Descriptive geometry is a branch of geometry which uses a specific set of procedures to
The methods of descriptive geometry are based on projection, i.e. on a transformation where an image of a threedimensional object a Euclidean space is projected onto the Euclidean plane. Basic Types of ProjectionLines and planes which are basic elements of space, as well as all other spatial objects, are observed as sets of points.Hence, it is sufficient to define the projection of a point. There are two basic types of projection  central and parallel. Central projectionLet a plane Π and a point S be given in space such that the point S is not at infinity and not lying in the plane Π (S ∉ Π).central projection to the plane Π from S. The point S is called the center, and the lines ST are called rays of the central projection. The plane Π is called the projection plane or picture plane.
Parallel projectionLet a plane Π and a line s be given in a space such that the line s is not parallel with the plane Π.^{1}Lines parallel with the line s are called rays of the parallel projection, and the plane Π is called the projection plane or picture plane. All other cases of parallel projection are called oblique parallel projections. ^{1} A line lying in a plane is also considered to be parallel with that plane. ^{2} In the extended Euclidean plane a parallel projection can be interpreted as a central projection with the center at infinity. From the fact that S^{∞} ∉ Π it can be deduced that the point S^{∞} is the point at infinity of some line that is not parallel with the plane Π.
Monge's method (two picture protocol)The loss of one dimension, in the projection of a spatial object to a plane, creates various problems when reconstructing the 3Dmodel from its projection.For example, all triangles whose vertices lie on the same rays of a central or parallel projection will have the same projection.
If you have only one projection of a triangle in a plane Π what can you conclude about the projected object from its image?  It can be a projection of a truncated threesided pyramid, in the case of a central projection, or a threesided prism, in the case of a parallel projection.  It can be a triangle.  There are also some other more complicated possibilities.
Such problems, when dealing with one projection, which is important for gaining an accurate image of an object, are solved in different ways: choosing a suitable object position in reference to a given projection, highlighting certain points and lines of the object etc. However, when the metrical and positional data of the spatial object are more important than the accuracy of its image on a drawing, as is often the case in engineering, then it is appropriate to use two orthogonal projections on two orthogonal planes. For example, if we add another orthogonal projection in the case above of the orthogonal projection of the white and yellow triangle, as on the picture below, and simultaneously observe both projections, then the graphical data about them is considerably more useful. Now, it is clear that there are two different triangles, one (yellow) above another (white).
Monge's method consists of:  orthogonal projection onto two orthogonal planes where one is chosen as the drawing plane,  rotating the orthogonal projection of the object that does not lie in the drawing plane by 90^{o} around the intersection of the planes into the drawing plane,  in this way we gain two connected projections of an object in the drawing plane which allows a unique and accurate reconstruction of the 3Dmodel. The projection planes are usually taken to be vertical and horizontal, where the vertical plane of projection is chosen as the drawing plane, and the horizontal plane of projection is rotated counterclockwise. The pictures below represent animated illustrations of this procedure when the given object is a point T.

Today Monge's method is integrated in almost all 3DCAD programs. The computer 3Dmodelling is brought to the stage where the
twodimensional object on the screen can be easily changed, so that we have the feeling that it is only rotating while we are observing it.
In fact only the provisioned projection parameters are changing fast. Therefore, since Monge's method is very applicable in computer
3Dmodelling, the detailed study of this basic procedure and its properties is useful for future engineers.
Created by Sonja Gorjanc 3DGeomTeh  Developing project of the University of Zagreb.
Translated by Helena Halas and Iva Kodrnja.