## 7 Discussion
### Application Challenges and Limitations  
In the comprehensive exploration of AR presented in this paper, this research delves into various AR techniques such as marker-based AR, markerless AR, projection-based AR, superimposition AR, and outlining AR. The assessment of AR's utilization across numerous domains, including education and training, healthcare and medical applications, manufacturing and industrial processes, retail and marketing, as well as urban planning and smart cities, brings to the first discussion segment: application challenges & limitations. AR, despite its significant advancements and widespread applicability, faces some critical challenges that limit its full exploitation. These hurdles vary depending on the type of AR technique in use and the domain of application. Marker-based and Markerless AR systems, for instance, often encounter difficulties related to the stability and accuracy of object recognition and tracking. Lighting conditions, occlusions, and camera quality significantly impact these systems' reliability, making them less feasible in environments where precision is paramount, such as healthcare and manufacturing. Projection-based and Superimposition AR, on the other hand, face issues concerning the alignment of virtual and real-world elements. This incongruuity may lead to an ineffective or distorted AR experience. Furthermore, these techniques need high computational power and advanced graphics capabilities, limiting their widespread adoption, especially in resource-constrained environments such as handheld devices. One of the AR's main challenges lies in the accuracy of real-time depth sensing and perception. Errors or delays in rendering could lead to insufficient or incorrect information, adversely impacting the user experience and the applicability in fields such as education and training, where real-time interactivity is key.  
Considering AR devices, challenges also persist. For instance, headsets and smart glasses may offer immersive experiences but can be physically uncomfortable for prolonged use and pose social acceptability issues. Handheld devices suffer from limitations in display size and interaction modalities. Projection-based systems require controlled environmental conditions for optimal performance, limiting their usability in varying settings. AR contact lenses, while promising, are still in nascent stages, with challenges related to comfort, safety, and ethical considerations yet to be addressed comprehensively. Moreover, complexities related to user interface design, privacy concerns, and the cost of implementing AR solutions are pervasive challenges across all applications and technologies. They impose significant restrictions on AR's broader adoption and usability in domains like retail and marketing or urban planning and smart cities. While AR holds immense potential in transforming numerous domains, these limitations necessitate concerted efforts in research and development to mitigate the identified challenges and unlock AR's full potential. The subsequent sections will delve into the technological advancements and emerging trends that are set to shape the future of AR, followed by a discussion on the contributions, benefits, and implications of these developments.