Portraits of ancient emperors date back to a very early period, but the exact date is uncertain.
During the Tang Dynasty, Yan Liben created the "Portrait of the Emperors of the Past Dynasties", which is the earliest collection of portraits of emperors that can be examined. However, the authenticity of the portraits of the emperors before the Tang Dynasty is debatable, because the painter did not see these emperors and could only draw them based on historical records and his own understanding.
From the Tang dynasty onwards, the portraits of some emperors began to gain credibility, especially those of Tang Taizong, a contemporary of Yan Liben. Judging from the "Walking Diagram", Tang Taizong's posture is depicted relatively realistically. However, the portraits of later Tang emperors were mostly painted by later generations, and their credibility gradually decreased.
From the Song Dynasty onwards, the portraits of court painters began to be handed down, and were enshrined in various shrines of the gods, Tianzhang Pavilion, and the palace of filial piety of Qinxian. These portraits have a high degree of credibility because they were painted by court painters of the time based on the true image of the emperor.
In conclusion, portraits of ancient emperors have different credibility in different historical periods. While some of the early portraits may not have been accurate, the quality and credibility of the portraits have gradually improved over time.