## 4 Conceptual Framework for Augmented Reality Application
### Augmented Reality  
After the geometric and process modeling, an assessment is conducted to determine whether the collected data align with the intended objectives of the application. In cases where geometric models are exported, it is essential to monitor for any potential loss of geometry, which may necessitate revisiting the previous phase. Once the geometric and process models are deemed satisfactory, the subsequent step involves implementing the model for visualizing the digital twin in augmented reality.  
During this phase, the selection of the object tracking system and visualization equipment, such as the display device, is determined. The primary goal of tracking systems is to calculate the real scene's orientation based on captured images, ensuring accurate positioning of virtual objects that will be incorporated into the background [6; 70]. The selection of tracking technology depends on the sort of environment, the sort of data, and the availability of required budgets. These tracking techniques can be categorized as sensor-based, vision-based, or hybrid, depending on whether they rely solely on sensors, solely on visualization, or on a combination of both methods [71]. Some of these techniques are as follows:  
* Vision-based tracking--using markers--The tracking technique using markers is characterized by using cards containing a symbol inside. The markers must be previously registered in the system so that the software can recognize their pattern and connect with the virtual object. It is considered the simplest technique [72]. Some factors may interfere with marker recognition, such as (i) ambient lighting, (ii) camera orientation of the marker, and (iii) registration of markers with similar patterns [73]. A requirement of this technique is the need for a visual range for the marker to be recognized by the system [71].
* Vision-based tracking--markerless--This technique recognizes the image through \"natural\" markers, that is, through objects in the scene. In this technique, any part of the natural environment can be a marker. The advantage of this technique is that it does not introduce elements that are not part of the scene into the environment, such as a printed marker. However, the tracking to be developed tends to be more complex and specific [74]. In this technique, the tracking of objects in the environment can be performed with the characteristics of the points, lines, edges, and textures present in the objects.
* Sensor-based tracking--Systems using the sensor-based tracking technique may rely on acoustic, optical, mechanical, inertial, and magnetic sensors. For this, they can use the following methods: Global Positioning System (GPS), Wi-Fi, Bluetooth, ultrasonic sensors, infrared, and radio frequency identification (RFID), among others [71].  
Regarding the benefits of visual tracking, one can enumerate accuracy, stability, and widespread applicability. Nevertheless, it comes with the drawback of necessitating a physical marker for marker-based tracking and being vulnerable to variations in ambient lighting conditions. Conversely, markerless tracking can be employed in diverse environments and alleviates registration interruptions by leveraging natural features. However, it entails a trade-off between real-time performance and registration accuracy. Sensor-based tracking exhibits the advantage of rapidity and simplicity in its registration algorithm, albeit occasionally incurring higher costs and restricting natural interactivity due to the need for extra facilities and calibration. The hybrid technique complements each component to achieve maximal functionality.