Introduction:

Digital Water Marking is the process of embedding a pattern of bits or information in to a digital signal which is used in order to convey the hidden data. The signal can be any audio, videos or picture.

What is a watermark?
A watermark is a secondary image which is overlaid on a primary source image. This helps in protecting the image.

Purpose:
The purpose of digital watermarks is to provide copyright protection for intellectual property that is in digital format.

Types of Watermarking:

* Visible Watermarking
* Invisible Watermarking

Visible Watermarking

As its name implies, the information is visible in the picture or video. It can also be called as printed watermarks. The information may be a text or logo which identifies the owner of the media.

In the picture below, a television broadcaster added its logo to the corner of its transmitted video, is an example of visible watermarking. Below is a sample image where you can experience watermarking, you will find a small image at the right bottom corner which is overlaid above the primary picture/video.

It's important to overlay the watermark in such a way that it is difficult to remove.

Invisible Watermarking

Invisible watermarks are designed to be completely invisible. In the case of Audio, it's completely inaudible. An important application of invisible watermarking is in the area of copyright protection systems, which helps in preventing unauthorized copying of digital media.
An invisible watermark is an overlaid image which cannot be seen but can be detected algorithmically.

Watermarking Lifecycle phases:

Life cycle of a watermark begins with embedding of watermark and ends with the detecting or retrieval.

A watermarking system is divided in to three different stages.

~ Embedding - An algorithm accepts the host and the data to be embedded and produces a watermarked signal

~ Attack
Watermarked signal is then stored or transmitted to another person. If this person makes a modification, this is called an attack. The term modification may not be malicious; the term attack arises from the copyright protection application, where pirates attempt to remove the digital watermark through modification. There are many possible modifications, for example, lossy compression of the data, cropping an image or video or intentionally adding noise.

~ Detection- An algorithm which is applied to an attacked signal in order to extract the watermark from it. If the signal is not modified during the transmission, then the watermark is still present and can be extracted. In robust watermarking applications, the extraction algorithm should be able to correctly produce the watermark, even if the modifications were strong. In fragile watermarking, the extraction algorithm should fail if any change is made to the signal.

How can an image be watermarked?

Visible watermarks on images can be easily achieved thorough image editing software's like imagemagick or any other, which have the watermark functionality. Invisible watermarks on images can be achieved through some proprietary software's.

Applications of Digital watermarking

* Watermarking has its applications in image/video copyright protection.
* Watermarking has been used to send "sensitive'' information hidden in another signal
* Broadcast Monitoring (Television news often contains watermarked video from international agencies).
* Fingerprinting (Different recipients get differently watermarked content).