Watermark originally refers to the traditional method of printing and painting with woodcut in China. Later, it mainly refers to a digital information protection method, which adds some digital information to data multimedia (images, sounds, video signals, etc.). ) to realize the functions of document authenticity identification and copyright protection.

The embedded watermark information is hidden in the host file, which does not affect the visibility and integrity of the original file. In other words, making a semi-transparent mark on a picture can be realized by PS tools or some picture reading software.

The development history of watermark usually uses watermark in banknotes, certificates, securities, food stamps and so on. In order to prevent forgery. For example, in the left margin of the 20 15 version of 100 RMB, you can see the head image through the light, which is the watermark. The earliest watermark was created by paper makers in the Tang Dynasty in China at the end of the 8th century or the beginning of the 9th century.

At that time, silk thread was woven on the bamboo curtain. The pattern is more prominent than the curtain surface, so the fiber interweaving in the corresponding parts is finer and the light transmittance is higher when making paper. The watermark effect is obtained. /kloc-In the 3rd century, Italian craftsmen also woven letters, numbers or patterns on the paper-making network, leaving watermarks as trademarks in the paper-making industry. On the paper machine, the wet paper is pressed with decorative paper (watermark paper) to form a watermark.