4.1 Refer to conservation theory
Compared with traditional restoration methods, the virtual imaging restoration is a new one that has not been discussed yet. Actually, not all innovative methods can be used in restoration. Any method intends to be used in restoration must be carefully considered with the following fundamental theories of conservation.
Authenticity is the core concept in the modern theories of cultural heritage. The methods, whether physical or virtual, in the restoration of incomplete cultural heritage, should be based on convincing evidence. All algorithms of digital restoration methods are based on the principle of authenticity, which calculate the appearance of missing parts and carry out a completely virtual cultural heritage. But is completely virtual cultural heritage what we want? Compared with the completely virtual cultural heritage, people would rather see the real incomplete one. Actually, the completely virtual restoration deviates from the authenticity that people want the cultural heritage to present. In contrast, the virtual imaging restoration gives people the real state of cultural heritage to a greater extent. What people see in front of their eyes is the real incomplete cultural heritage that joined with the virtual missing part.
Is that virtual restoration better than physical restoration? It is undeniable that physical restoration often reinforces cultural heritage, while virtual restoration only makes cultural heritage look complete. But is physical restoration still necessary when the existing incomplete cultural heritage is stable and not need to strengthen it? In fact, physical restoration is exposed to more complicated authenticity problems, that is because the authentic it theory requires not only the authenticity of the appearance but also the authenticity of materials and processes , which is very hard to achieve. For example, the authenticity of the restoration on ancient metals is difficult to achieve, for we can hardly completely reproduce their processes. That is a challenge brought by the multidimensional information of physical restoration. The physical materials carry various information such as morphology, material and process, which undoubtedly increases the difficulty of restoration. Obviously, the more complex the information they carried, the more content deviating from the authenticity may be introduced for restoration objects. On the contrary, refer to the virtual imaging restoration, we only need to take into account the authenticity of the morphology, which is easier to achieve under the technical conditions nowadays.
Therefore, the virtual imaging restoration can restore cultural heritage in a more cost-effective way with less authenticity offset, while physical restoration may introduce more inauthenticity. From this point of view, the virtual imaging restoration may be a better choice for incomplete cultural heritage.
4.1.2 Reversibility and minimum intervention
Reversibility is a widely used but relatively vague concept. In simple terms, reversibility refers to recover the restored object to the previous form for any future need . However, for physical restoration, absolute reversibility is almost impossible since the treatments implemented on the body of the objects will produce irreversible effects. As people have realized that true reversibility can never be achieved, some alternative concepts developed, such as repeatability  and removability . Both of the two concepts accept the reality that the additional materials have irreversible effects on the restoration objects. They only require that the treatments do not cause too much hindrance to restoration in the future. In a sense, repeatability and removability are both the products of the compromise between reality and the ideal that people wish to achieve complete reversibility.
Minimum intervention is also a universal concept. It recognizes that the physical restoration will bring about irrevocable adverse effects so that when people have to intervene in cultural heritage for certain restoration purposes, they should try to minimize the intervention to the restoration objects . In fact, the minimum intervention is the product of people's pursuit of authenticity. People hope that the restoration will not impair the authenticity of heritages and be non-intervention, which is obviously impossible, therefore born the minimum intervention.
Irreversible intervention to cultural heritage caused by physical restoration may result in future regret, but the virtual imaging restoration can satisfy the rigorous pursuit of people to perfectly restoration. The virtual missing part in virtual imaging restoration has no physical contact with the restoration object, strictly speaking, the lights that form the virtual image do not even reach the restoration object. Therefore, this kind of method is entirely reversible and non-intervention, which also makes the real reversibility and non-intervention have the value of re-discussing.
By following the Venice Charter in 1964, people have paid more and more attention to the concept of discernibility. They believe that “replacements of missing parts must integrate harmoniously with the whole, but at the same time must be distinguishable from the original so that restoration does not falsify the artistic or historic evidence ”. However, there are some discrepancies in people's understanding of the discernibility in real restorations. 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 . Others want to make it indistinguishable to the naked eye, but from special angles or by the special treatment, it is distinguishable. Such as dyeing the outer surface of ancient bronzes to make it indistinguishable but doesn’t dye the inner surface to distinguish the restored area , or adding the fluorescent agent to restoration material to make it distinguishable under the ultraviolet . However, in considering the purpose of the exhibition, the restoration traces should be recognizable, because the main group of visitors is the ordinary people, rather than the professionals who have access to view cultural heritage through ultraviolet lights or from the inside of cultural heritage. We cannot sacrifice ordinary peoples’ rights to knowing the original state of cultural heritage. In contrast, the virtual imaging restoration can present both the complete and incomplete state of the cultural heritage by controlling the appearance and disappearance of virtual images, allowing the restoration area to be easily discerned by viewers.
4.2 Scope of application
The virtual imaging restoration can be well applied to the restoring of some cultural heritage. For instance, it can replace the commonly used plane patterns restoration for incomplete cultural heritage, of which the restoration areas are easy to discern but a little abrupt (Fig. 6). By contrast, the virtual imaging restoration ensures that the restored patterns are realistic and easy to discern by controlling the projecting of virtual images. It is especially suitable for cultural heritage that is usually viewed from a fixed angle such as painting, fresco, textiles and paper (that is because the ideal viewing angle range of the virtual imaging restoration is limited, only from the front can get a better viewing effect, from other angles it will lead to obvious dislocation).
Since the virtual imaging restoration is entirely supported by the virtual image, there is no physical support provided. Therefore, it is only suitable for objects that are sufficient to maintain their stability.
In addition, this method is flexible in showing the state before and after restoration, so it is also very suitable for pretesting the restoration plans and providing a flexible reference for actual restoration work, which could reduce the possible operating errors.