A watermark is a recognizable image or pattern in paper that appears as various shades of lightness / darkness when viewed by transmitted light ( or when viewed by reflected light, atop a dark background ), caused by thickness variations in the paper. There are two main types of watermark, the Dandy Roll process, and the more complex Cylinder Mould process. A watermark is very useful in the examination of paper because it can be used for dating, identifying sizes, mill trademarks and locations, and the quality of a paper.
Watermarks vary greatly in their visibility; while some are obvious on casual inspection, others require some study to pick out. Various aids have been developed, such as watermark fluid that wets the paper without damaging it. Encoding an identifying code into digitized music, video, picture or other file is known as a digital watermark.
Digital watermarking is the process of possibly irreversibly embedding information into a digital signal. The signal may be audio, pictures or videos, for example. If the signal is copied, then the information is also carried in the copy. In visible watermarking, the information is visible in the picture or video. Typically, the information is text or a logo which identifies the owner of the media. The image on the right has a visible watermark. When a television broadcaster adds its logo to the corner of transmitted video, this is also visible watermark. In invisible watermarking, information is added as digital data to audio, picture or video, but it cannot be perceived as such (although it is possible to detect the hidden information).
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