The Types of Digital Watermarks and Their Uses
The Types of Digital Watermarks
All digital watermarks can be separated into two broad categories: visible watermarks and invisible ones. As the categories' names imply, visible watermarks can be easily seen, and invisible ones are hidden from sight. Visible watermarks are also known as overt watermarks, and invisible ones are sometimes called covert watermarks.
Visible watermarks usually take the form of a semi-transparent image that's overlaid on the original image. The image usually contains the name of the photographer or company that holds the copyright for the image, although it can contain additional items, like the year or a copyright symbol. Since the image is semi-transparent, it's clearly visible but also lets the viewer see the original image.
When superimposing an image as a visible watermark, it's important to make sure the superimposed image can't easily be cropped from the photograph. The semi-transparent image that's superimposed should either cover most of the picture or a vital part of it, such as a person's face in a portrait or wildlife in a nature shot.
Invisible watermarks are embedded into an image and intended to remain unseen under normal viewing. They're only visible via specialized software. There are several different types of invisible watermarks:
- superimposing an image but keeping it extremely transparent so that it's not noticeable
- flipping the lowest-order bit of specific pixels, which only works if the photograph won't be modified and, even then, is usually too basic to be effective
- spatial watermarking, which applies a watermark to a specific color band so that the watermark only visible when the colors are separated, such as for printing
- frequency watermarking, which applies a watermark to a specific frequency so that the watermark is only visible when that frequency is separated out
Of these, spatial and frequency watermarking are the most common invisible watermarks used.
When using frequency watermarking, the watermark is best applied to either a low frequency or a frequency that's critical to the image. High frequencies are often lost in compression and scaling, and the watermark will be lost if the frequency it's applied to is lost.
Uses of Visible Watermarks
By stating the copyright holder's name, visible watermarks help protect an image from copyright infringement. Their usefulness in tracking down illicit uses of an image, however, is limited. Not only is it difficult to search for all uses of a semi-transparent watermark, but these watermarks can be removed by software. Visible watermarks are getting better at resisting image transformation, but a determined and the knowledgeable thief will still see the watermark and, with the right software, possibly be able to remove the semi-transparent image.
The strength of visible watermarks lies in their immediate claim of ownership. The best visible watermarks clearly label the image with the copyright owner's name and, thus, eliminate any commercial value for people who would use the image illicitly. Because they state the copyright owner's name, they can also be used for promotional purposes.
The Uses of Invisible Watermarks
Invisible watermarks are used to prove an image's authenticity and identify the rightful copyright holder. Since they're harder to identify and remove than visible watermarks, invisible ones are especially helpful when searching for and prosecuting illicit postings of an image. They can help prove that a person violated a copyright claim, which can make prosecuting a copyright lawsuit and collecting royalties easier.
Invisible watermarks can also be used to track down occurrences of an image and identify the original source of each occurrence. The photographers, copyright holder's, distributor's and consumer's name can all be embedded in an invisible watermark of an image.
Copyright Protection Options for Photographers
Photographers have always needed to take steps to protect their work. In today's digital age, these watermark options give photographers several ways to prevent, identify and prosecute illicit uses of their images.