the signal. Therefore, within the contrast threshold of the HVS, certain changes to the original image (adding a watermark) can be achieved by embedding some hidden information without changing the visual effect. Generally, there are several basic requirements for digital watermarking: 1 imperceptibility; 2 provability: 3 robustness: 4 security.
The universal digital watermark model consists of two phases: the embedding of the watermark and the extraction or detection of the watermark. The design of the embedding stage mainly solves two problems: 1 generation of digital watermark. It can be a string of pseudo-random numbers, digital signs, texts, images, etc., or it can be generated by encrypting strings, icons, etc. related to the author: 2 embedded algorithm. The goal of the embedding scheme is to find a better compromise between digital watermarking in imperceptibility and robustness.
The detection phase is mainly to design a detection algorithm corresponding to the embedded process. The result of the detection is the original watermark (such as a string or icon) or the 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 the watermark, most watermarking schemes currently use the key when only the master or the key is used to read the watermark.
There are many methods for digital watermarking. According to the different methods of digital watermark loading, digital watermarks can be divided into two categories: spatial domain digital watermarking and transform domain digital watermarking. According to whether the watermark is needed to detect the watermark, the digital watermark technology can be divided into three types: 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 digital watermarking technique is applied by changing the amount of ink in the unit area on the printing plate, that is, the number of dots to represent the change of the layer and color of the continuous tone image. In the anti-counterfeiting of printed matter, it is necessary to consider the process of adding screen in printing, that is, the digital watermarking technology 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 the anti-counterfeiting of printed products meets some special requirements in addition to the above-mentioned basic requirements:
1 visual invisibility and machine readable. When using printing or printing equipment, the watermark added at various resolutions cannot be displayed due to the resolution. At the same time, it does not affect the quality of the original printed matter only through the specific testing equipment and computer software. Identification.
2 resistance color conversion, file format transformation In the A / D and D / A conversion process, the digital watermark algorithm needs to resist gamma correction and color distortion. In addition, in the process of image circulation, color distortion will also occur due to the aging of printed matter. At the same time, it must be able to resist the conversion and common operations (rotation, cropping, scaling, etc.) between image file formats (such as BMP, TIFF, JPEG, etc.).
3 There are no special requirements for printing equipment, no change in the printing process, and no increase in printing costs.
2, principle and algorithm
The principle of digital watermarking technology for print anti-counterfeiting is basically the same as that of digital watermarking technology for multimedia, except that in the embedding process of watermarking, screening should be considered, and in the process of extracting and detecting watermark, the information of printed matter should be carried out. Scanning, there is an analog/digital (A/D) conversion process in this process. This requires that algorithms based on halftone digital watermarking techniques must account for these factors in the corresponding embedding and detection process.