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But in text files there is nowhere to ‘hide’ the watermark.Also, whilst these watermarks act to identify the owner of the original they don’ t tell you anything about who was actually authorized to use them. Several patents appear to have been granted over the use of digital watermarks as a method of demonstrating that a document that is encoded as a picture is genuine (but not necessarily original).
However, it is not a very useful method if you are trying to protect information such as text or similar.
This is because to hide these kinds of watermarks in a file you need to be able to alter lots of bits without anyone being able to notice.
The principle is to ‘scan’ the images or sounds and recognize the pattern, thus ‘prove’ the ownership of the original so that the owner can prosecute those found making or storing illegal copies.
This type of watermark is often used in the film and music industries to identify pirated copies.
Watermarks are known to have existed in Italy before the end of the 13th century. The more common type, which produces a translucent design when held up to a light, is produced by a wire design laid over and sewn onto the sheet mold wire (for handmade paper) or attached to the “dandy roll” (for machine-made paper).
The rarer “shaded” watermark is produced by a depression in the sheet mold wire, which results in a greater density of fibres—hence, a shaded, or darker, design when held up to a light.The first type of digital watermarks you will see are those which are visible or obvious, and are intended to be so.These are usually images that are superimposed upon a still picture or a moving picture.Watermarks are often used commercially to identify the manufacturer or the grade of paper.They have also been used to detect and prevent counterfeiting and forgery.These rely upon mathematical calculations performed on the picture, which has embedded in it, a unique numeric identification.This has a similar characteristic to the watermark manufactured into paper documents.They are also used to identify the owner (or copyright controller/or the document).Static watermarks may also be used so that when a document is copied, a previously ‘hidden’ watermark is revealed which shows that the copied document is not an original.That is why documents using this technique are normally professionally printed, and distributed as paper documents rather than in electronic form.There are other printing techniques that produce ‘raised’ printing, use magnetic inks or inks that will change color if they get wet (either with water or other liquids), but they are very specialized and are used together with watermarks to provide higher levels of document protection and copy prevention.