3D scanning and digital documentation
Fixed and handheld scanning was performed to record the 3D shape of the statue and its missing parts (Fig. 3). First, the exterior of the statue (scanned 101 times) and the five fragments (scanned 34 times) were digitized using a fixed high-precision scanner based on stereo vision. However, the fixed high-precision scanner based on the binocular lens could not reach deep enough to record the shape of the excavated hole from the bottom to deep inside. Therefore, a handheld mid-precision scanner based on a single lens with depth scan capability was used to record the shape of the excavated hole.
Each image acquired by on-site scanning provides only misaligned raw shape information. Therefore, the post-processing of the raw scans was performed to complete a single model from the raw scan data. The processing was executed in the following order: filtering, aligning, registering, merging, filling holes, and RGB texture mapping. In particular, registering, which is the most important stage in scan processing, was processed with an average of 66% overlap ratio and within an error value of 45 μm.
Moreover, the high-precision scanning of the exterior of the statue and the mid-precision scanning of the excavated hole needed to be integrated into a single model because they created separate 3D models. Accordingly, a deviation analysis between the two scanning models to evaluate the registering error revealed that most deviations were within ±0.1 mm and the average RMS was 0.28 mm. This is similar to the accuracy of general precision scanners, so the convergence model was created based on an ICP algorithm.
The converged 3D model comprises 36,556,604 poly-faces with an average point distance resolution of 0.2 mm. According to Fig. 4, the 3D scanning result clearly reveals the complete shape of the statue and the polygon mesh and RGB texture mapping models show excellent quality. In particular, the final 3D scanning result had a high resolution with millions of polygons, which well revealed the surface texture, manufacturing technique, and the detailed shape of the damaged area.
Based on the 3D scanning, the stone Buddha statue was measured as 410-mm wide, 310-mm thick, and 580-mm high, and its calculated area and volume were 7,250 cm2 and 20,040 cm3, respectively. Also, the excavated hole was measured as 188-mm wide, 155-mm thick, and 195-mm high, and its area and volume were measured as 1,228 cm2 and 2,401 cm3, respectively. In particular, the cone-shaped excavated hole showed traces of chisels whose average sizes were 10.5 and 15.4 mm. Therefore, the excavated hole may have been carved using these two chisels.
Establishment of restoration scope
A 3D deterioration map was created to define the scope of the conservation treatment and the restoration. According to the map, cracks and missing parts were observed around the head, and the body was damaged mainly owing to cracks (Fig. 5). Out of the total area, the deterioration rates of missing parts were 4.8% (348.3 cm2) in the eye and head and 0.7% (52.9 cm2) in the body.
The general rule of the conservation treatment and restoration of cultural artifacts states minimum and passive intervention only under inevitable conditions. Therefore, in this study, among the damaged areas, minimum scope was selected for aesthetic restoration and exhibition. The cracks in the statue were left without conservation treatment, as they were not severe in depth and therefore determined not to cause any severe physical and structural problems.
However, the missing parts have a critical impact on the aesthetic value of the statue. Therefore, some parts that could be recreated using symmetry modeling or assured significant evidence through a historic research were restored. In particular, the whole stone fragments that were stored separately were used for the restoration. Accordingly, for the restoration, the big missing parts in the head, the surrounding area of the Baekho, the right ear, and the eye were selected, accounting for 64.2% (257.1 cm2) of the total surface area of the missing parts (400.1 cm2).
Virtual restoration modeling
The haptic modeling system used to create the virtual model of the missing parts enables intuitive modeling because it helps recognize conflicts and interference of data. Compared with the existing method that uses a mouse, this method is much better and excellent for creating a complex shape. In particular, a user can feel a force feedback similar to that experienced in actually carving a virtual object because the system uses a voxel engine to create a model. This system is mainly used in the medical field, design, car industry, exhibition technology, and restoration of cultural artifacts [33, 36–39].
In this study, the digital virtual restoration of the four selected parts was performed based on a haptic modeling system. In the process of modeling, Boolean operation was applied for combining several complex models into a new complete model. Boolean operation is an essential method for original-based modeling requirements such as the restoration of the cultural artifacts, and therefore, the operation was mainly applied to the symmetry modeling of the missing part.
During the examination of the virtual restoration modeling process of missing parts in the head (Fig. 6), first the original stone fragments were placed by referring to the shape and pattern of the stone Buddha statue. Then, a reference model for the missing part was selected and copied using symmetry modeling. However, since the original statue was created manually, it was not perfectly symmetrical. Therefore, Boolean operation was performed to modify the model, matching it exactly to the original shape. Because symmetry modeling could not be applied to the center of the crown part, a statue similar to the study object was selected as a reference and the estimated restoration was applied.
The restoration of the right ear has aesthetic well as functional purpose to support the original fragment. Accordingly, for the virtual restoration, the original fragment was placed in the right ear and a shape of the missing part was acquired through the symmetry of the well-preserved left ear. Next, the angle and height from the front view was adjusted to match those in the left and completed after partial revision (Fig. 7a, 7b). Finally, the aesthetic restoration of the surrounding area of Baekho and the right ear was performed by filling up the inner part and retouching the outer part. In particular, their restoration was completed after several modifications in consultation with art history experts because the overall impression of the stone Buddha statue can change owing to minor shape differences (Fig. 7c).
Design mock-up and assembly simulation
Design mock-up helps to intuitively understand the influence of interference caused by components and to reduce completion time by solving a making-related problem in advance. Also, preliminary verification through design mock-up minimizes unnecessary consumption of materials and time in the restoration process and maximizes work efficiency. In this study, to enhance the restoration completeness by the non-contact approach, digital–analog based assembly simulation was performed by creating a design mock-up. Material extrusion 3D printing, which can minimize time, shape error, and costs, was used to create the design mock-up.
To summarize the assembly simulation process, first an alternative model for the original statue was printed based on the 3D scanning model to minimize direct physical contact with the statue. Also, heuristic-based assembly suitability was examined several times by printing the virtual restoration model. When an error was detected during the verification, the virtual restoration model was modified and reprinted to enhance the assembly completeness. However, when no error or aesthetic issue was detected, a design mock-up was directly applied to the statue for further verification, and after using the photopolymerization 3D printing, the restoration process was completed (Fig. 8).
When the printed virtual restoration model was applied to the mock-up statue, the interference between the two surfaces was severe owing to the non-structured form (Fig. 9a). This indicates the high quality of the virtual restoration model, but it was insufficient for the restoration of the original artifact. Therefore, the area in the virtual model, where interference occurs, should be corrected so that the printed output fits the missing part of the statue. In addition to the revision of the virtual model, the assembly suitability to the 3D-printed mock-up statue was intuitively checked and verified by directly modifying the mock-up of the virtual restoration model (Fig. 9b). The 3D model, which was verified for digital–analog assembly suitability, was completed as the final design mock-up through the outer design modification (Fig. 9c).
In a quantitative analysis of the shape difference between the virtual restoration model and the final design mock-up of the head, the volume of the virtual restoration model was measured as 461 cm3 and the volume of the design mock-up that was assembled on the original stone Buddha statue was measured as 437 cm3 (Fig. 10a). The volume reduction by 24 cm3 (5.2%) is the amount of interfering surface removal along the assembly interface. In the deviation analysis of the final design mock-up against the virtual restoration model, the size of the joining surface removed was as much as 5.41 mm, with the RMS and standard deviation being 0.67 and 0.59 mm, respectively (Fig. 10b). Approximately 66.6% of the virtual restoration model, whose tolerance range was within ±0.1 mm, was not modified. The interfering surface of the joining interface was removed for 33.4% of the virtual restoration model. This deviation-mapping result enabled the visualization of the modification range and the deviation amount of the initial virtual restoration model, providing quantitative information about the digital–analog simulation process.
Restoration of the missing parts and their use for exhibition
The Athens Charter (1931), Venice Charter (1964), and Nara Document on Authenticity (1992) were prepared to establish the principles for the restoration of cultural artifacts, and countries around the world have restored their cultural artifacts according to their own rules following international recommendation . In general, damaged cultural artifacts can be restored only when their historical and cultural value can be retrieved through sufficient and direct historical research. Any restoration that might distort the value or cause a controversy should be limited. Therefore, restoration should be minimum and only when it is inevitable, using materials that are identifiable and maintain harmony when applied to the original. In this regard, it is a principle to use a traditional technique and the original material for the restoration. However, if the traditional technique is not applicable and the original materials cannot be supplied, modern technologies and materials that have been scientifically and experientially proven can be used.
The study object, i.e., the stone Buddha statue owned by Chuncheon museum, was made of zeolite; however, zeolite quarrying was illegal in the source area according to the local forestry law. Therefore, in this study, 3D printing technology was used for the restoration to serve an educational effect in exhibition and enhance aesthetic and functional completeness. The material extrusion 3D printing used to create the design mock-up played a critical role in analyzing the shape of the missing parts to design the assembly planning. However, this technology uses a material having relatively high surface roughness and weak physical properties to functionally support the original stone fragments owing to its light weight. To complement this, photopolymerization 3D printing technology was used to create the final restoration model.
The model was printed using opaque UV-hardened plastic whose color was similar to that of the original statue, with a layer thickness of 0.1 mm (Fig. 11a). After about 15 h of printing, the supporters were removed from the printed output and the surface that was in contact with the supporter was carefully smoothed with soft sandpaper with more than 3000 mesh. Finally, additional UV hardening was applied to enhance the surface strength of the output (Fig. 11b).
The finished 3D printing output had a smooth and glowing surface, which was disparate with that of the original statue (Fig. 11c). The initiator used in photopolymerization can cause yellowing from the surrounding light. Therefore, to modify the surface texture of the 3D-printed output and prevent yellowing, the surface was colored with acrylic painting and varnished to prevent the colored layer. In addition, the surface strength of the original statue, with which the 3D-printed output was interfaced, was enhanced by consolidation treatment, and the 3D-printed output was joined to the missing part of the statue using acrylic resin and cyanoacrylate instant adhesive (Fig. 12a).
In this study, various digital data and mock-up of the original statue were obtained using 3D scanning and printing technologies. This wide variety of contents shifted the exhibition, which used to consist of simple display of artifacts, into an educational exhibition that focuses on the restoration procedure. Combined with unique storytelling, the exhibition has provided a new experience to museum visitors (Fig. 12b). This is an important case, which shows that proven modern technologies and materials can be used for the restoration, education, and exhibition of cultural artifacts.