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Digital watermarks are regarded as the "last line of defense" against multimedia piracy.
Therefore, from the perspective of watermark technology itself, it has broad application prospects and huge economic value.
The development of today's society has shown two obvious characteristics: digitalization and networking.
Digitization refers to the storage form of information, which is characterized by large information storage and easy editing and copying; networking refers to the transmission form of information, which has the advantages of fast speed and wide distribution.
The past 10 years have seen explosive growth in the use and distribution of digital media information.
People can obtain digital information and online services quickly and conveniently through the Internet.
But at the same time, piracy has become easier, and the management and protection of digital content has become an urgent problem that the industry needs to solve.
Digital information is essentially different from analog information, and traditional solutions for protecting analog information are no longer effective for digital information.
Coupled with some general-purpose processors, such as PCs, hardware-based media protection solutions are easily broken.
The commonly used encryption technology can actually only protect the content of the media during the transmission of information from the sender to the recipient.
After the information is received, all data is transparent to the user during the reuse process and no longer receives any protection.
Under this situation, digital watermarking, as a potential solution, has been favored by many scholars.
The basic idea of ??digital watermarking is to hide additional information with a certain meaning as a mark in the original media data, such as audio, video, images, etc. This information is closely integrated with the original data and transmitted together.
At the receiving end, the watermark signal is extracted through a computer and used for various purposes. Possible applications include digital signatures, digital fingerprints, broadcast surveillance, content authentication, copy control, and secret communications.
Digital watermarking is regarded as the "last line of defense" against multimedia piracy.
Therefore, from the perspective of watermark technology itself, it has broad application prospects and huge economic value.
General framework diagram of digital watermarking Basic framework of digital watermarking A typical watermarking system consists of an embedder and a detector, as shown in the figure.
The embedder (formula (1)) generates a real watermark signal based on the information M to be transmitted, and hides it into the media data x to obtain a watermarked signal y.
For security reasons, the generation of watermark signals usually relies on the key K.
There may be a certain amount of information loss when y passes through the transmission network and becomes y′ when it reaches the detector. This channel is uncontrollable and unknowable to both the embedder and the detector, and can be called an attack channel.
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The detector is responsible for extracting information from y′, such as equation (2).
For detection that does not require a host signal, we call it blind watermarking, and the opposite is called non-blind watermarking.
Due to application requirements, blind watermarking has always been the mainstream of research.
Characteristics of digital watermarking Although the idea of ??digital watermarking is simple, in order to achieve the purpose of application, it must meet certain performance indicators. The relatively important characteristics include: ● Fidelity (fidelity): also often called invisibility, refers to
It is the degree to which watermark embedding causes changes in host signal quality.
Since the host signal is mostly multimedia data for people to watch, the watermark should have high fidelity and at the same time increase the security of the watermark itself.
● Robustness: refers to the survivability of watermarks during media data editing and processing.
Various operations on media data can cause the loss of host signal information, thereby destroying watermark integrity, such as compression, filtering, noise addition, shearing, scaling and rotation, etc., as well as some malicious attacks.
● Information capacity (data payload): refers to the amount of information that can be transmitted by a watermark signal under a certain fidelity.
Practical applications require watermarks to transmit multiple bits of information.
● Security: In an application, someone always wants to embed, detect or remove watermarks, and others must be restricted from doing the same operation. This is the security of watermarks.
To achieve security, important information must be kept secret, such as watermarking, usually using a key.
● False alarm rate: refers to the probability of incorrectly detecting a watermark in a host signal that does not contain a watermark.
Obviously, only when the false alarm rate is low enough can the system be used safely and reliably.
Designing a watermark must select the appropriate technology around the above performance indicators.
Some properties are incompatible with each other, like invisibility, robustness, and information capacity, and tradeoffs must be made.
Watermarks are divided into visible watermarks and invisible watermarks based on their fidelity. As the name implies, visible watermarks can be detected by the human eye.
According to the robustness of watermark, it can be divided into robust watermarking and fragile watermarking.
Robust watermarks can resist a certain degree of signal processing; while fragile watermarks are characterized by any change to the media information that will destroy the integrity of the watermark, making the watermark undetectable.
Therefore, robust watermarking tries its best to ensure the integrity of the watermark information, and fragile watermarking tries its best to ensure the integrity of the media information. They each have their own uses.