# Dynamic Mixed Reality Assembly Guidance Using Optical Recognition Methods
## 2 State of the Art  
### Literature Review  
In recent years, XR has been gaining increasing attention in industry, as demonstrated by many contributions in the field [16; 17]. For this reason, the study conducted in [18] proposes a framework to be used as a guideline for improving the usability of XR systems in manufacturing. The work presented in [7] aims at comparing the effects of oral, paper, and AR instructions for assembly tasks, showing that operators using AR make fewer errors, while also achieving a better quality of the product assembled, together with lower stress level and more independence. As also highlighted in [19], the more complex the assembly task is, the more appropriate is the use of AR instructions [7]. This evidence is also confirmed by an experiment conducted in [11], which shows that the difference in terms of execution times and errors in performing simple tasks with and without the help of AR is not as significant as when performing more complex tasks. Consistently with these findings, another research [20] indicates that the use of AR does not help when the assembly task is simple or when understanding the information takes more time than the assembly. Results of these experiments thus demonstrate that AR instructions do not always outperform paper instructions but provide better results for complex assembly tasks.  
As far as MR is concerned, in [21] a system combining virtual and augmented reality to assist and give real-time support in manual operations is presented; the study demon strates the ability of the systems to guide users in carrying out the task even without any prior knowledge on the assembly. The research conducted in [22] aims at designing and developing a virtual reality (VR) and AR system as a learning and training platform, finally showing an enhanced participant's understanding when using the proposed method. The work developed in [23] presents a MR application for mechanical design and assembly planning, aimed at visualizing and manipulating product parts in a virtual environment. In [24], a MR approach based on the development of a hybrid tracking system for the synchronization of the virtual and the real environment shows a significant improvement of the user performance in the assembly task execution, compared to the use of the VR only. The paper presented in [25] deals with the design and development of a concept for assisted assembly using MR and collaborative robots, with a significant decrease in assembly and training time in real production. The work presented in [26; 27] introduces a prototypical MR to provide assembly-related information as a support to the worker during the process, in order to ensure a zero failure performance; the testing of the system involving a comparative study with existing text-based instructions resulted in a significant increase in the final quality of the assembled product. Other contributions propose MR systems for supporting operators with specific assembly cases, as in [28], which deals with printed circuit board assembly lines or [29], in which motherboard assembly is tested; results of the application of these approaches show significant benefits for operators in terms of usability and effectiveness. Furthermore, in [30; 31], design requirements for MR interfaces are presented and applied to the development of a MR system to support workers in the wire-harness process by means of animated instructions, pictures, and videos. The test of the application shows promising results in terms of reduction of operator errors and forgetfulness.