What standard does the Preface to New Yuefu put forward for poetry writing?

Quality and diameter, straight and tangent, core and solid, light and smooth. Bai Juyi clearly pointed out in the preface to New Yuefu that the standard of poetry writing is quality and diameter, straightness and tangency, core and reality, and smoothness, which means that poetry must be authentic, easy to understand and easy to sing in order to be considered as the ultimate.