## 3 Implementation of Automatic 2D Drawing Generation Program
### Automatic Generation of 2D Drawings  
Generation of Elevation, Section, and Plan View  
(1) Generation of elevation  
The elevation basically represents the right and left elevation shapes in the front and rear. The line indicating the elevation can be divided into two types: the intersection of the object faces and the absence of such intersection. Furthermore, a contour is formed at the boundary of the protrusion. For example, for curved surfaces, as shown in Figure 3, there is no line formed at the intersection of the object faces, but the part where outlines should be displayed depends on the elevation direction. The contour between two faces is visualized according to the direction of line of sight. When the elevation is displayed on the interface screen of system, camera is aligned with the X (left), -X (right), Y (front), and -Y (back) axes, and the projection method is displayed in the orthogonal mode on the screen. When the elevation is shown on the screen visualizing the IFC model, the invisible objects behind the faces of the objects visible in the elevation are masked and rendered invisible. To display the elevation on the computer screen, the triangular faces of the obscured objects are drawn with the background color. This may result in unwanted lines in addition to those visible on the elevation appearing in the background color. Therefore, it is possible to display the elevation on the screen in a fast and effective way by not using a separate removal operation for the obscured object. Figure 2: Fundamental data structure used to project a 3D shape onto a 2D drawing. Generation of section  
A section view is created by cutting the BIM model appropriately for the section view face desired by the user and by isometrically projecting the section view face in a direction perpendicular to the section view. A building can be cut in a reference plane as follows:  
1. The camera is positioned on the basis of coordinates of the cutting reference plane, and all objects intersected by the cutting plane perpendicular to the cross section are cut. 2. The boundary line of the cut faces becomes the boundary line displayed on the screen. As in the case of creating an elevation, when a cross section is displayed on the screen for rapid cross-section representation, the building object's cross section is represented such that the objects behind the cross section are covered. This study provided an interface for setting a reference line of the cross section at the user's desired location in a plane, and a cross section was then created using the reference line. Floor plan generation is a special form of section generation. Floor plan view is cut from the 1.2 m reference line upward from the bottom boundary surface of each layer and isometrically projected downward. Unlike the section generation calculation method, information such as the outline of the bottom face of the objects was used for creating the floor plan, in order to reduce the calculation time in the floor plan generation process. Information on the door opening direction can be generated in IfcDoor. The IfcDoor entity contains only location information and information about the opening direction. For the examination of the detailed form and type of inquiry, the Profile representation in the IfcDoorStyle parameter can be referred to. IfcDoor has attribute information, which is called separate footprint. Although this information is not expressed as a 3D shape, it contains information that can indicate the opening and closing directions of door, etc. in 2D drawings. The development program uses the property information to create the floor plan of the door object, as shown in Figure 4.  
Figure 3: Perimeter and outline of the elevation. The floor plan information generated in this manner can be expressed using the Direct X 3D library in a 3D scene instead of a 2D scene, as shown in Figure 5. There are many limitations in expressing rich 2D information, and future studies should focus on overcoming them. Figure 4: Derivation process of IfcDoor data.