Descriptive geometry is a subject that studies the theory and method of representing shapes on a plane and solving space geometry problems. 1 103, Li Jie of the Song Dynasty in China wrote "Building French Style", in which the architectural drawings basically conformed to the geometric laws, but the painting theory had not yet formed. 1799, Frenchman g gaspard monge published a book, Descriptive Geometry, and proposed to use polyhedral orthographic drawings to represent space shapes, thus laying a theoretical foundation for descriptive geometry. Since then, scholars from all over the world have constantly put forward new theories and methods in projection transformation and axonometric drawing, which has made this subject more and more perfect.

The basis of descriptive geometry is projection method, which comes from the physical phenomenon that light illuminates spatial shapes and leaves shadows on the plane. The elements that form a projection are projection lines, objects and projection planes. Projection method is divided into central projection (all projection lines pass through a projection center point) and parallel projection method (all projection lines are parallel to each other). You can draw a perspective view with central projection and an axonometric view with parallel projection, both of which have good stereoscopic effect. Parallel projection method can be divided into orthographic projection method and oblique projection method according to whether the projection line is perpendicular to the projection plane. Among them, the orthogonal projection method is used to project the spatial shape on the horizontal projection plane, and the height value is added to the figure on the projection plane next to the projection of the corresponding point and line, which is called the elevation projection figure. It is usually used to represent three-dimensional images of objects, perspective views commonly used in architectural engineering, axonometric drawings commonly used in mechanical engineering, and elevation projection drawings commonly used in topographic survey, water conservancy, civil engineering, geology and mining. Perspective, axonometric view and elevation view are all single-sided projections. Because a space shape has three dimensions: length, width and height, its projection only reflects the dimensions of two dimensions. In order to describe the space shape comprehensively and accurately, it is necessary to adopt polyhedral orthographic map (a map that represents the shape and position of a space shape by several orthographic projections).