"Copy" Yungang Grottoes "Permanent" Buddha Statue
The cave is weathered and peeled off every year; experts use digital technology to collect data, and then present the cave through 3D printing
The Yungang Grottoes, located in Datong, Shanxi, have stood tall for 1,500 years.
But through careful measurement, those who protect it know that every year in its natural state, more than 10 cubic centimeters of sandstone per square meter is peeled off.
The seemingly insignificant deformations are very considerable when placed on the scale of ten years, a hundred years, or a thousand years.
Although the means of cultural relic protection are becoming more and more abundant, the degradation of grottoes follows the laws of nature and cannot be completely contained.
The protector is using other methods to retain these exquisite and vicissitudes of Buddha statues.
Yungang Grottoes Research Institute is using digital technology to record and reproduce all the information of the caves in the digital space, and then use 3D printing to reproduce these caves and other large and realistically.
There are 45 large caves and more than 200 small caves in Yungang at the southern foot of Wuzhou Mountain, stretching for one kilometer from east to west. Among them, there are about 59,000 Buddha statues in various forms.
These magnificent grottoes blending Chinese, Indian, Greek, Roman and other Chinese and Western styles have been integrated with the mountains for 1,500 years, overlooking Datong.
Today, the Yungang Giant Buddhas that have been "copied" have stepped out of the mountains and entered Qingdao, Beijing, Hangzhou and other places, and plan to travel around the world in the future.
Night Shot of Cave Eleven
At 7 pm on October 20, Pan Peng flashed a flashlight with his colleagues and walked into the eaves of Yungang Cave Eleven.
The cave eaves is a three-story antique wooden building, built to protect the grotto, it is attached to the outside of the grotto, equivalent to a shell.
Equipment can also be installed in the "shell" to adjust the dry humidity for the grotto.
The temperature in Datong dropped sharply that day, and it was already below 10°C at night.
They were wearing fleece cardigans and jackets. The air-conditioning still invaded their bodies, and a warm water bottle was essential.
A shadowless lamp and shed flashing light were cast into the work area to illuminate the dark grotto.
Li Zehua and Wang Chao took SLR cameras to shoot the last remaining part of the east wall, casting huge shadows on the inner wall.
In order to fill in the light evenly, the camera is equipped with a special ring LED fill light.
In the work preparation area in the eaves of the cave, there are more than a dozen shed flashing lights, shadowless lights, and black chargers for various equipment.
The place where they stood was a scaffold more than ten meters above the ground. The scaffold was covered with wooden boards that would shake when walking.
The steel pipes of the scaffolding rise from the ground and run through the grottoes, dividing it into one layer every 1.5 meters.
The digital collection of Cave Eleven was started after the National Day holiday this year. The work on the first floor of the cave roof was just completed a few days ago, and the first floor planks were removed immediately.
It is now the second floor from the top, and the top of the cave is when you look up.
Wang Chao walked carefully across the board, not far from his feet, half of the Buddha's head was already exposed on the edge of the board.
During the day of the day, two other collections were carried out in the cave: handheld 3D scanning and standing 3D scanning.
Pangbo holds a three-dimensional laser scanner, the size of a barcode scanner at a supermarket cashier, and moves it in front of the wall little by little, and the scanned picture appears on the computer connected to it in real time.
A black hole was formed where he didn't receive it. He scanned the computer while he was checking, to make up for the missing.
The station laser scanner is activated when the entire floor is basically collected, positioned at one point, and automatically scans the surrounding space in 360 degrees.
Stationary laser scanners have extremely high requirements for stability, and there should be no shaking around during scanning. Except for Wang Jiaxin, who is responsible for the operation, all the others exit the cave.
Close-range photogrammetry, handheld 3D laser scanning and station-type 3D laser scanning, the three collection methods have their own advantages.
"The advantage of photography is that the pixels are relatively high and the texture is very clear, but because it is two-dimensional to three-dimensional, there is an error in the spatial structure. The laser scanning space standard can calibrate the photogrammetry, making the three-dimensional structure more accurate." Yungang Grottoes Pan Peng, the person in charge of the digitization project of Cave 11 of the Institute's Digitization Room, explained.
Stationary 3D scanning can record the spatial scale of the entire cave, just like customizing a standard size frame and loading the previously collected 3D images into it.
The final output is an accurate three-dimensional color model.
The technical solution combining these three methods was formed by the Yungang Grottoes digital team after more than 10 years of exploration.
There are about 59,000 Buddha statues in Yungang, the highest is 17.3 meters, the smallest is only 2 cm high, and the size of a thumb.
Collecting pictures and data inch by inch, they are making these Buddha statues "rebirth" in the digital space, always maintaining their appearance and color today.
The birth of the 3D Buddha
After an overnight rush, the data collection on the second floor of Cave Eleven ended.
The two SLR cameras in the close-up photography group each had to press about 10,000 shutters per day, and collected nearly 20,000 photos in total.
After a project is completed, more than 500,000 photos will be collected, and the shutter is basically invalidated.
The most difficult part is the blind spot in the Buddhist niche. They have to switch to a card camera, "concave" various postures, stretch out behind the ears of the Buddha statue, and take pictures from the side. Keep your feet safe and keep your hands away from cultural relics.
On the morning of October 21, the worker master entered the site and removed the wooden boards on the second floor.
After dismantling, the planks will be moved to the third layer for laying.
The whole process lasts one to two days, and the data collection of the entire layer takes one and a half to two days. In addition, the work of each layer takes 4 days to complete.
"A total of nine floors were built, and there are seven floors. That is about 28 days." Pan Peng gave it a second thought, "We must hurry up on the construction period, otherwise it will be very cold."
He predicted that by the end of November, the collection in the cave would stop.
Lessons from the past are right here. In the last few years at the end of the year, they did the data collection of Cave Twelve. At that time, the temperature was so low that the camera automatically turned off, and the equipment went wrong one after another.
The data processor Zhao Xiaodan's working environment is slightly better, and he doesn't need to be frozen in the cave.
In the office, she is responsible for importing the photos taken from the front to the computer, quickly browsing and checking, removing unqualified photos, and then importing them into image processing software.
The software will automatically extract the feature points of each photo, extract the color information and geometric features, and turn them into point sources separated by 0.02 mm.
The dots are connected into a line, and the three lines form a surface. The surface and the surface form a space to complete the preliminary processing from two-dimensional photos to three-dimensional models.
In the office of the digitization room of the Yungang Grottoes Research Institute, five computers are responsible for this work, and each computer’s display is connected to up to five hosts to meet the computing power.
This "stacked Arhat" configuration can process 10,000 to 20,000 photos taken that day in one night, and continue to process the data collected overtime the night before during the day.
Whenever a new acquisition project is launched, these computers buzz around the clock.
What worries Zhao Xiaodan the most is that sometimes after counting the night, the calculated model went wrong. They called it "running away".
Sometimes, the huge amount of calculations will cause the computer to crash, and you can only start from scratch.
However, the Yungang Grottoes Research Institute is building the first advanced computing center in the cultural relics system. They will "replace shotguns for guns" and have larger and faster computing power.
At this time, in a cooperative factory in Shenzhen, dozens of 3D printers printed the data of the three-dimensional model into hundreds of three-dimensional modules.
After that, the artist and the worker master worked together to color a 3D model that is more than ten meters tall and the size of the cave.
This process will last 8 months.
Last summer, Zhao Xiaodan, who is a graphic designer, spent two months in Shenzhen, coloring the 12th cave model that had previously collected data. “The picture is very spectacular.”
Warning from Notre Dame Cathedral
In June of this year, the Museum of Art and Archaeology of Zhejiang University ushered in a complete Yungang cave.
Through 3D printing and assembly, the twelfth cave was "copy+pasted" to Hangzhou.
This is the world's first detachable 3D printed digital grotto jointly completed by the Cultural Heritage Research Institute of Zhejiang University and the Yungang Grottoes Research Institute. Both parties hope that this grotto will be exhibited globally in the future.
The first isometric 3D printed grotto was the third grotto that was permanently landed in Qingdao City Media Group Plaza in 2017.
The third cave is the largest grotto in Yungang. The entire project has undergone data collection, data processing, 3D printing, structural design and construction, print assembly, light source design and installation, sandblasting and coloring, and other processes, which lasted 2 years.
"If you want to see Yungang Grottoes, you must come to the site, so some people don't know what Yungang Grottoes look like for a lifetime." said Wang Jiaxin, a technician in the digital room of Yungang Grottoes Research Institute, "The best way is to let the cultural relics come to everyone. Around."
However, at the beginning, the purpose of digitizing the grottoes was not for display, but for the urgent need for cultural relic protection.
Intuitively, Yungang Grottoes may have visible changes every ten years.
Modern technology confirms this feeling. Since 2012, in conjunction with the Yungang Wuhua Cave Eave Protection Project, the Yungang Grottoes Research Institute has continuously monitored the amount of sand falling on the columns of the 9th and 10th caves.
The monitoring results show that before the eaves of the cave, more than 10 cubic centimeters of sandstone will be peeled off on an average of 1 square meter per year.
On the innermost north wall of some grottoes, the wall peeling caused by the seepage of the mountain is particularly violent. The statues have been blurred, and even degenerated into almost a flat surface. It is impossible to know what it used to be.
Keeping the current digital data will make up for this regret in the future.
In April last year, Notre Dame de Paris suffered a fire.
Fortunately, the laser scanning digital acquisition completed before has used more than 1 billion data points to record the entire picture of Notre Dame, which can provide a basis for reconstruction.
This incident reminds cultural heritage institutions around the world that in order to cope with unexpected events, they must establish digital archives as soon as possible.
Li Zhirong, deputy dean of the Cultural Heritage Research Institute of Zhejiang University, believes that the core purpose of the current digitization of cave temples is to understand the basic status of the cultural relics of the cave temples in my country, and use digital methods to integrate all the information on the grottoes on the land of China in the 21st century. write it down.
In terms of short-term goals, she hopes that in 5 years, China will be able to obtain a large database containing detailed and basic information of cave temples, and each cave temple will be able to harvest solid digital files, so that other affairs can be upgraded.
Virtual repair and digital return
According to Ningbo, director of the Digitalization Office of Yungang Grottoes Research Institute, Yungang Grottoes is the most difficult type of grotto to carry out digitalization.
Yungang Grottoes, as a masterpiece of high-relief grottoes, are huge in volume, "a mountain is a cave, and a cave is a mountain".
Moreover, the spatial structure is complex, with both the Indian dome roof and the Chinese palace style of the Northern Wei Dynasty. There are exquisite carvings of Greek, Roman, Indian, and Chinese architecture, characters, animals, flowers and plants.
"The huge volume, the complicated space, and the exquisite carving make it difficult to collect, store, and apply the entire 3D digital work," Ningbo said.
At the earliest, Yungang Grottoes wanted to borrow the digital collection method of Dunhuang to make it, and took a detour, because the main objects of Dunhuang digitalization were murals, and the main body of Yungang Grottoes was high-relief sculptures.
"The big cave statue has encountered many practical problems from technology to engineering. After more than 10 years of exploration, we have basically solved the problem of 3D digital collection at Yungang Grottoes."
In this process, the Yungang Grottoes made full use of external forces.
For example, Beijing Jianzhu University was the first to solve the problem of digitizing the facade of Yungang Grottoes. Zhejiang University, Wuhan University and Yungang Research Institute cooperated to provide sufficient technical support.
As Yungang Grottoes are typical among the grotto temples in the country, “when they came to Yungang, they felt that the digital collection problem in Yungang was solved, and most of the problems in the country’s grottoes would also be solved.” Ningbo said. The "Digital Yungang Joint Laboratory" was established to solve the problems of grotto collection, preservation and application.
When a database is established in the grottoes, more historical shortcomings will be made up.
A typical case is the Longmen Grottoes in Luoyang, Henan Province, which used digital technology to "restore" incomplete cultural relics.
The statue of Avalokitesvara on the south wall of the front room of the Ten Thousand Buddhas Cave in the Longmen Grottoes is graceful and elegant, with delicate and smooth carvings. It is known as the "most beautiful voice of the world" in Longmen.
Last year, the Longmen Grottoes Research Institute performed a virtual restoration of it, which not only reproduced the Bodhisattva's face, but also restored the color.
The basis of virtual restoration mainly comes from four aspects: old photos, three-dimensional data measurement, color detection and analysis report, and reference for images of the same period and the same type.
Researchers use 3D scanning surveying and mapping data to first infer the volume, length, width, and arc of the missing part, and then combine the old photo data and the laws of the same period and the same type to reconstruct the model.
Since there is no first-hand information, the restoration of painted paintings is also quite difficult.
The naked eye can only see the remaining red and black, but the color detection and analysis detected a small amount of gold, which means there was gold.
According to the color detection of other caves and the comparison of unearthed cultural relics, and considering the fading effect, the colored painting of the statue was restored.
Nowadays, the Longmen Grottoes through APP AR scans, documentaries, 3D printing and other forms, allowing viewers to see the full face of the "most beautiful world music" on site and online, and understand the story behind the virtual restoration.
Another possible application is to "digitally return" the cultural relics lost overseas.
For example, the heads of some Buddha statues were stolen in history and changed hands and became the collections of foreign museums.
The recovery of stolen cultural relics remains a difficult problem.
However, before being returned, if the digital information of the stolen cultural relics can be collected, the Buddha can be restored through digital means.
Constantly "start from scratch"
According to statistics, there are more than 300 cave temples and stone carvings among the key cultural relics protection units nationwide.
The cave temple has a long history, lasting for nearly two thousand years. Even though it has the advantage of avoiding the destruction of thunder and fire, it still suffers from natural diseases such as weathering and water seepage, as well as historical man-made damage, theft and loss.
Not long ago, Dong Guangqiang, director of the Digital Center of the Maijishan Grottoes Art Institute, stated at the Yungang Grottoes' Digital Protection Forum for the Grottoes. Currently, the Maijishan Grottoes has a digital center, but there is no professional team, and there is no training and equipment increase. Keep up with sync.
The number of tourists to the Maijishan Grottoes has increased year by year, reaching 800,000. However, since most of them are small caves, most of the internal space is only 2 to 3 cubic meters, and they are usually closed.
Dong Guangqiang said that he hopes to complete three-dimensional scanning of some key caves based on factors such as the exquisiteness of the statues in the caves, and establish a digital display center for Maijishan Grottoes cultural relics in appropriate locations to provide more tourists with a digital visit experience.
With more small and scattered cave temples across the country, it is even more difficult to have professional digital technology and teams.
Ningbo believes that the number of grottoes in the country is huge, and in the short term, there will be problems of heavy digital work and tight time. At the same time, digital collection and storage standards are not perfect, and future data ownership will also face some problems to be solved.
The national cave temples are uniting through some platforms.
For example, in September this year, the China Cultural Relics Conservation Foundation launched the "Digital Rebirth-Digital Protection Project of Cave Temples along the Silk Road". The three major caves of Yungang, Longmen, and Maijishan participated in the first batch to explore more digital protection and utilization methods. .
Its long-term goal is to promote the overall digitalization of the caves along the Silk Road on the basis of the current digital work of cave temples.
The project period is expected to be 6 years. The first three years will focus more on digital record storage, virtual restoration, digital return, and the establishment of digital standards. The next three years will focus more on exhibitions, public education, public welfare activities, and cultural value interpretation and dissemination. , Fundraising and dissemination run through the whole process.
As a cave temple with an early digitalization and a more mature technology, Yungang Grottoes is going out to help other cultural relics to establish digital archives.
In recent years, the team has taken a full set of equipment to enter the Changling Yi'en Temple in Beijing's Ming Tombs, Guangren Wang Temple in Yuncheng, Shanxi, Wutaishan Nanchan Temple, Yongle Palace and Huayan Temple.
Changling Yien Temple is the most difficult project. They stayed for two months and spent more time than collecting a cave in Yungang Grottoes.
Pan Peng recalled that the difficulty of Changling Yi'en Temple was mainly due to its large volume and complex structure. The exterior used drone photogrammetry and the interior used a three-dimensional scanner.
The floor of the main hall was paved with ancient bricks, and steel columns could not be directly erected. They designed and customized an air column with an air cushion at the bottom. The steel column can be raised to a height of more than ten meters, and the scanner is placed on it to scan the architectural details at high places.
Since the beginning of the digital collection of the facade of the Yungang Grottoes in 2005, about one-third of the data collection of the entire cave has been completed.
However, with the iteration of technology, this work often needs to "start from scratch."
More digital technologies such as cloud computing, big data, virtual reality, knowledge graphs, artificial intelligence, etc. are being put in front of cultural protection workers.
Li Zhirong believes that in order to be open to technology, the intervention of new technologies will expand the means of protection of the cave temple, upgrade the protection process of the cave temple, and will also raise the digital standard.
Pan Peng said that the digitization of Cave 5 was carried out four or five years ago. From now on, the technology at that time has fallen behind and the data accuracy is not as good as it is now.
He predicts that the data collected by the current equipment can be maintained at an advanced level in five to ten years.
But after that, a new round of collection may begin again.
Beijing News reporter Ni Wei
A10-A11 version photography / Beijing News reporter Pu Feng