Fang Zheng's method was actually very simple: ancient disguise techniques. Modern so-called "imitation makeup" was something similar.
Of course, ancient disguise techniques had evolved as well. In the beginning, information wasn't so widespread, so disguises didn't need to be that precise. You only had to alter a few distinguishing features. After all, portraits back then weren't realistic, and details about prominent figures were passed on by word of mouth. What they really looked like could only be known by seeing them in person.
That was why, in ancient times, people often recognized tokens rather than faces. There were no photographs, after all. Who knew what you looked like?
That was also why protagonists in so many Wuxia novels could steal a token and escape without a hitch.
Under those circumstances, there was no need to pursue perfect replication through disguise techniques. There was neither the demand nor the tools for it.
Only later, when information became easier to exchange and technology advanced, did disguise techniques become more sophisticated.
Fang Zheng hadn't developed this disguise technique himself. He had learned it from someone else, back when he had been emperor in the Xun Qin World. Around three hundred years after his ascension, a great thief known as the Hundred Transformations Star Lord emerged in the martial world. This person's greatest skill was becoming anyone. From 0.8 meters tall to 1.8 meters tall, male to female, young to old—whatever he pretended to be, he resembled it perfectly. Not only that, he could imitate other people's aura and Internal Force, so the authorities had never managed to catch him.
That could be considered a remarkable natural talent.
Of course, the way this fellow was eventually caught was rather amusing. Once people got too full of themselves, they started floating. The Hundred Transformations Star