## 4 Discussions and Suggestions
### Developing Markerless Systems and Smartphone-Based AR  
Suitable types of AR systems should be selected based on course content, prior to the design and development of the system. It is recommended to develop markerless systems for a larger application range to benefit students with additional convenience. Systems can be accompanied by functions, such as instant online assessments, synchronized assessments, and exchange capabilities to help students to learn what has been taught and develop their critical thinking ability.  
The development programs of AR systems are discussed based on three categories: The AR development tool, the AR type, and the AR teaching device. Most of the ACE education research uses open source tools to create AR systems, such as AR-media(tm), ARToolKit(r), and Junaio(r). The most selective AR type is the marker-based AR, with less options in developing markerless AR. It is worth  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Inductive Categories** & **Sub-Categories** & **Quantity** & **Sample Research** \\ \hline \multirow{6}{*}{Systems} & Difficult to create AR model or contents & 2 & [47] \\ \cline{2-4}  & Small and not immersive AR device screen & 1 & [35] \\ \cline{2-4}  & Lack of system stability & 1 & [24] \\ \cline{2-4}  & Tied easily after extended holding marker-based AR & 1 & [49] \\ \cline{2-4}  & mobile device in the air while sliding it & & \\ \hline \multirow{6}{*}{Curriculum} & Insufficient study time or too much course content & 5 & [46] \\ \cline{2-4}  & May increase students’ learning difficulty after using & 1 & [35] \\ \cline{2-4}  & AR for a long period of time & & \\ \cline{2-4}  & Can be difficult in design-concept assessment & & [17] \\ \cline{2-4}  & compared to using AR in a paper-based approach & 1 & [36] \\ \cline{2-4}  & Difficulty in working simultaneously with AR and & & \\ \cline{2-4}  & concentrating on the lecture & 1 & [36] \\ \cline{2-4}  & More effective in a 3D physical model than an AR & & \\ \cline{2-4}  & model in helping students to understand the & 1 & [29] \\ \cline{2-4}  & transformation from 2D images to 3D objects & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: AR challenges in ACE education.  
noting that marker-based AR systems require students to maintain their own position relative to the markers in order to present virtual information. Consequently, students might feel fatigued after long operating hours [49]. Similar problems have also been noted in architectural practice. For example, due to the complexity of construction sites, it can be difficult to place AR markers properly, and system recognition issues frequently occur [57]. Future research and development should focus on using markerless AR programs, since markerless AR is considered to be more interactive than marker-based AR [8; 12]. In summary, both the design and arrangement of the marker are crucial in marker-based AR systems, with problems related to the limitations of distance, environment, and movement (Table 5). Researchers can select suitable types of systems for future AR development.  
Scholars in the field of architectural engineering have reported that tablet computers and wearable devices are superior AR devices to smartphones, and will therefore have great potential as hand-held AR devices [58]. However, smartphones continue to be the most commonly used devices in ACE education currently. This seems to be a reasonable finding, as smartphones are used ubiquitously by today's university students. Compared to other devices, smartphone-based AR can be more convenient for immediate responses to various occasions [59].