The purpose of setting the rules for this photo is to facilitate identification, so the requirements for the five senses are strict. Let's take tooth exposure as an example. How much exposure? Dew doesn't affect the recognition at all, but what if there is too much dew? It is troublesome to identify, and there will be problems. If the rules need to fill this loophole, it is forbidden to show your teeth.
There are eyebrows and ears, that is, ears and eyebrows are exposed, and the face should be exposed.
Such photos are all white, and the whole world may achieve a unified pure white, but there are thousands of blue and red, and they cannot be unified in any case. This kind of photo is the standard of identification setting.