## 3 Framework
### Creating 3D Models with UV Map and Texture Image  
The first step is to build raw 3D models without texture through BIM software. Even though there might be original material information when creating a 3D building model in BIM software, the material information in the BIM is only for the information delivery to indicate the variety of designed material, rather than actually including the colored texture to the model surface. To create a 3D model whose surface material could be projected on a real object by a video projector [38], a material adding, or texture mapping procedure is required, either in the BIM software or 3D animation and rendering software. For example, the process of applying material to the object's surface could be further done in the same BIM software such as Revit, after completing the raw 3D building model [39]. Meanwhile, to create physical mass models as the base of projection, white-colored physical mass models are made in proportion to the 3D building model.  
The next step is to create UV maps and texture image files. UV mapping is the process of translating a 3D surface with volume and shape onto a flat 2D image, where the 2D image is referred to as a UV map [40]. The U and V refer to the horizontal and vertical axes of the 2D space, as X, Y and Z are already used in 3D space. During the UV mapping process, a polygon mesh needs to be produced, which is the collection of vertices, edges, and faces that make up a 3D object. After producing the polygon mesh, the seams on which the unwarping is based will be clearly defined. This step is normally automatically processed by UV mapping software. A UV map can be either have material texture, which can be used straight as the texture file, or without material texture, in which the texture file should be drawn based on the defined edge curves in the UV map. Furthermore, the UV map can be added to the raw 3D model, which is stored in the same file with the 3D model and can also be exported as an image file.  
Depending on the software chosen, there might be a few ways to produce a UV map with the corresponding texture file of a 3D model. In this context, the software could have integrated functions of UV mapping, material adding, and 2D painting. There is existing software that offers professional and convenient material adding for building design, and the entire complicated building texture is not easy to be drawn in a UV map. Therefore, as shown in Figure 2, the way we suggest is to add the material to 3D models to change the surface appearance before the UV mapping process, to add color, detail, and texture to the 3D objects created in the 3D modeling software. Therefore, both the UV  
Figure 2: BIM-based SAR Implementation Framework for Building Design Prototyping.  
map and texture file would be generated by UV mapping a material-added 3D model. Commercial software, such as Autodesk 3 ds Max and Blender provide both functions of material adding and UV mapping. The texture baking function might also be provided in this software to generate the texture file of a 3D object whose material is added by the other software such as Unity [38; 39]. In addition, a supplemental method could be used if the textured model is not fully prepared or there are slight changes on the designed texture. Since a 3D model is clearly unfolded at the seams and laid out flat on a 2D plan, once the UV mapping is complete, users can produce a custom image on the \"pattern\" of a UV map through 2D painting. This is similar to filling the color chunk on the UV map, which is linked to the respective area of the building model. This process makes it possible to produce models that are rich in color and detail with graphics software such as Photoshop. It is worth mentioning that Blender could also perform texture painting on the UV map and textures are displayed on the 3D object in the meantime. [41; 42].