## 5 Discussion  
We tested the feasibility of the proposed two-process framework by incorporating a projection mapping tool (i.e., the SAR rapid prototyping method) with a BIM model. Traditional AR relies on the sensor detection of the HMDs or hand-held devices for 3D position registration between the devices and the real world, leading to deviation results from sensor errors during operation. However, with the BIM-integrated SAR approach proposed in this study, 3D position registration is achieved through reference point assignment between the virtual and physical models in the real world. Once registration is complete, the corresponding 3D coordinate information provides a stable and accurate projection output, without the need for a wearable device or a continuous signal transmission from the sensors. The use of this method for design review would have great potential to provide a more interactive and intuitive means for design visualization. During the design review process, there could be several scenarios of the building design to be displayed, such as different materials of the building, positions of building components (e.g., windows), and overall structural designs. These design alternatives can be quickly projected on a physical mass model of a building, supporting collaborative design  
Figure 7: Comparison of projection results on planar/curved surfaces with various building textures. (**a**,**b**,**e**,**f**) Building textures of stone; (**c**) Building texture of brick; (**d**) Building texture of column pattern. between project participants. For example, textured models of varying design alternatives can be generated and visualized one by one using a projector during collaboration, allowing more interactive design comparison. If needed, users are able to make a change on building design using a BIM software, which can be simply and quickly inserted to the projection mapping software, and thus the updated design alternatives can be shown promptly on the physical model. For this process, only the presenter needs to operate the equipment, and the users do not have to hold or wear any devices during design collaboration activities. As a result, non-technical users can easily and intuitively review building models without feeling the discomfort or dizziness of wearing an AR device. Users can even move around to view models from different angles, similar to how a real building would be observed. According to the results of qualitative analysis in the case study, high-quality details from various perspectives were able to be easily observed, such as areas with a lot of visual contents and the building model edges. Through the proposed methodology, project participants could also share a common reference point in the projected SAR environment. This could be potentially beneficial for effective model-based collaboration between participants, since it would enable them to communicate face-to-face with a shared view of the model. There are, however, some challenges remain when using the projection mapping tool for design collaboration. First, manual matching with at least six reference points (the basis of a stereo object) is only suitable for simple models, such as a cube. Certain building models may have a very complicated structure that contains many interleaved edges and a variety of small components of various shapes. In such cases, with more reference points being required for better matching, manual calibration would not be suitable, and a number of the components might not perfectly match due to their complex structures. Second, for projects with numerous design alternatives, especially with replaced physical models, or where the design is frequently changed, manual calibration might also be inappropriate since recalibration of the reference points would be required. Third, when constructing a 360-degree view of a building model, edge blending might occur if there are overlapping areas from multiple projectors.