## 2 Literature Review
### SAR for Design Collaboration  
Spatial augmented reality (SAR) provides the experience of augmented reality by changing the appearance of the physical world with an optical projector (Figure 1). Compared to AR technology, SAR separates the display technology from a user's body and embeds it into a physical model [21; 28; 29]. For SAR, the real world is augmented by a display that is integrated directly into the user's physical environment--not merely through the user's field of vision--to maximize the user's level of immersion [30; 31]. The images may appear in 2D, placed on a flat display surface, or may be 3D and float over a planar surface, or even 3D and float over an uneven surface. Seeing a design in a projection-based mixed reality environment helps users to better understand how a virtual product actually looks like in the real word. Moreover, not only will users obtain an augmented display of various prototypes themselves, but they can also interact with their customers in a creative and improved manner [32]. It is also worth mentioning that when SAR is applied to moving objects, the position of those objects is required in real time to facilitate dynamic mapping [33].  
The designed displays of SAR have been applied to various domains to explore how SAR systems can be applied to enhanced design mock-ups. In particular, there have been applications in the automotive industry, which uses virtual content to design and assess new components for cars [34]. For instance, virtual content may be projected on a mass car model to help during a collaborative design session. Similar to the automotive industry, architectural design uses 3D building models to benefit from SAR technology for design collaboration. [PERSON] et al. created a SAR system that combined the rapid prototyping of a physical model and an illuminating touchable interface for the aesthetic design of an automobile. The physical mass model of automobile was placed on a turntable to offer rotational movement, and the tangible interface was projected on a vertical planar screen [35]. [PERSON] et al. applied the SAR system when designing an automotive dashboard. The dashboard's designs were fully projected on a mock-up model, and the texture could be changed to reflect particular design decisions. By changing the projection with the displays of several designs, users were able to compare and evaluate snapshots of possible layouts, even though the mapping did not have high fidelity [36]. [PERSON] et al. also demonstrated the application of SAR when designing appliances for an oven, where the design alternatives were projected quickly and easily on a real-scale oven. This application demonstrated that using SAR allows non-technical stakeholders to effectively take part in a collaboration process of a product's design [37]. With the potential of SAR becoming a new visualization platform, collaborative design in the building industry would likely benefit from this technology as well, with the combined usage of the 3D BIM model. Since this integrated application of SAR and BIM for design collaboration has not been tested so far, we have proposed a framework of BIM-based SAR for a detailed individual building design collaboration.