In recent years, the distribution of works of art, including pictures, music, video and textual documents, has become easier. With the widespread and increasing use of the Internet, digital forms of these media (still images, audio, video, text) are easily accessible. This is clearly advantageous, in that it is easier to market and sell one's works of art. However, this same property threatens copyright protection. Digital documents are easy to copy and distribute, allowing for pirating. There are a number of methods for protecting ownership. One of these is known as digital watermarking.
Digital watermarking is the process of inserting a digital signal or pattern (indicative of the owner of the content) into digital content. The signal, known as a watermark, can be used later to identify the owner of the work, to authenticate the content, and to trace illegal copies of the work.
Watermarks of varying degrees of obtrusiveness are added to presentation media as a guarantee of authenticity, quality, ownership, and source.
To be effective in its purpose, a watermark should adhere to a few requirements. In particular, it should be robust, and transparent. Robustness requires that it be able to survive any alterations or distortions that the watermarked content may undergo, including intentional attacks to remove the watermark, and common signal processing alterations used to make the data more efficient to store and transmit. This is so that afterwards, the owner can still be identified. Transparency requires a watermark to be imperceptible so that it does not affect the quality of the content, and makes detection, and therefore removal, by pirates less possible.
The media of focus in this paper is the still image. There are a variety of image watermarking techniques, falling into 2 main categories, depending on in which domain the watermark is constructed: the spatial domain (producing spatial watermarks) and the frequency domain (producing spectral watermarks). The effectiveness of a watermark is improved when the technique exploits known properties of the human visual system. These are known as perceptually based watermarking techniques. Within this category, the class of image-adaptive watermarks proves most effective.
In conclusion, image watermarking techniques that take advantage of properties of the human visual system, and the characteristics of the image create the most robust and transparent watermarks.
1.1 THE DIGITAL WATERMARK
Digital watermarking is a technology for embedding various types of information in digital content. In general, information for protecting copyrights and proving the validity of data is embedded as a watermark.
A digital watermark is a digital signal or pattern inserted into digital content. The digital content could be a still image, an audio clip, a video clip, a text document, or some form of digital data that the creator or owner would like to protect. The main purpose of the watermark is to identify who the owner of the digital data is, but it can also identify the intended recipient.
Why do we need to embed such information in digital content using digital watermark technology? The Internet boom is one of the reasons. It has become easy to connect to the Internet from home computers and obtain or provide various information using the World Wide Web (WWW).
All the information handled on the Internet is provided as digital content. Such digital content can be easily copied in a way that makes the new file indistinguishable from the original. Then the content can be reproduced in large quantities.
For example, if paper bank notes or stock certificates could be easily copied and used, trust in their authenticity would greatly be reduced, resulting in a big loss. To prevent this, currencies and stock certificates contain watermarks. These watermarks are one of the methods for preventing counterfeit and illegal use.
Digital watermarks apply a similar method to digital content. Watermarked content can prove its origin, thereby protecting copyright. A watermark also discourages piracy by silently and psychologically deterring criminals from making illegal copies.
1.2 Principle of digital watermarks
A watermark on a bank note has a different transparency than the rest of the note when a light is shined on it. However, this method is useless in the digital world.
Currently there are various techniques for embedding digital watermarks. Basically, they all digitally write desired information directly onto images or audio data in such a manner that the images or audio data are not damaged. Embedding a watermark should not result in a significant increase or reduction in the original data.
Digital watermarks are added to images or audio data in such a way that they are invisible or inaudible Ñ unidentifiable by human eye or ear. Furthermore, they can be embedded in content with a variety of file formats. Digital watermarking is the content protection method for the multimedia era.
1.3 Materials suitable for watermarking
Digital watermarking is applicable to any type of digital content, including still images, animation, and audio data. It is easy to embed watermarks in material that has a comparatively high redundancy level ("wasted"), such as color still images, animation, and audio data; however, it is difficult to embed watermarks in material with a low redundancy level, such as black-and-white still images.
To solve this problem, we developed a technique for embedding digital watermarks in black-and-white still images and a software application that can effectively embed and detect digital watermarks.