What do the three facets and five tones in gouache mean?

Five tones: light and dark boundary, bright part, dark part, projection and highlight. The so-called "three big faces" are black, white and gray. "Black" refers to the backlight part of the object; "White" refers to the light-receiving part of an object; "Gray" refers to the sidelight part of an object. "Five major tones" refer to highlights (the brightest spots), bright parts (the light-receiving parts other than highlights), the dividing line between light and shade, dark parts (including reflections) and projections. Another theory is to remove "projection" and add "reflection", which are the "five major tones"

An object has three bright and dark states under illumination, which are called three major surfaces, namely, bright surface, middle surface and dark surface.

The five tones are highlight, gray part, light-dark dividing line, reflection and projection.

Three facets refer to the three facets that make up the perspective solid geometry. Without these facets, it is impossible to make a solid perspective view. . . The five tones are exactly what Lou said, black, white, gray, bright and dark, which are the basic elements in painting, making the work more hierarchical, that is, it can be summarized into five elements to deal with a painting, that is, five tones.