Digital watermarking is the technology of hiding information in digital media content such as images, videos about the content itself. It is similar to the idea of steganography where the message is hidden inside some digital content but differs in the purpose of hiding the message. Because in steganography hidden messages are not related to the content but In the case of watermarking a message related to the actual content is hidden inside the content.So watermarking is simply the technique of embedding some identification information known as watermark into the digital data.Watermarking has been around for a long time., in the form of watermarks found initially in plain paper and subsequently in paper bills. However, the field of digital watermarking was only developed during the last couple of decades and it is now being used for many different applications.
There are two types of digital watermarking Visible Digital Watermarking and Invisible Digital Watermarking. In Visible Digital Watermarking data is embedded as the watermark. This can be a logo or a text that denotes a digital medium's owner.In the case of Invisible Digital Watermarking data embedded is invisible or, in case of audio content, inaudible.
Some Desirable features of Watermarking are Robustness, Imperceptibility, Reversibility, Lossless embedding, Security etc.
By robustness it means the watermarked image is able to resist the attacks or distortions introduced by malicious data processing. This feature makes watermarked images acceptable for legal purposes.
A watermark is called imperceptible if the original signal and marked signal is indistinguishable.
The modifications introduced on watermark insertion should be below the perceptible threshold. After embedding, the watermarked image must be visibly appealing and identical to the original media.
Reversibility is a measure of the extent to which watermark signal removal is possible from the
watermarked media.In certain applications as forensic or medical, the watermark removal is desirable once the purpose is served. After authentication the image can be restored to their original form by removing the Watermarking.
The embedding process usually transforms the images to some domain as cosine transform or wavelet transform for adding the watermark signal. Distortions are normally introduced as an after effect of this conversion. A good watermarking system should be lossless, in that it should not distort the original Contents.
Watermarks must exist undetectable. Security means that even after the presence of the watermark is known to a malicious attacker it must not be possible for them to remove the same from the host media.
Application of Digital watermarking
Digital watermarking can be used for multiple purposes, such as:
- Copyright protection
- Content Authentication
- Covert communication
- Broadcast monitoring,
- Owner identification
- Transaction tracking
Copyright protection is the most prominent application of Watermarking. Who did first is the issue that is addressed by this application. In the internet age there is always the risk of published digital data be stolen and reused without obtaining any legal permission. Many times information is tampered or modified to the levels that are unacceptable. In such cases of misuse the original creator of the document could establish his ownership rights.
Content Authentication assures the data integrity. Digital images may be subjected to editing or removal of certain pixels areas. There may not be even any visible difference between the original and manipulated image in first sight to a viewer. However small these modifications or manipulations are the integrity of the data should be verifiable.
A hidden piece of information can be transmitted in a host media to a destination without being noticed. This is similar to Steganography where the information is hidden in a cover media. The watermark must be imperceptible to the eyes of the viewer. If the effects of watermarking are too strong it will attract the attention of attackers who may eventually succeed in retrieving the embedded information.
One of the first applications for watermarking was broadcast monitoring. It is often crucially important that we are able to track when a specific video is being broadcast by a TV station. This is important to advertising agencies that want to ensure that their commercials are getting the air time they paid for. Watermarking can be used for this purpose. Information used to identify individual videos could be embedded in the videos themselves using watermarking, making broadcast monitoring easier.
Another very important application is owner identification. Being able to identify the owner of a specific digital work of art, such as a video or image can be quite difficult. Nevertheless, it is a very important task, especially in cases related to copyright infringement. So, instead of including copyright notices with every image or song, we could use watermarking to embed the copyright in the image or the song itself.