## 4 Case Study  
### Case Procedures of BIM-Based SAR  
The first step was to create a 3D building model, UV map, and texture file. The 3D raw model of a four-story building was built in Autodesk Revit and exported as a FBX file format, which is compatible with Autodesk 3 ds Max. The material information was added to the raw building models in Autodesk 3 ds Max. The UV maps and texture plans were also generated in this software, as shown in Figure 3a-c. The UV maps could be added to different 3D model file formats, compatible with the projection mapping tools; and the texture plans were exported as image files.  
Then, two-size mass models were made as the same proportional size as the virtual building model, where the proportion of dimensions are 78:79:100. The mass models were made of thick white cardboard, and the size of the two mass models are \(8.3\times 8.4\times 10.1\) cm, and \(5.8\times 5.85\times 7.4\) cm, respectively. The MadMapper was selected as the projection mapping software. The projector selected was Optoma ML550 with a throw ratio of 1.5/1 (D:W), which is suitable for the mass model size. This case study was conducted with the PC of Intel(R) Core (TM) i5-8250U CPU, 8 GB installed RAM, 256 GB SSD, Intel(R) UHD Graphics, and Windows 10 (64 bits). The connection port used between PC and projector was HDMI.  
The next step was to set up the position of the projector and the mass models. The \"Show Test Pattern\" function in MadMapper ensures the model is fully covered under the projection area, as illustrated in Figure 3d. Then, the virtual raw model file attached with the UV map and texture file was imported. The results displayed on the input channel in MadMapper is shown in Figure 3e,f. The file format of the 3D model should be an OBJ file due to the compatibility requirements of  
Figure 3: Case procedures. (**a**) 3D building model with material added; (**b**) UV map generated in 3 ds Max; (**c**) Texture plan generated in 3 ds Max; (**d**) Setting projector and physical model; (**e**) 3D raw model with UV map in MadMapper; (**f**) Textured model in MadMapper; and (**g**) Geometric matching.  
MadMapper. In MadMapper, virtual textured models can be moved, zoomed in or out, and rotated in the control panel.  
The last step was geometric matching and projection. Geometric matching was proceeded by using the calibration function of MadMapper. It offers a manual calibration method that matches six points in the virtual model on the control panel of MadMapper with the correspondent six points on the physical mass model. As a result, the six points in both the virtual model and physical model can be marked and connected through a red line emitted from the projector (see Figure 3g). After matching the six reference points, the projected model image was automatically dragged to the expected position on the mass model, with the virtual model being dragged to the pointed position in the software panel. Moreover, micro adjustments could be further performed manually at the matching point for trimming the image to fit the object edge perfectly, so that the projection could be achieved.  
As can be seen in Section 4.2, the realness level of the projected model and the similarity to the designed virtual model were satisfied. Although the imaging throwing technology of the projector is one of the core factors that would affect projection quality, the medium-class professional projector we selected for this case study was good enough overall to present the 3D model clearly and vividly. (Due to the working mechanism of the digital light processing (DLP) projector, there will be banding or line flicker when shooting the projection. Therefore, the captured results have nonnegligible anamorphosis from the actual scene observed by the eyes, and the performance is better for in situ observation).