3.1 Restorations of the bronze mirror and the mural
The bronze mirror and the mural before and after virtual imaging restoration are shown in Fig. 6 and Fig. 7. Before restoration (the projection screen played a pure black picture), the bronze mirror and the mural presented the original incomplete state. When the screen projected the digital images, the artifacts were restored to the complete state. The pure black picture and the digital image can be switched through the remote control, so visitors are allowed to freely control the restorations. In other words, virtual imaging restoration is a discernible method. In real restorations, there are some discrepancies in people's understanding of the discernibility. Some people want it to look the same from a far distance and different from a near place, such as the very common tratteggio retouch [22]. Others want to make it indistinguishable to the naked eyes, but distinguishable from special angles or by the special treatment. Such as dyeing the outer surface of ancient bronzes to make it indistinguishable but does not dye the inner surface to distinguish the restored area [23], or adding the fluorescent agent to restoration materials to make it distinguishable under the ultraviolet [24]. However, the main group of visitors is the ordinary people rather than the professionals who have access to view artifacts through ultraviolet lights or from the inside of artifacts. The ordinary people’s rights to knowing the original state of artifacts cannot be sacrificed. In contrast, the virtual imaging restoration can present both the complete and incomplete state of the artifacts, allowing the restoration areas to be easily discerned by visitors.
The virtual images and the artifact were perfectly joined together, which looks like a kind of in-situ restoration. Based on the rigorous simulation of the missing parts, the outlines of the virtual images were perfectly matched with the incomplete artifacts. The fine patterns of the virtual images were also accurately docked with that of incomplete artifacts.
Of course, there were still some shortcomings in our virtual image restorations. As shown in Figures 4 and 5, the first is that the virtual images have a certain degree of ghosting, of which the reason has been explained in Section 2.2. In the future, the use of thinner transparent sheets can make the ghosting be eliminated to an ideal degree. Second, the color differences between virtual images and artifacts were still relatively large. This is mainly because the imaging plate only reflects part of the light of the digital image, there are some differences between the color of the virtual image and that of the digital image. Theoretically, by adjusting the color parameters of the digital image, the color of real artifacts can be approximated. Third, as shown in Fig 6 and 7, although virtual images can still match the artifacts when viewed obliquely, from a very high or very low viewing angle, virtual images will be misaligned with the artifacts to a certain extent. This is probably due to the thickness of the imaging plate is still somewhat large. Finally, because virtual images were two-dimensional, depth of field cannot be produced. Therefore, the current virtual imaging restoration is only suitable for artifacts with flat surfaces, such as paintings, frescos or murals, textiles, and papers.
Fig. 4. Images of the bronze mirror before (a) and after (b) virtual imaging restoration.
Fig. 5. Images of the mural before (a) and after (b) virtual imaging restoration.
Fig. 6. Images of the bronze mirror viewed from an inclined (a), very high (b), and very low (c) viewing angle.
Fig. 7. Images of the mural viewed from an inclined (a), very high (b), and very low (c) angle.
3.2 Illumination test
Fig. 8. Illumination of reflected light on the surface of artifacts before and after virtual imaging restoration
Although in principle, the light of virtual images will not reach the artifacts, we still conservatively tested the illumination of the reflected light from the bronze mirror before and after virtual image restoration. The illuminance data is shown in Figure 8. It can be seen from the figure that the reflected light from the bronze mirror were not changed before and after the restoration, which proved that the virtual imaging restoration is a completely reversible restoration method.
3.3 Questionnaire surveys on the evaluation of virtual imaging restoration
Table 1 shows the results of single choice questions and scoring questions of the questionnaire. In Question 1, all respondents said that this was the first time they have seen the application of virtual imaging technology in the restoration of artifacts, which means that the work is innovative. The results of Question 2 show that 83.33% of scholars and all ordinary visitors believed that virtual imaging restoration can improve their visiting experience or understanding of artifacts, which affirms the value of our work. In Question 3, restoration scholars gave 4.33 points for the visiting experience of virtual imaging restoration, while ordinary visitors gave 4.47 points, confirming the good visiting experience of virtual imaging restoration. Most restoration scholars (83.33%) and all ordinary visitors thought that this method is worth recommending (Question 4), and the recommendation scores are 4.58 and 4.62 respectively (Question 5). This indicates that most people welcomed the virtual imaging restoration and were willing to recommend it. In the scoring of the effect of virtual imaging restoration (Question 6), restoration scholars gave 4.17 points, while ordinary visitors gave a higher score of 4.57. The difference in this score possibly mean that ordinary visitors depended more on intuitive feelings, but restoration scholars considered more evaluation factors. 75% of the scholars agreed that virtual imaging restoration does well in preserving the authenticity of artifacts, while the remaining 25% were uncertain (question 7). To a certain extent, it reflects that there were some disputes on the understanding of authenticity. All scholars agreed that virtual imaging restoration performs well in terms of reversibility (question 8), which is supported by the obtained scientific data (Fig. 8). Similarly, all scholars also agreed that virtual imaging restoration performs well in terms of discernibility.
Table 1. Questionnaire results of single choice questions and scoring questions.
Question
|
conservation scholars
|
ordinary visitors
|
1. Is this the first time you have seen the application of this technology in the restoration of artifacts?
|
Yes
|
No
|
Not sure
|
Yes
|
No
|
Not sure
|
100%
|
0%
|
0%
|
100%
|
0%
|
0%
|
2. Can this restoration method improve your understanding or visiting experience of artifacts?
|
Yes
|
No
|
Not sure
|
Yes
|
No
|
Not sure
|
83.33%
|
0%
|
16.67%
|
100%
|
0%
|
0%
|
3. Give a score to your visiting experience with virtual imaging restoration, from 1 (very insufficient) to 5 (very good).
|
4.33
|
4.62
|
4. Is this restoration method recommended?
|
Yes
|
No
|
Not sure
|
Yes
|
No
|
Not sure
|
83.33%
|
0%
|
16.67%
|
100%
|
0%
|
0%
|
5. Give a score to how much you recommend it, from 1 (very unrecommended) to 5 (very recommended).
|
4.58
|
4.62
|
6. Give an overall score to the effect of virtual imaging restoration, from 1 (very insufficient) to 5 (very good).
|
4.17
|
4.57
|
7.※Do you think this restoration method does well in preserving the authenticity of artifacts?
|
Yes
|
No
|
Not sure
|
/
|
75%
|
0%
|
25%
|
8. ※Do you think this restoration method does well in reversibility?
|
Yes
|
No
|
Not sure
|
/
|
100%
|
0%
|
0%
|
9. ※Do you think this restoration method does well in discernibility?
|
Yes
|
No
|
Not sure
|
/
|
100%
|
0%
|
0%
|
※questions only for scholars
/ vacancy
Table 2 shows the comparison results of virtual imaging restoration with other methods in several aspects by restoration scholars. The visualization results of Table 2 are shown in Figure 9, which is represented by a radar chart.
Compared with physical restoration. As shown in Table 2 and Fig. 9(a), most scholars believed that the visiting experience of virtual imaging restoration is better than physical restoration, which may be that virtual imaging restoration can bring more freshness and interactivity. Most scholars agreed that virtual imaging restoration has more advantages in authenticity than physical restoration, which is consistent with our point of view. On the one hand, virtual imaging restoration can present the true incomplete state of artifacts to a greater extent. On the other hand, physical restoration may be exposed to more complicated authenticity problems. Authenticity judgments may be linked to a great variety of sources of information, including form and design, materials and substance, traditions and techniques, and other internal and external factors [14], which undoubtedly increases the difficulty of restoration. Obviously, the more complex information the restoration materials carried, the more content deviating from the authenticity may be introduced to restoration objects to probably confuse visitors or restorers. In the case of re-restorations, the added materials may interfere with the judgment of restorers. The restorers usually focus on presenting the truth, but in the process they may lose track of it or may have to settle for a compromise [13]. Such was the dilemma in the Tirsted Church, where about 50% of the non-original pictorial contents were removed when the Gothic wall paintings were re-restored for the fourth time in 2000 [13,25]. On the contrary, for virtual imaging restoration, people only need to take into account the authenticity of the appearance, which is easier to achieve under the technical conditions nowadays. Therefore, the virtual imaging restoration can restore artifacts in a more cost-effective way with less authenticity offset, while physical restoration may introduce more inauthenticity. In terms of reversibility, most scholars believed that virtual imaging restoration is better than physical restoration. This is because absolute reversibility is almost impossible for physical restoration since the treatments implemented on the body of the objects will produce irreversible effects [15]. In contrast, virtual imaging restoration is completely reversible. A majority of scholars also agreed that virtual imaging restoration is better in terms of discernibility. This is possibly owing to the harmony and discernibility of physical restoration areas are contradictory, which makes it difficult for physical restoration to achieve significant distinguishability while maintaining the harmony of the restoration areas. Also, in terms of the economy, most restoration scholars agreed that virtual imaging restoration is better than physical restoration. This may due to because virtual imaging restoration saves materials and labor costs in comparison with physical restoration. For the scope of application, unexpectedly, up to one-third of scholars believed that the scope of application of virtual imaging restoration is wider than physical restoration. These scholars may think that quite a lot of artifacts are not suitable for physical restoration, and virtual restoration is a better alternative. However, we think they still overestimate the scope of application of virtual imaging restoration, because it is currently only suitable for artifacts with flat surfaces.
Table 2. Comparison results of virtual imaging restoration and other methods.
Restoration Methods
|
Visiting Experience
|
Authenticity
|
Reversibility
|
Discernibility
|
Economy
|
Scope of Application
|
Physical Restoration
|
+
|
6
|
+
|
8
|
+
|
8
|
+
|
8
|
+
|
7
|
+
|
4
|
-
|
3
|
-
|
2
|
-
|
3
|
-
|
2
|
-
|
3
|
-
|
4
|
○
|
3
|
○
|
2
|
○
|
1
|
○
|
2
|
○
|
2
|
○
|
4
|
Digital Restoration
|
+
|
9
|
+
|
4
|
+
|
7
|
+
|
7
|
+
|
6
|
+
|
5
|
-
|
0
|
-
|
2
|
-
|
0
|
-
|
0
|
-
|
5
|
-
|
4
|
○
|
3
|
○
|
6
|
○
|
5
|
○
|
5
|
○
|
1
|
○
|
3
|
Digital Projection Restoration
|
+
|
7
|
+
|
4
|
+
|
6
|
+
|
6
|
+
|
5
|
+
|
5
|
-
|
0
|
-
|
4
|
-
|
0
|
-
|
0
|
-
|
2
|
-
|
3
|
○
|
5
|
○
|
4
|
○
|
6
|
○
|
6
|
○
|
5
|
○
|
4
|
Patterns Restoration
|
+
|
11
|
+
|
7
|
+
|
9
|
+
|
9
|
+
|
6
|
+
|
4
|
-
|
0
|
-
|
2
|
-
|
0
|
-
|
0
|
-
|
4
|
-
|
2
|
○
|
1
|
○
|
3
|
○
|
3
|
○
|
3
|
○
|
2
|
○
|
6
|
AR Restoration
|
+
|
5
|
+
|
3
|
+
|
4
|
+
|
6
|
+
|
7
|
+
|
3
|
-
|
3
|
-
|
1
|
-
|
1
|
-
|
1
|
-
|
2
|
-
|
5
|
○
|
4
|
○
|
8
|
○
|
7
|
○
|
5
|
○
|
3
|
○
|
4
|
the numbers represent the number of people
+ virtual imaging restoration is better than the compared method
- virtual imaging restoration is worse than the compared method
○ the two methods are equivalent or incomparable
Fig. 9. The results of restoration scholars comparing virtual imaging restoration with other methods. (a) Compared with physical restoration; (b) compared with digital restoration; (c) compared with digital projection restoration; (d) compared with patterns restoration; (e) compared with AR restoration. + means that virtual imaging restoration is better than the compared method, - means that virtual imaging restoration is worse than the compared method; ○ means that the two methods are equivalent or incomparable.
Comparison with digital restoration. The results in Table 2 and Figure 9(b) show that virtual image restoration has obvious advantages over digital restoration in visiting experience. This possibly due to the scholars agreed that virtual imaging restoration presents the true appearance of artifacts, while digital restoration only shows digital artifacts. For authenticity, up to half of the scholars agreed that the two methods are equivalent or incomparable. This is a confusing result, in comparison with physical restoration, most scholars believed that virtual imaging restoration is better in terms of authenticity. However, when comparing with digital restoration, scholars seemed more neutral or hesitant. We think that this is a manifestation of authenticity disputes. As for reversibility and discernibility, the statistical results are more biased towards virtual imaging restoration. We believe that since digital restoration does not directly involve artifacts, the meaning of reversibility and discernibility is not very prominent for digital restoration, which may influence the judgment of scholars to some extent. In terms of economy, scholars had clear and contradictory views, that probably based on different cost estimation models. For the scope of application, the results are unexpected that most of the scholars chose "+". This is possibly that some scholars did not fully understand the application limitations of virtual imaging restoration.
Comparison with digital projection restoration. The results in Table 2 and Figure 9(c) show that, compared with digital projection restoration, the most prominent advantage of virtual imaging restoration is the visiting experience. This may be that virtual imaging restoration is more innovative, while digital projection restoration is to some extent just a kind of projection. For authenticity, the statistical results are again confusing, since three options are preferred equally by scholars. We believe that the two are equivalent in terms of authenticity because they both show the real state of artifacts and restore the appearance of artifacts through virtual methods. The statistical results are the same in terms of reversibility and discernibility. Half of the scholars chose "+" and the others chose "○". Since the light from the projector is in direct contact with the artifacts, which may be harmful to artifacts, especially for the artifacts with organic substances on’ surface [26]. Therefore, the reversibility of virtual imaging restoration should be better than digital projection restoration, and scholars who chose "○" may have overlooked the destructiveness of the light. Digital projection restoration can also be discernible by controlling the projection, which may be the reason why half of the scholars choose "○". The other half of scholars chose "+", perhaps because the current digital projection restoration has not fully highlighted the discernibility, although it has this potential. According to statistics, the advantages of virtual imaging restoration in terms of the economy and the scope of application are not outstanding. As virtual methods, both methods are relatively cost-effective. The scope of application of the two is limited. The former is suitable for artifacts with flat surfaces, and the latter is suitable for artifacts with damaged surfaces.
Comparison with patterns restoration. The results in Table 2 and Figure 9(d) show that virtual image restoration has huge advantages over patterns restoration in visiting experience, authenticity, reversibility, and discernibility. It is obvious that both methods are inexpensive and have a limited scope of application. Since pattern restoration is a bit crude, virtual imaging restoration may be used as a better alternative.
Comparison with AR restoration. One of the most impressive parts of AR technology is its rich visiting experience. In fact, AR restoration and virtual imaging restoration have different advantages in visiting experience. AR restoration is undoubtedly more advantageous in terms of interaction and immersion. With the development of AR technology, AR restoration will be more playable for artifacts. However, compared with AR restoration, virtual imaging restoration has the advantage of exhibiting artifacts more directly. Visitors of virtual imaging restoration observe the artifacts with naked eyes, while AR restoration requires AR equipment. Besides, AR restoration often requires certain training to experience, and virtual imaging restoration is more simple and direct. In an AR restoration case, visitors tried to touch the electronic screen to enlarge the artifacts and commented that the augmented object hides the real world [11]. It shows that visitors who view artifacts through AR subconsciously think that artifacts in front of them are electronic. In a sense, AR equipment cuts the direct connection between visitors and artifacts. The statistical results in Table 2 and Figure (e) do not highlight the advantages of AR restoration on visiting experience. It is possible that AR restoration is not a common technology at present, and most scholars have not experienced AR restoration in person, which affected the results to a certain extent. In the comparison of authenticity and reversibility, most scholars agreed that virtual imaging restoration and AR restoration are equivalent or incomparable in these two aspects, which is consistent with our expectations. Since the two methods are based on the combination of virtual and real, there is no much difference in maintaining authenticity and achieving reversibility. AR restoration is capable of being discernible, but half of the scholars in the statistical results believed that virtual imaging restoration is better on discernibility. This may be that in the virtual imaging restoration, discernibility is currently the only interactive function, which impressed the visitors. However, discernibility in AR restoration is only one of many interactive functions, which may dilute the attention of scholars to a certain extent. As for the economy, statistical results show that virtual imaging restoration performs better on the economy. Because AR restoration requires AR equipment and specific algorithms, which is usually expensive. As for the scope of application, most scholars agreed that virtual imaging restoration is more limited than AR restoration because AR restoration is not limited to be used on artifacts with flat surfaces.
In the free comments, the advantages of virtual imaging restoration proposed by the respondents include: “novel method and experience”, “inexpensive”, “good viewing”, “interactive and intuitive”, “convenient and easy to promote”, “you can see with naked eyes”, “the original information and the restoration information can be displayed well”, “good discernibility and reversibility”, “completely non-destructive restoration, providing a reference for the physical restoration of artifacts”. Correspondingly, some disadvantages are also highlighted in respondents' comments: “the viewing angle is limited”, “the types of artifacts are limited”, “the color difference is a little obvious”, “the virtual image has ghosting”, “it cannot be displayed in all directions”. There was also a comment from respondents that “it is a means of exhibition rather than restoration”. However, as already mentioned, the restoration is now not only a means to make artifacts more stable and complete but also to enhance people’s understanding of artifacts [13]. In fact, virtual imaging restoration as well as AR restoration and digital restoration all have the dual attributes of restoration and exhibition.
Suggestions for improvement from respondents include: “add images save function”, “improve the degree of integration of the device”, “add a dynamic demonstration of the restoration steps”, “optimize imaging clarity and color difference”, “add sound and other elements”, “realize the restoration of three-dimensional artifacts if possible”.
To sum up, the results of the questionnaire gave an overall positive evaluation. Virtual imaging restoration performed well in terms of visiting experience, authenticity, reversibility, discernibility, and economy, but it is only suitable for limited artifacts. Besides, virtual imaging restoration needs to address shortcomings such as ghosting, color differences, and limited viewing angles. There is room for improvement in the functions, interactivity, and integration of devices.
3.4 The future of virtual imaging restoration
As a low-tech and low-cost method, virtual imaging technology presented encouraging results in the restoration of artifacts. Although virtual imaging restoration still has some limitations, its advantages of cost-effective and easy to implement, making it an exciting and potential application in suitable artifacts, especially for small museums. The current shortcomings of virtual imaging restoration, such as the ghosting, the limited viewing angles, and color differences, are expected to be solved by replacing the thinner imaging plate and adjusting color parameters. Moreover, richer functions and interactive content can be added to further develop this method. In addition, due to it is flexible in showing the state before and after restoration, it is also very suitable for pretesting the restoration plans and providing references for actual restoration work, which could reduce the possible operating errors.