You don't need complicated mathematical knowledge, but you can understand it by simple proportion and geometry: for example, this line is about three-quarters of that line, and this surface can basically be summarized as an equilateral triangle.

Gypsum geometry >: Still Life Combination > Gypsum Head > Portrait. Draw Portrait at the checkpoint.

Everyone who paints starts from zero, and so does Leonardo da Vinci. ...

It is not easy to learn your own definition well, but it is a long process anyway. Some teachers teach you to master basic skills for two or three years, and many people are stopped at the entrance door all their lives. There is no teacher to read books and watch videos.