## 4 Discussion  
For the 3D digital recording of cultural heritage objects, it is critical to select an appropriate technology considering the purpose, scope, object size, and required representation resolution [41]. Although there are cases where digital recording achieved using only a single technology is feasible, in most cases, the best results are achieved when various technologies are integrated in a complementary manner [42]. In particular, 3D convergence modeling is common when data fusion with photogrammetry is actively employed to counter the poor accessibility of 3D scanning technology or enhance the realistic effects of RGB textures [43,44]. Both 3D scanning and photogrammetry are optimized to acquire high-quality data based on the external geometry of the target object. Nevertheless, both technologies show drawbacks in digitally recording internal shape information that cannot be observed by the human eye. The use of X-ray CT is essential because information on the internal shape of an artifact is critical for archeological interpretation and conservation research. In this study, convergence modeling was performed to merge the optical scanning and X-ray CT models into a single 3D model. The optical scanning result of the replica of the prunus vase was a high-resolution mesh model with an average point spacing of 0.21 mm. The surface material texture and real color of the vase were well reflected; however, the thickness and internal geometry were not accurately reflected. To overcome this limitation, X-ray CT scanning was performed. In particular, the internal surface was converted into a mesh model with an average point spacing of 0.80 mm via surface determination processing. Figure 11: Three-dimensional convergence modeling results. The 3D mesh models obtained via optical scanning and X-ray CT were different in fine dimensions; hence, the X-ray CT model was aligned and registered with the optical scanning model with high dimensional accuracy. Then, the registration accuracy was evaluated through deviation analysis and visualization. The results demonstrated that the two models showed deviations within \(\pm\)0.5 mm, with an average RMS deviation of 0.13 mm. This RMS deviation was smaller than the point spacings of the optical scanning and X-ray CT models, indicating that the deviations had no significant effects on the merging process. Subsequently, mesh-based convergence modeling was performed. Convergence modeling utilized the external geometry of the optical scanning model as much as possible and supplemented the unscanned internal geometry using the X-ray CT model. To minimize geometry interference caused by overlapping areas of the models, the external surface of the X-ray CT model was removed, and the model was merged with the optical scanning model to obtain a single mesh model. Through mesh defect optimization, a 3D convergence model with an average point spacing of 0.45 mm was obtained. The convergence model showed the external geometry of the optical scanning model, exhibiting a volume higher (by 1.8%) than that of the X-ray CT model. Our results showed that the internal structures of cultural artifacts that cannot be determined by optical scanning can be determined using X-ray CT. To improve modeling effectiveness, digital documentation based on a single method can be augmented with multiple technologies. Notably, 3D convergence modeling can be employed in various applications, including the digital shape analysis, simulation, restoration, and replication of exhibition content. Therefore, transitioning to fusion modeling using RGB textures and voxel information is necessary in the future. Based on the findings of this study, we propose four main future directions to advance 3D convergence modeling: (i) Validate the convergence modeling technology for artifacts with various shapes. This study, being exploratory, focused on a prunus vase replica with relatively simple geometry, which limits the generalizability of the method.