Visible: A visible watermark is very robust because, while it is not part of the foundation image, the watermark's presence is clearly noticeable and often very difficult to remove. An easy example of a visible watermark is a television identification logo, which is usually in the lower right- or left-hand corner of the screen. The watermark can either be solid or semitransparent, and removing it from a recording would require significant cropping, which would be instantly noticeable. For those of you who have any experience with Adobe Photoshop or another pixel-editing program, you instantly know the difficulty you would have removing the watermark in the image above without it being noticed (Figure 5.1).
Invisible: An invisible watermark's purpose is to identify ownership or verify the integrity of an image or piece of information. An invisible watermark is imperceptible, but can be extracted via computational methods. Sometimes you will see the term data hiding; this refers to an invisible watermark that actually contains information either about the watermark itself or the image or data in which it is embedded. Usually, when extracting an invisible watermark, a password is required; this password is called a watermark key. It is important to note the difference between a watermark key and an encryption key. A watermark key simply affects the watermark; an encryption key is the seed that affects the whole of the information to be encrypted. As described in the previous section about fragile watermarks, the invisible watermark is meant to be hidden, although it does not always have to be fragile. In the case of identifying ownership of a piece of information, an invisible, robust watermark would be preferable.