In painting, light is expressed through the change of light and shade levels, which makes painting submerged in the free rhythm and harmonious contrast formed by light and shade, and makes light and shade show unique charm in contrast.

It is a physical phenomenon in the objective world that the illumination of light changes the brightness of three-dimensional objects. Light and shade expression is one of the methods of artistic modeling. This modeling method, as early as the Renaissance, has formed a relatively complete system after being summarized by Da Vinci and other art masters. Using the method of light and shade can make the painting image have three-dimensional sense, space sense, texture, color sense and light sense, and at the same time, it can also give the painting image rich levels, realistic effect and implicit charm, and enhance its artistic appeal. Objects show different faces according to their own structures. Under the irradiation of light, these surfaces form different angles with the light source, showing a certain relationship between light and shade. It shows different hues (hue is the amount of light reflected from the surface of an object). It can be seen that the differences and changes in tones are formed according to the changes in dignity.

Observe the size of the object by light and the law of color change, based on the following four points:

1, the angle between the surface of the object and the light.

2. The intensity of the light source and the distance from the object.

3. The distance between the object and the painter.

4. The depth of the object color.

The light and shade changes of an object after receiving light can be summarized as follows: three major surfaces: the light receiving surface, the side light surface and the backlight surface.

Five tones: bright, semi-bright, dividing line between light and dark, dark and reflection.

The bright part and the semi-bright part belong to the bright side. The bright part is often called "bright tone", and the semi-bright part is often called "middle tone" or "gray tone". The boundary between light and shade, dark part and reflection belongs to the dark side, which is often called "dark tone" The light tone, dark tone and middle tone are also called black, white and gray.

Bright part: it is the place where the object is irradiated by direct light, with large amount of light, light color and little change. Smooth and hard objects will have "highlights" in direct light, but not all objects have them.

Semi-bright part: it is the part of an object irradiated by side light, showing relatively clear and distinct characteristics. The change of tone is complex and subtle. The dividing line between light and dark: it is the dividing line between light and dark. Through the dividing line between light and shade, the two parts are closely linked and strictly separated. It should be noted that the boundary between light and shade is not a line, but a relatively narrow surface and a deeper tone. Because it receives weak ambient light and is close to the light-receiving part, the timbre feels the deepest. Grasping the dividing line between light and shade during sketching is of great help to the generalization of the basic tone of light and shade and the expression of three-dimensional sense.

Dark part: formed by an object with its back facing the front light source. In the case of the same inherent color of the object, the tone of the dark part is darker. Reflection: it is the result of the influence of ambient light on the side. Drawing the reflection part well is helpful to the expression of three-dimensional sense. Under normal circumstances, the brightness of reflection will not exceed the light-receiving part, which is very soft and subtle.

Projection is the shadow part of the object, at the edge of the object. As far as lighting is concerned, everything has a projection. Drawing a good projection helps to express the three-dimensional sense and spatial sense of the object. The hue of the projection is deeper, the front edge of the projection is clearer, and the further it fades, the boundary edge becomes blurred. There is a division of light and dark tones that divides projection into five tones, namely: bright part, semi-bright part, dividing line between light and dark, reflection and projection.