This should start with the history of signature and painting. According to ancient historical records, the earliest time to draw a sign was in the Tang Dynasty. At that time, if you left your personal notes on documents, calligraphy, paintings or contracts, it was called "pledge", which is equivalent to "signature" and "signature" now.
At that time, all kinds of fancy cursive scripts were generally used in other contracts and documents, except for the throne for the emperor, which was forbidden. These signatures are collectively called "Flower Oath". Officials in Song Dynasty usually only wrote their names at the end of official documents, so they were also called "affixes". For example, Wang Anshi, the famous prime minister of the Song Dynasty, wrote the word "stone" in his signature, leaving only one after another, and directly omitting the word "mouth" below. Later, some officials thought his signature looked like the word "anti", so they added a circle in the middle instead of the word "mouth".
So the problem came. At that time, most people were illiterate. What should they do if they need to sign? Many people have ideas. They asked the people to imitate Wang Anshi directly and draw a circle instead. This is the origin of the so-called "flower duck". In fact, when the government takes photos, people usually need to use fingerprints as auxiliary evidence, so the photos themselves may be just a symbolic meaning.
On the other hand, because it is the first time for many people to touch the brush, it is difficult to draw a complete circle, so the patterns they draw have certain particularity. For example, Ah Q wanted to draw a circle, but it turned out to be melon-shaped, so in fact, the word "gambling" itself has special recognition significance for ordinary people.