Digital watermarking is defined as the imperceptibly altering a work in order to embed information about that work. In the recent years copyright protection of digital content became a serious problem due to rapid development in technology. Watermarking is one of the alternatives to copyright-protection problem.

Digital watermarking can be classified as visible and invisible. The visible watermarks are viewable to the normal eye such as bills, company logos and television channel logos etc. This type of watermarks is easily viewable without any mathematical calculation but these embedded watermarks can be destroyed easily. In the case of invisible watermarks, the locations in which the watermark is embedded are secret, only the authorized persons extract the watermark. Some mathematical calculations are required to retrieve the watermark. This kind of watermarks is not viewable by an ordinary eye. Invisible watermarks are more secure and robust than visible watermarks.

The main characteristics of Digital watermark are:
Robustness: The watermark should be able to withstand after normal signal processing operations such as image cropping, transformation, compression etc.
Imperceptibility: The watermarked image should look like same as the original image to the normal eye. The viewer cannot detect that watermark is embedded in it.
Security: An unauthorized person cannot detect, retrieve or modify the embedded watermark.

Depending on the ability of the watermark to withstand normal signal processing operations, digital watermarking can be categorized as robust, fragile and semi-fragile watermarking. Robust watermarks are detectable even after some image processing operations has been performed on the watermarked image such as image scaling, bending, cropping, and so on. Robust watermarks are mainly used for copyright protection. Fragile watermarks became invalid even if a slight modification is done to the watermarked image. Fragile watermarks are mainly used for authentication purpose. Semi-fragile watermarks allow some acceptable distortion to the watermarked image. Beyond this acceptance level if any modification is done to the watermarked image, the watermark will not be detected.

Books: Digital Watermarking and Steganography: Fundamentals and Techniques by Frank Y.
Web resources: Digital Watermarking
Power Point Presentation (PPT): Digital Watermarking

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