10.6092/issn.1973-9494/5440
Due to natural disasters, economic development, tourism development and other factors, many precious heritage buildings have been in endangered situation. How to protect, research and develop these heritage resources effectively has become very urgent and important. Three-dimensional (3D) digital technology plays a more and more important role in protecting and using cultural heritage. The article will take the synthetic study on the mode of virtual construction, recovery, simulation and exhibition of Lingzhao Xuan (a heritage building which stopped construction for some reason in the Palace Museum) as an example to explore and summary an effective interdisciplinary cooperation mode. Besides, we broaden and deepen the concept of “virtual recovery”, and add the concept “virtual simulation” by means of virtual design and the new achievements which are created by such mode for the first time. This research is aimed to provide reference for the standard application of 3D digital technology and perfect the protection work of heritage buildings.
application/pdf
1973-9494
5440
2
en
Liyu Fang
Conservation Science in Cultural Heritage
3D real time rendering
The Interdisciplinary Research of Virtual Recovery and Simulation of Heritage Buildings. Take Lingzhao Xuan in the Palace Museum as an Example
Text.Serial.Journal
https://conservation-science.unibo.it/
14
Articles
2014-12-30
2015-09-24
2014-12-30
2016-06-14
189-205
CSCH
Liyu Fang
2014/12/30
10.6092/issn.1973-9494/5440
189
Department of IT Imaging&Digital Media The Palace Museum, Beijing
1973-9494
2
3D real time rendering
en
205
Wang Weihua: Digital and Physical Exhibition of Cultural Heritage in Museums, Southeast Culture, 2011, pp. 91-95.
The Interdisciplinary Research of Virtual Recovery and Simulation of Heritage Buildings. Take Lingzhao Xuan in the Palace Museum as an Example
https://conservation-science.unibo.it/article/download/5440/5153
14
T   HE INTERDISCIPLINARY RESEARCH OF VIRTUAL RECOV-
    ERY AND SIMULATION OF HERITAGE BUILDINGS. TAKE
    LINGZHAO XUAN IN THE PALACE MUSEUM AS AN EXAMPLE

    Fang Liyu
    Department of IT
    Imaging&Digital Media The Palace Museum, Beijing, China



   Keywords: cultural heritage, virtual reality, 3D real time rendering


   1. Introduction

    3D digital technology, an effective key technology in digital museum, has been ap-
plied to the actual work of the heritage protection, research and development. And
the VR technology based virtual restoration techniques has become popular in recent
years in protecting and researching heritage buildings. It can create the virtual scene
of present situation, recovery, or simulation of the heritage building by building the 3D
digital models of them. The advantages of using computer graphics to conserve and
research heritage building are minimal damage, reversible process, visible results and
authentic.
    There are more and more cases about virtual restored heritage building. Their processes
are also similar: first is to collect the heritage buildings shapes, textures and other informa-
tion, and then combine academic research to create digital 3D model, and finally displayed.




                                                                                                         CONSERVATION SCIENCE IN CULTURAL HERITAGE
    However, virtual recovery of heritage buildings is in a dilemma nowadays: the cultur-
al heritage digitization staff (hereinafter referred to as digital heritage protection group),
and actual cultural heritage protector and academic researchers (hereinafter referred
to as actual heritage protection group) as the two camps, they do not establish an ef-
fective coordination mechanism.
    Because of professional limitations, whether the actual or digital heritage protec-
tion group, it is difficult for them to obtain detailed needs from each other without
active communication.
    Professional physical evidence and historical research are the basic premises to
restore and recovery. Without these premises, digital heritage protection group will not
have enough time to devote to the most valuable digitized research points, and the
results are also greatly reduced.
    In a new era with rapid technological changes, the plight of this poor communication
for heritage buildings’ protection and research is a serious obstacle. To break this dead-
lock, the most direct and effective way is to create interdisciplinary cooperation mode.
    “Stone from other hills, can last jade.” Aiming at a virtual heritage recovery, simula-
tion and exhibition project, personnel in two different academic camps should coop-
erate and understand each other initiatively, and then conduct research and make
achievements. They need to master both concepts of 3D digitalizing and actual pro-
tecting methods of heritage buildings, set a common standard and create a win-win
situation together.

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                                                                                                          So, how to carry out interdisciplinary collaboration effectively? Through ten years’
                                                                                                       3D digitalize working, the Palace Museum have explored and summed up a more
                                                                                                       effective interdisciplinary cooperation mode. Besides, the “virtual recovery” concept
                                                                                                       of heritage buildings has been widened and deepened and the “virtual simulation”
                                                                                                       concept has been joined for the first time in project as a means of scientifically virtual
                                                                                                       design heritage building. This paper will combine the latest Lingzhao Xuan’s virtual re-
                                                                                                       covery and simulation experience, trying to summarize concepts and values of virtual
                                                                                                       recovery and virtual simulation of heritage building, as well as the effective interdisci-
                                                                                                       plinary collaboration mode of operation and the significance. It will provide a reference
                                                                                                       to regulate the application of 3D digital technology to benefit the protection work.


                                                                                                         2. Concepts and values of interdisciplinary virtual recovery and simulation of
                                                                                                       heritage building
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                           First, we must make clear some concepts of the specialized vocabulary.
                                                                                                           Recovery: means “restitution”, which refers to restore the existing building to its
                                                                                                       original appearance and condition by removing various attachments on heritage build-
                                                                                                       ing, replacing the missing parts, cleaning, painting and other repair methods.
                                                                                                           Simulation: suppose the original appearance; make the characteristics clear by imitating.
                                                                                                           Chinese cultural relics protection guidelines states clearly that all protective meas-
                                                                                                       ures must comply with the principles of not changing the original historic state. The
                                                                                                       principle includes two aspects: preservation status and restitution.
                                                                                                           In International Charter for the Conservati-on and Restoration of Monuments and
                                                                                                       Sites, conservation principles such as “minimum intervention”, “identify”, “reversibility”
                                                                                                       are also proposed, which should be followed carefully. Renovation of heritage build-
                                                                                                       ings includes repair and scientific recovery and so on. It is the most important heritage
                                                                                                       conservation means. The aims of restoring ancient buildings are not only to prevent
                                                                                                       damage and prolong its life scientifically and technologically, but must also maximize
                                                                                                       preservation of their historical, artistic and scientific values.
                                                                                                           Based on these principles, the entity of cultural heritages cannot be restored or
                                                                                                       even simulated (design) on a large scale in actual operation. But the historical, artistic,
                                                                                                       technological and other important values of cultural heritage cannot be fully demon-
                                                                                                       strated or intuitively reflected by its own entity.
                                                                                                           The emergence of digital archaeology museum makes the scope of conserva-
                                                                                                       tion and recovery’s concepts expand, and the computer display has been added
                                                                                                       into the purpose of repair and recovery work and an auxiliary means of physical
                                                                                                       display has also been added.
                                                                                                           In the 3D digital context, virtual recovery has the same meaning as the definition of
                                                                                                       recovery in real environment, but in order to facilitate the description, the concept of
                                                                                                       virtual simulation is broadened. For the heritage buildings whose original appearance
                                                                                                       cannot be verified, virtual simulation means virtualizing and assuming the original ap-
                                                                                                       pearance by mimicking the characteristics that have already been made clear. For un-
                                                                                                       finished heritage buildings, virtual simulation means virtualizing and assuming the fin-
                                                                                                       ished appearance by mimicking the characteristics that have already been made clear.
                                                                                                           There are important values in use and education in launching research into the
                                                                                                       computer-aid recovery and simulation technology.
                                                                                                           The value in use is shown in two aspects. The first one is to guide the work on the


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recovery of cultural relics and to promote the protection of the heritage building entity
through the process of computer-aid recovery of cultural relics. The second one is to
create the 3D digital model of recovery and simulation of cultural relics to be the origi-
nal materials in virtual exhibition.
    The value in education is the ultimate objective of protecting cultural relics. And re-
search is the lifeline of such protection. The value of every protection project of cultural
relics is weighed by the quality of protection and propagation. A great collection will
have real value only when it is added to people’s colorful life.
    What virtual recovery and simulation can achieve is to show the public large
amounts of research materials (in written language or 2D image) by simple means of
VR interactive works and 3D application (such as 3D digital scene). Thus the value in
the heritage building can be shown in a more vivid, straightforward and interesting way.
Besides, as a beneficial supplement, it will become one of the effective ways for people
to understand cultural heritage in depth.
    Interdisciplinary cooperation mode is the mode to combine professional strength of
two different subjects together to study on virtual recovery and simulation. Its value lies
in the innovation of achievement. Considering the difference of restored and simulated
ancient architecture, the majors involved is different too. The most important value of
it is to combine the heritage building’s digitalization and entity protection together to
increase work productivity and get more comprehensive and deeper achievements.


   3. Case analysis: the background and objectives of the Lingzhao Xuan project

    Next, the paper will take the virtual recovery and simulation of Lingzhao Xuan as an
example to explain the way to apply the virtual recovery and simulation to cultural relics
and the utilizations of these achievements.




                                                                                                     CONSERVATION SCIENCE IN CULTURAL HERITAGE
    The Palace Museum based on the Forbidden City, which is the priceless ancient
cultural heritage of Chinese nation and the whole human being. From 2011 to 2013,
the Institute for Digitization of the Palace Museum Heritage, which is the main strength
of the project, cooperates with multiple departments and social forces to apply the
virtual recovery, simulation and exhibition to the heritage building Lingzhao Xuan in the
Forbidden City (Figure 1.).
    Lingzhao Xuan, located in Yanxi gong of the Palace Museum, was first built in 1909
by the government of the Qing Dynasty. It is commonly known as “Crystal Palace”. This
crystal palace, using the steel-structure technology (which is very trendy at that time),
was built to avoid catching fire because the previous architectures here had frequently
caught fire. It was used to store water and raise fish, and it was the unique place to
enjoy the sight of fish. But soon after, the construction was stopped because of the
lack of money. This architecture has been undergoing the vicissitudes afterwards; it is
unfinished and so dilapidated that it is difficult for the audience to appreciate the char-
acteristics of the times and the architectural feature.
    There are important values of utilization and education in applying the virtual recov-
ery and simulation to Lingzhao Xuan. It can not only help us to explore more deeply
into the history of this unique architecture in the Forbidden City, but also explain the
characteristics of the times and the architectural feature of Lingzhao Xuan by virtual
exhibition which is easy to understand for the audience.
    The objectives of this project are to do research about the present situation’s virtual


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                                                                                                       construction (Figure 2.), virtual recovery and stimulation (Figure 3.) of Lingzhao Xuan
                                                                                                       by using 3D digital technology, mainly VR technology. And these will be exhibited in the
                                                                                                       virtual form of the VR work “Lingzhao Xuan” (Figure 4.) to make sure the achievements
                                                                                                       of this project to be timely spread and be effectively used.
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                                                Figure 1: Photo of Lingzhao Xuan in 2012




                                                                                                            Figure 2: VR rendering picture of Lingzhao Xuan’s present situation in virtual scene




                                                                                                                Figure 3: VR rendering picture of Lingzhao Xuan in virtual simulation scene



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                   Figure 4: Photo of “Lingzhao Xuan” VR work’ live show




  4. The interdisciplinary method of virtual recovery and simulation of heritage
building

    The guideline for the virtual recovery and stimulation of the heritage building Ling-
zhao Xuan is shown in the following table. The tasks are mainly divided into three
steps: to draw up the plan of recovery and simulation; to do 3D digitalization; to utilize
the achievements.
    In these three steps, aiming at specific heritage building, different majors need to
be participated. The members of actual heritage protection group are personnel major-
ing in the scientific technology about the protection of the cultural relics, architecture,
and the court history and so on. The members of the digital heritage protection group




                                                                                                     CONSERVATION SCIENCE IN CULTURAL HERITAGE
are personnel majoring in photography, CG modeling, VR programming and so on.
Personnel majoring in broadcasting, director and music are also related to the virtual
exhibition. The simple division of labor is in the following table. The most difficult thing
is, considering the difference of majors, the actual cooperation needs many extra ef-
forts to explain the objectives, the technical means and the division of tasks.




              Group        Major                      Task stage

                           Scientific technology
              Actual       about the protection of
                           the cultural relics        Draw up plan;
              heritage
              protection
                                                      3D digitalization
              group        Architecture

                           Court history




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                                                                                                                     Digital       Photography                3D digitalization
                                                                                                                     heritage
                                                                                                                                   CG modeling                Draw up plan;
                                                                                                                     protection
                                                                                                                     group         VR programming             3D digitalization;

                                                                                                                                   Director                   Use achievements
                                                                                                                     Virtual
                                                                                                                     exhibition    Broadcasting
                                                                                                                     group                                    Use achievements
                                                                                                                                   Music

                                                                                                                          Table1: The simple division in the Lingzhao Xuan project



                                                                                                          4.1 Draw up a plan
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                           To make sure the scientificalness of virtual recovery and simulation, the first thing
                                                                                                       need to be done is to collect the related data about Lingzhao Xuan from the profes-
                                                                                                       sional personnel and survey the Lingzhao Xuan scene. Experts on material science
                                                                                                       and ancient architecture in the Palace Museum are responsible to provide the related
                                                                                                       literatures, material characteristics and architecture drawings and other data and sur-
                                                                                                       vey the whole appearance and details. Besides, they need to analyze and refine the
                                                                                                       function of building unit, write detailed report on present situation and draw up the
                                                                                                       proposed plan of recovery and simulation. Experts on the court history need to aim at
                                                                                                       the function of Lingzhao Xuan and combine it with historical background at that time
                                                                                                       to provide important theme giving reference to the VR work. Experts on the digitalized
                                                                                                       protection is responsible to collect the present digital images about Lingzhao Xuan and
                                                                                                       decide original plan of virtual recovery, simulation and exhibition from three aspects:
                                                                                                       protection of entity, academic research and virtual exhibition.
                                                                                                           It can be seen from the survey of scene that Lingzhao Xuan has masonry and steel
                                                                                                       hybrid bearing structure and it is a western-type iron building with five top towers. The
                                                                                                       first floor of underground main building and the edge of bank can be the places to
                                                                                                       conserve water. As for the building on the first floor on the ground, the interior walls are
                                                                                                       covered with incomplete ceramic tiles and the outside white marble walls are engraved
                                                                                                       and decorated. There are pavilions at every corner of the second floor. There are doors
                                                                                                       and windows on every corner but only the iron window frame and door frame remain.
                                                                                                       The building units of Lingzhao Xuan such as iron and steel units are mostly rust and
                                                                                                       have serious defect. The stone walls are broken partly and weathered and brick walls
                                                                                                       are loose partly or falling off. The ceramic tiles have serious defect and the glass units
                                                                                                       remain little, which are taken apart to be stored elsewhere. There are three floors in
                                                                                                       this building but without stairs to connect these. Besides, there are a lot of information
                                                                                                       which can be used in recovery and simulation remaining on the spot, for example,
                                                                                                       there are some regular holes remaining on the window frame and some plinth remains
                                                                                                       on the ground and so on.
                                                                                                           The materials used in recovery and simulation mainly include architectural draw-
                                                                                                       ings, survey report about the heritage building scene, digital images, historical docu-
                                                                                                       ments and academic research data.
                                                                                                           Because Lingzhao xuan’s construction hasn’t been finished yet, all the present 14
                                                                                                       architectural drawings that can be collected are drawn by ancient building design de-

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partment of the Palace Museum in September, 2009, which contain rather completely
units and size of every side of Liangzhao Xuan (Figure 5.). It is hard to do the recovery
because the design drawings have been missing and other related literature and re-
search data are very few.




                       Figure 5: A drawing of Lingzhao Xuan( 2009)

   Because preliminary data are few, more supporting data are needed to do this re-
covery and simulation. The project group studied out a simple preliminary plan in the
end of the first step: firstly to make 3D virtual scene of Lingzhao Xuan’s present situa-
tion and make sure it can be the high precision version to record present situation; at
the same time to analyze the details of present situation as many as possible and col-
lect related research data more broadly. The restored and simulated 3D virtual scene
will not be made until all these data have been prepared fully.




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE
  4.2 3D digitalize

  4.2.1 Supplementary materials

   The supplementary materials are divided into two parts.
   The first part includes historical documents, present academic research achieve-
ments and contemporaneous data about Chinese and Western architecture, which are
collected by the actual heritage protection group.
   The second part includes comprehensive 2D digital images of Lingzhao Xuan, parts
of 3D digital images and manual measurement about building’s size data and so on,
which are done by the digital heritage protection group. These works need the coor-
dination and cooperation between photographers and CG modelers. Photographers
collect 2D images, CG modelers collect important data about building’s size and 3D
scanning personnel collect 3D images.
   Because the 3D virtual scene of Lingzhao Xuan needs to reach the storage level of
high-precision record of the Imperial Palace’s present situation, there are two require-
ments for photography. The first one is that the lens for the shoot should be perpen-
dicular to the building’s plane. The second one is that every plane’s photograph should
be taken according to the required precision. The larger plane should be divided into


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                                                                                                       several pars and shoot successively. Generally speaking, these 2D digital images
                                                                                                       should be collected in the same precision. But considering their achievements will be
                                                                                                       the source materials of VR works on virtual exhibition, thus the architectural details
                                                                                                       which need to be given close-up in VR works should be pointed out to photographs
                                                                                                       by CG modelers on the spot that these details should be collected in higher precision
                                                                                                       such as the holes and bulges in window frames and the holes on the ground.
                                                                                                           Such requirements for photography in shooting images of heritage building make it
                                                                                                       rather difficult in real shooting because it is limited by heritage building’s exhibition and
                                                                                                       the characteristics of the building itself.
                                                                                                           Lingzhao Xuan is a three-layer western building, whose height makes it hard to set
                                                                                                       the camera. Besides, there are no units such as stairs to get to the second floor of the
                                                                                                       building because of the unfinished construction, which makes the vertical shooting of
                                                                                                       some units’ images become impossible, only the digital images with geometric defor-
                                                                                                       mation and color that is not exactly correct are available.
                                                                                                           It is one of the major difficulties that being unable to shooting the images of all ar-
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                       chitectural details in strict accordance with the photography requirements and to get
                                                                                                       the best 2D image materials.
                                                                                                           There are great achievements in putting 3D digital image into practice. But 3D digi-
                                                                                                       tal image technology will not be used in project on a large scale for the time being
                                                                                                       because of its immaturity. However, out of scientific research, a set of relief units’ 3D
                                                                                                       digital images are collected to get the effective mode to use 3D digital image data on
                                                                                                       units’ 3D virtual scene. The digital heritage protection group uses 3D laser scanner
                                                                                                       to collect the 3D digital image of a white marble high relief in the north-west side of
                                                                                                       Lingzhao Xuan (Figure 6.).




                                                                                                                         Figure 6: 3D digital image of high relief of Lingzhao Xuan




                                                                                                          4.2.2 Construct 3D virtual scene

                                                                                                          The constructions of 3D virtual scene are divided into three steps. The first thing is
                                                                                                       to construct the 3D virtual scene of present situation. The second thing is to do the vir-
                                                                                                       tual recovery, which is based on the 3D virtual scene of present situation. And the third
                                                                                                       step is to do the virtual simulation, which is based on the restored 3D virtual scene.
                                                                                                          In the whole process, the team will explore and integrate the research achieve-
                                                                                                       ments related to Lingzhao Xuan continually and keep discussing and correcting until a
                                                                                                       scientific and reasonable plan is made for the recovery and simulation finally.


                                                                                                 196
  A Construct present situation’s virtual scene

    Firstly the standard for construction aimed at 3D models of this project should be
discussed and set by CG modelers and VR programmers. Before the 3D digital models
building begins, VR director need to draw up a VR script so that it will easier for CG
modelers divide 3D models’ precisions according to the script and the project devel-
opment period will be shorten. In order to guarantee VR work’s quality and operation
speed, VR programmers will provide the standard for construction related to VR effect,
such as model segmentation, making bump map and light map.
    Because the 3D virtual scene of Lingzhao Xuan needs to reach the storage level of
high-precision record of the Imperial Palace’s present situation, the main building and
surrounding circumstances are 3D digitalized by manual modeling. CG modelers can
construct 3D virtual scene of present situation based on sufficient image materials.
    There are four major difficulties of constructing 3D virtual scene.
    The first one is the problem of building’s size. Architectural drawings are idealized
drawings and the image materials collected have the problem of geometric deforma-
tion, thus they cannot be the very correct standard of size.
    The best solution is get from experiment: getting the building’s outline from the ar-
chitectural drawings to be the basic standard for model construction. As for some small
units such as relief, they can take the size measured data on the spot as the standard.
These can make sure the constructed 3D models to be the same as far as possible.
But still, the error can only be controlled on the centimeter level.
    The second one is about precision. There are many building units of Linagzhao
Xuan, thus how to keep the unity of precision becomes a problem.
    The best solution for the time being is as follows: firstly to set the corresponding
standards between building size and texture resolution. The highest precision is A
level, which requires the actual size range of 1 square meters should correspond at




                                                                                                  CONSERVATION SCIENCE IN CULTURAL HERITAGE
least 2048 x2048 resolution; the lower level is B level, which requires the actual size
range of 1 square meters should correspond at least 1024 x1024 resolution, and so
forth. Next, the unit models should be cut reasonably to meet the requirements of
standard precision. In the making process, A-level precision version will be made first
and then VR version will be made according to VR work’s requirements for precision
and making.
    The third problem is about the geometric deformation and color correction. Since
some image materials have some problems such as geometric deformation and color
errors, how to correct these becomes a problem.
    The present solution: use graphics processing software (Photoshop) to apply ge-
ometric deformation and color correction to images. But inevitably there are some
limitations in such software correction which lead to inaccurate geometry and color.
Therefore better solution is still needed to be found out.
    The fourth one is about the effective application of 3D image data. On the premise
of reserving details of every pattern, the collected 3D image data of high relief reaches
up to 1,000,000 polygons and the error range is on millimeter level. It can be regarded
as the most precise method now to record and exhibit heritage building. But because
of the limitations of computer performance, such high data cannot be integrate into the
restored 3D virtual scene of Lingzhao Xuan and it needs a heavy workload of reducing
patches and making maps based on the data.
    The effective solution now: use these data as blueprint and construct models again


                                                                                            197
                                                                                                       in a reasonable way, which can keep error level, keep almost the same and finish the
                                                                                                       making of models and maps effectively.
                                                                                                          Theoretically, 3D image data can solve the above 3 problems. But considering pre-
                                                                                                       sent technology level, if personnel construct the whole 3D scene by means of combi-
                                                                                                       nation of “3D scan and manual model construction”, the workload will be heavy. If the
                                                                                                       data can be used more efficiently, this method will become the best to collect and use
                                                                                                       data of heritage building. Thus using data efficiently is an important research area that
                                                                                                       worth exploring.
                                                                                                          3D virtual scene of present situation with high precision is not only the 3D modeling
                                                                                                       data base of recovery and simulation, but also the important remaining materials of
                                                                                                       digital heritage. The final VR rendering picture of Lingzhao Xuan in virtual construction
                                                                                                       scene is shown in figure 2.


                                                                                                          B Construct recovery and simulation’s virtual scene
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                          The virtual recovery and simulation can be done on the basis of 3D virtual scene of
                                                                                                       Lingzhao Xuan‘s present situation. This part of work is a little more difficult because of
                                                                                                       few bases for recovery. It is done by actual heritage protectors, CG modelers and VR
                                                                                                       programmers. Actual heritage protectors are responsible for extracting effective basis
                                                                                                       for recovery and simulation from the collected data, deciding the final recovery plan
                                                                                                       and guiding digital heritage protectors to make and correct models. CG models are
                                                                                                       responsible for making 3D models of recovery and simulation. VR programmers are
                                                                                                       responsible for making VR effect to simulate the designed scene effect in the recovery
                                                                                                       plan.
                                                                                                          Firstly carry on virtual simulation in strict accordance with recovery plan and on the
                                                                                                       basis of 3D virtual scene. It should be done according to the principles of cultural rel-
                                                                                                       ics’ restoration and the original appearances and patterns of cultural relics should not
                                                                                                       be changed. That is to clean up some surface attachments on Lingzhao Xuan such
                                                                                                       as blots, weeds and so on; to repair the obvious missing or broken parts; to add some
                                                                                                       units such as glass. Finally the restored virtual scene is constructed.
                                                                                                          The simulation work next is very challenging.
                                                                                                          The first challenge: there are some obvious unfinished parts in the remaining units.
                                                                                                       Then how to carry on simulation?
                                                                                                          Solution: finish the unfinished parts virtually according to the pattern of remaining units.
                                                                                                          To take the high relief in the north-west side of the first floor (Figure 7.) as an exam-
                                                                                                       ple, it can be seen on the spot that the pine and cypress pattern is engraved, thus the
                                                                                                       unfinished flower patterns can be engraved according to the finished patterns. It can
                                                                                                       be concluded that such simulation plan is much more precise according to the 3D vir-
                                                                                                       tual scene of present situation and VR rendering picture of 3D virtual simulated scene
                                                                                                       (Figure 7.) and wireframe (Figure 8.).
                                                                                                          The second challenge: there are some units with similar structure in present situa-
                                                                                                       tion. Then how to simulate these units?




                                                                                                 198
          Figure7: VR rendering picture of 3D virtual scene of present situation (left)
                           and 3D virtual simulation scene (right)




               Figure8: Wireframe of 3D virtual scene of present situation (left)
                            and 3D virtual simulation scene (right)

    Solution: analyze all the units with similar structure and extract elements from these
units and combine them together to make a new 3D model containing all these impor-
tant elements. And then correct all 3D models with such structure to 3D models with
new structures.
    For example, the iron pavilions are constructed according to such idea for simula-
tion. The structure of central iron pavilion is preserved more completely than others




                                                                                                   CONSERVATION SCIENCE IN CULTURAL HERITAGE
and it is probably the finished pattern. But the remaining structures of iron pavilions on
four sides are different from each other. Therefore the structure of central iron pavilion
is taken as a reference and the characteristics of another four iron pavilions’ units are
also combined together to infer scientifically the probable structure and then models
can be made to carry on simulation.
    The third challenge: According to the completeness of architectures and the re-
cord in literature, it is obvious that there should have been some units such as doors,
windows and stairs of Lingzhao Xuan, but now they are missing. How to carry on
simulation when there are no concrete descriptions about these units whether on the
spot or from data?
    Solution: The obvious missing units must be simulated completely to guarantee the
completeness of architectures. The miss parts should be completed according to the
principles and characteristics of Chinese and Western building and simulation of the
architectural styles of contemporaneous similar architectures. And then models should
be constructed to carry on simulation.
    For example, the simulation of doors and windows can refer to the drawings of
contemporaneous Haiyan tang which has the most similar drawing as Lingzhao Xuan
(Figure 9.). According to the 3D virtual scene of present situation and the VR render-
ing picture of simulated 3D virtual scene of the windows in Lingzhao Xuan’s first floor
(Figure 10.), the simulated structure of windows is very close to the whole architectural
style. Although the original structure is not available, the 3D models for simulation


                                                                                             199
                                                                                                       can to some extent provide the breakthrough points for experts and a more complete
                                                                                                       architecture history for tourists.
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                                            Figure 9: Drawing of contemporaneous Haiyan tang




                                                                                                                   Figure10: VR rendering picture of present situation’ virtual scene (left)
                                                                                                                               and virtual simulation scene’ window (right)

                                                                                                           The fourth challenge: how to simulate the water in Lingzhao Xuan’s pool? This
                                                                                                       problem can be divided into two parts. The first part is: how to make VR water effect
                                                                                                       in the pool? The second one is: how can water play role in “Crystal Palace”? Water
                                                                                                       highlights the important architectural function of Lingzhao Xuan, but the depth and
                                                                                                       source of water and others are unknown. There is some evidence of quite large
                                                                                                       outlet of pipe on the wall on underground first floor and there are some parts of
                                                                                                       pipe lying on the first floor on the ground. Apart from the pool on the first floor un-
                                                                                                       derground, was there spring on the ground?
                                                                                                           The first problem will be explained at length in the following parts, but there is no
                                                                                                       conclusive evidence on the spot or the literature.
                                                                                                           To guarantee the scientificalness of simulation, the simulation were only done on
                                                                                                       the water in the pool on the underground first floor. After simulating water in VR en-
                                                                                                       vironment, architecture experts analyzed visually on the basis of water’s 3D virtual
                                                                                                       scene and inferred the depth of water scientifically. These experts also inferred that
                                                                                                       there should be sideboards on the bank (Figure 3.) according to the usual practice of


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Chinese architecture. Other units related to water and their functions can only wait to
be simulated until further research.
    In virtual recovery and simulation, actual heritage protectors can have a more visual
understanding of details of architecture and infer some new ideas that don’t think of be-
fore through construction of virtual recovery and simulation. Thanks to the cooperation
of the whole team, Lingzhao Xuan’s construction is “finished” in the virtual world. The
ultimate virtual scene is as figure 3. The complete Lingzhao Xuan looks like a white
pearl embellished in The Forbidden City.


   4.3 The use of achievements based on the virtual recovery and simulation

  Many scientific research achievements have been gained after the virtual recovery
and simulation of Lingzhao Xuan.

   4.3.1 Digital heritage

   First achievement of digital heritage is precious digital materials. In the project, nearly
8500 digital images (with a total size of 34.7GB) of Lingzhao Xuan’s present situation in
2011 comprehensively have been gained. There is 1.58GB in total size when construc-
tion documents (obj format) of 3D images and 2D images data. These are precious data
recording and researching the architecture and history of Lingzhao Xuan.
   Cultural heritage have been changing all the time. The state on a certain age is ir-
reversible no matter it is under slight natural aging or manual restoration. Therefore
these image materials and analytic results are very precious, which are the important
basis of comprehensive research of cultural heritage in the future.
   Second achievement of digital heritage is 3D virtual scene of heritage building. In




                                                                                                       CONSERVATION SCIENCE IN CULTURAL HERITAGE
the Lingzhao Xuan project, the virtual scene of Lingzhao Xuan’s present situation with
high precision provides the basis for further virtual research and use.
   There are multiple values in using virtual scene of present situation with high preci-
sion as the reflection of real environment. One is value in preservation; it can be pre-
served as the digital data with the basic materials. Another one is value in use; it can
be used in virtual exhibition or carrying on simulation and experiment on the behavior
and event that cannot be accomplished in real world.
   Virtual recovery and simulation have gradually become the important ways to sup-
port the protection and repair of cultural heritage. The key points of it are interdisci-
plinary research and scientific inferences. The characteristics and history of cultural
heritage can be expressed visually by simulating the functions of cultural heritage and
historical scene in a most possible and reasonable way.

   4.3.2 Virtual exhibition—VR digital work

   In the Lingzhao Xuan project, virtual exhibition with the theme of Lingzhao Xuan is
developed based on the 3D virtual scene of present situation, recovery and simulation
and so on. It is done by VR programmers, CG modelers, VR directors, music profes-
sional personnel and broadcast professional personnel together.
   This work mainly describes the architectural style and characteristic of Lingzhao
Xuan itself and shows comprehensively Lingzhao Xuan’s value as the cultural herit-


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                                                                                                       age. Before the work is released, it is checked and reviewed by experts and scholars of
                                                                                                       cultural heritage’s protection and architecture in The Palace Museum and universities
                                                                                                       and then it is under repeated modification and improvement according to these ad-
                                                                                                       vices. The finished VR work Lingzhao Xuan is released to the public (Figure 4.), which
                                                                                                       is a shortcut for people to understand Lingzhao Xuan’s history and architectural style,
                                                                                                       not only meeting the aesthetic needs of normal tourists closely and comprehensively,
                                                                                                       but also meeting the research requirements of experts and lovers of cultural heritage.
                                                                                                       At the same time, it can go out of The Palace Museum to combine with the actual exhi-
                                                                                                       bition, which increases heritage building’s expressive force and artistic appeal.

                                                                                                         4.3.3 Academic research achievements

                                                                                                          Academic research achievements include two aspects. First of all, Actual heritage
                                                                                                       protectors had deeper understandings of details of every unit in Lingzhao Xuan and
                                                                                                       studied out the plan about Lingzhao Xuan’s virtual recovery and simulation through the
Fang Liyu - The interdisciplinary research of virtual recovery and simulation of heritage buildings




                                                                                                       cooperation of professional personnel in multiple disciplines, which provided effective
                                                                                                       reference for the protection of actual heritage building and new breakthrough point for
                                                                                                       further research.
                                                                                                          Second, this project put the application of graphics in the field of digital museum to
                                                                                                       a new level, which is a very effective experiment. In order to exhibit more vividly the
                                                                                                       visual effects of virtual recovery and simulation and highlight the importance of virtual
                                                                                                       recovery and simulation, 3D real time rendering technology is necessary.
                                                                                                           The application and research of 3D real-time rendering technology is done by VR
                                                                                                       programmers and CG modelers. The effect need to be discussed and finished together
                                                                                                       by VR directors, radio announcers, VR programmers and CG modelers. VR direc-
                                                                                                       tors decide the required VR effect and radio announcers decide the way of exhibition
                                                                                                       (which will affect the choosing of 3D real-time rendering technology).
                                                                                                          In developing VR work Lingzhao Xuan, in order to be closer to the descriptions in
                                                                                                       the historical documents and highlight the function of the architecture more authenti-
                                                                                                       cally, 3D real-time rendering effect which bases on OpenGL’s Shader language was
                                                                                                       adopted, including HDR (High Dynamic Range) effect(Figure 11.), water real-time ef-
                                                                                                       fect (Figure 12.) and glass real-time effect and so on. These can not only improve the
                                                                                                       rendering quality, but also make sure that the rendering speed will not too low. And
                                                                                                       then the rendering speed was increased by adopting technologies such as culling and
                                                                                                       MipMapping and so on.




                                                                                                                    Figure 11: Rendering picture of Lingzhao Xuan present situati-on’s
                                                                                                                           virtual scene without HDR (left) and with HDR (right)

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                Figure12: Lingzhao Xuan present situation’s virtual scene (left)
                      and virtual simulation scene (right, water surface)

   5 Conclusion

    It can be inferred from Lingzhao Xuan project that it is of great importance in ap-
plication of heritage building’s interdisciplinary virtual restoration and simulation. It can
create abundant digital heritage achievements with historical values, provide valuable
reference to actual cultural heritage’s protection and provide abundant digital works to
aid in the entity exhibition. Furthermore, the digital heritage achievements also can be
important source for the database of 3D digital models in the future. Virtual exhibition
can exhibit the achievements of virtual restoration, construction and design, which is
one of the effective ways to aid in the entity exhibition and is beneficial to the spread
of it. Such interdisciplinary pattern can be extended into other cultural heritage types.
    However, according to the process and effect of present restoration and simulation,
although the ultimate achievements are abundant, there are some concrete problems




                                                                                                      CONSERVATION SCIENCE IN CULTURAL HERITAGE
without being solved properly, such as: how to obtain the best 2D images? How to col-
lect 3D digital images on a large scale and use them efficiently? How to obtain better
plan to carry on restoration?
    Developments in the future lie in two aspects. One is the setting of interdisciplinary
restoration and simulation mode standard. Two camps should establish the concepts
of heritage building’s virtual restoration and simulation and set adaptable standards.
Boarder and more initiative participation are needed to make sure that the most pre-
cise and valuable restoration plan can be obtained from every project, such as the par-
ticipation of more academic researchers and public. Another one is to keep up closely
with the scientific developments and keep trying new technology means to apply more
multiple-field knowledge to digital museum’s work such as graphics and 3D scan tech-
nology and so on. With rapid development of technology, it can be foreseen that the
ongoing improvements of digital technology such as 3D digital images and VR in many
aspects such as simplicity, authenticity and interaction will bring simpler and more
precise means of virtual restoration, construction, simulation and exhibition, which will
bring more abundant achievements in heritage building’s protection and utilization.




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                                                                                                             Summary

                                                                                                          Due to natural disasters, economic development, tourism development and other
                                                                                                       factors, many precious heritage buildings have been in endangered situation. How to
                                                                                                       protect, research and develop these heritage resources effectively has become very
                                                                                                       urgent and important. Three-dimensional (3D) digital technology plays a more and
                                                                                                       more important role in protecting and using cultural heritage. The article will take the
                                                                                                       synthetic study on the mode of virtual construction, recovery, simulation and exhibition
                                                                                                       of Lingzhao Xuan (a heritage building which stopped construction for some reason in
                                                                                                       the Palace Museum) as an example to explore and summary an effective interdisci-

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plinary cooperation mode. Besides, we broaden and deepen the concept of “virtual
recovery”, and add the concept “virtual simulation” by means of virtual design and the
new achievements which are created by such mode for the first time. This research is
aimed to provide reference for the standard application of 3D digital technology and
perfect the protection work of heritage buildings.




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