## 1 Introduction  
For architectural design, effective design collaboration is vital, as it helps to promote teamwork through encouraging cooperation, as well as sharing stakeholders' ideas and project knowledge [1]. Architects often work with not only peers but also professionals from other relevant backgrounds to address complex problems that are synthesized by multidisciplinary issues. Effective design collaboration can foster more creative design proposals, which might not be possible with the limited capability of architects [2]. Recently, the use of building information modeling (BIM) has facilitated close cooperation between different project stakeholders, including architects, engineers, contractors,and clients during the design phases of construction projects [3]. The technical core of BIM consists of 3D imagery and information management for buildings [4]. Since all the data is accessible in 3D, BIM could further promote the use of visualization as a method to exchange ideas and share knowledge within and amongst various stakeholders in a project, and some BIM viewer software are developed and available for users' communication [5]. Despite the advantages of visualizing architectural concepts using 3D models, existing BIM visualization platforms are not effective for sufficient design information sharing [6]. In particular, even though 3D models could be built by BIM software, participants would still have to image and map the models that are on 2D display mediums into the 3D real space, which relies on the spatial awareness of the participants [7]. To address this issue, previous studies have attempted to promote the process of translating modeling data into more intuitive physical experience by mixing simulated models with an actual environment [7]. In this context, augmented reality (AR) has been introduced as an alternative visualization platform to effectively convey 3D models into realistic insights via an extended assistance of BIM visualization [4]. AR is generally used to enhance real-world objects and spaces by using digital information [8]. In AR, data that stimulates and enhances the real world is presented and integrated into the user's observation [9], which shares resources between an AR display and users. These shared resources help to recognize relevant issues and inspire a new way of solution generation [10; 11]. Due to these aspects, AR has also gained attention for design review and collaboration. AR is suitable for developing an interactive 3D communication environment for design, allowing users to explore the greater potential of design scenarios and the ability to evaluate 3D BIM models more intuitively before being physically built [12]. Incorporating AR and BIM promotes collaborative attributes and realistic 3D visualization. AR also works as an extension or a supplement of BIM to immerse abstract 3D images into the user's view of the real world, which reaches a deeper level of reality [13]. Two types of AR display devices have been used for design collaboration: (1) hand-held devices such as tablets and mobile phones, and (2) head-mounted displays (HMDs) such as Microsoft HoloLens [14; 15]. Generally, in collaborative AR systems, virtual models displayed to users by hand-held devices or HMDs are consistent, and users can communicate with each other with details displayed on the screen [16]. However, existing AR display devices have been criticized from the drawbacks of (1) technical limitations of real-time tracking caused by the detecting sensors, (2) users' limited vision and uncomfortable wearing experience, and (3) unsatisfying visual quality of deviated perception and display issues due to screen display [17; 18; 19; 20]. Recently, a new form of AR called spatial augmented reality (SAR) has been introduced to address these issues. SAR uses a digital projector to present graphics on physical objects to augment real-world objects and spaces, where display devices are separate to users, as opposed to the body-attached displays of AR [21].