Cultural relics are the country's valuable historical wealth, historical research, scientific guidance and other aspects of great significance. However, with the passage of time, damage to cultural relics is often unavoidable, which includes both the natural weathering of the relics themselves and accidental destruction.
In April 2019, France's Notre Dame de Paris suffered its worst fire ever, with the wooden structure at the top of the church completely destroyed, leaving only stone remnants behind, and a huge amount of damage. Fortunately, the digital 3D point cloud model of Notre Dame was electronically archived prior to its destruction, providing sufficient and accurate data to support the reconstruction of the damaged parts of Notre Dame, which can be restored one by one based on the existing data.
Laser scanning and digitized 3D archive model can help heritage conservation and restoration, and ensure the authenticity and accuracy of restoration. Relevant researchers summarized the application of 3D laser scanner in the field of cultural relics protection in China from the aspects of technology development, working principle and achievements. Zhang Xiaoqing et al. studied the application of 3D printing in the reconstruction of cultural relics from the aspects of 3D laser scanning and modeling related technology. This paper describes the basic process of digitalization of cultural relics protection from the point of view of the combination of three-dimensional laser scanning and 3D printing two technologies, in order to solve the traditional cultural relics digitization information retention and accurate restoration of the problem.
First, the principle of work
Laser scanning using non-contact scanning method, can quickly obtain the target surface of the massive data, in the protection of cultural relics has outstanding advantages. Three-dimensional laser scanning is mainly through the high-speed laser scanning measurement method, in the form of a point cloud to obtain the array of geometric image data on the surface of the object. The data obtained by traditional measurement is ultimately two-dimensional form, through the drawings presented. Unlike traditional measurements, 3D laser scanning acquires data in a three-dimensional form, which not only contains information on planar position and elevation, but also RGB color and reflectance information of the object under test. Therefore, the information obtained by 3D laser scanning is very comprehensive.
The 3D laser scanner utilizes a laser as the emitting light source to measure the object in accordance with a certain horizontal and vertical scanning resolution, and adopts a non-contact method to obtain the surface data of the measured object. The principle of spatial point coordinates calculation is shown in the figure below. The laser is used to obtain the distance S from the object to be measured to the scanning center, and then the precision clock-controlled encoder built inside the device is used to synchronously measure the horizontal scanning angle observation value α and longitudinal scanning angle observation value β of each laser pulse, and the spatial three-dimensional coordinates of the measured point can be calculated through the spatial three-dimensional geometric relationship using a line element and two angular elements. X, Y, Z coordinates of the spatial point. The data acquired by 3D laser scanning is usually in a separate coordinate system within the equipment, with the X-axis in the transverse scanning plane, the YY-axis in the transverse scanning plane perpendicular to the X-axis, and the Z-axis perpendicular to the transverse scanning plane.
Above Principle diagram of spatial point coordinates calculation
3D printing technology is to print three-dimensional objects, the use of slicing and then rebuild the restoration of trajectory, high temperature melting raw materials to follow the principle of trajectory stacking to restore the digital model.3D printing technology appeared in the mid-1990s, in fact, is the use of light curing and paper lamination and other technologies such as the latest additive manufacturing device. It is basically the same working principle with ordinary printing, the printer using metal, ceramics, plastics, sand and other different materials such as raw materials, connected to the computer, through the computer-controlled slicing trajectory, the raw materials layer by layer according to the trajectory of the bottom-up stack, and ultimately the computer on the scanning of digital three-dimensional electronic model into a physical object.
Second, the analysis of technical advantages
Three-dimensional laser scanning has a fast speed (nearly 1 million points per second), do not need to contact the target (non-contact measurement), large amount of data, high accuracy and other characteristics. Among them, non-contact measurement can obtain massive data on the surface of the measured object, effectively avoiding contact damage to cultural relics in the process of cultural relics digitization. At the same time, no matter how complex the structure of the measured target is, it can be acquired in accordance with the original appearance. The measurement accuracy of 3D laser scanning can reach millimeter level or even sub-millimeter level, which can accurately record the original appearance of cultural relics and obtain high-precision 3D digitized models for archiving. The use of 3D printing technology, can be scanned after the archived high-precision digitized cultural relics for appropriate adjustments and modifications, the use of 3D printers to print the model, and then polishing, coloring and other post-processing of the model, to obtain a copy of the cultural relics. 3D printing technology is the advantage of the model into a fast speed, the model of the high precision of the restoration can be seamlessly connected to the laser scanning. Therefore, the combination of three-dimensional laser scanning and 3D printing technology used in the digitalization of cultural relics and restoration has a unique advantage.
Third, the basic process
The use of three-dimensional laser scanning and 3D printing combined to help cultural relics digitization and restoration of the field is the current mainstream development trend. The measured target has a short distance, high accuracy requirements, applied to the protection of cultural relics of three-dimensional laser scanning is usually selected handheld or station-carrying and the principle of phase difference to obtain the target of the instrumentation. The basic process mainly includes target deployment, data acquisition, model splicing, model processing, model slicing, 3D printing and other aspects, as shown in the figure below. Take Zhaozhou Bridge as an example to illustrate the process flow at different stages.
3.1 Target Laying
Uniformly lay spherical targets around the bridge, with no less than 3 pairs of targets between two neighboring stations. 3 pairs of spherical targets are located at different heights, staggered to avoid placing them on the same straight line, so as to avoid not being able to find the basis for alignment of the neighboring data with the same name at a later stage.
3.2 Data Acquisition
Select the appropriate angle, set up the station-carrying three-dimensional laser scanner, set up the scanning resolution on the device and the way the camera shoots, control the scanning time, and carry out 360 ° non-directional scanning of the target. Target scanning is completed, check the accuracy of the target ball scanning, confirm whether it meets the conditions of using the surface point cloud to fit the spherical coordinates, if the accuracy is not enough, you need to carry out high-resolution directional scanning of the spherical target. Repeatedly check until the conditions for fitting the center of the sphere are met.
3.3 Model Splicing
According to the working principle of laser scanning, the data acquired by neighboring stations are in different independent coordinate systems. It is necessary to use the principle of ICP and other related algorithms to unify the coordinate system of multi-station data, that is, by calculating the rotation matrix between the cloud data of neighboring stations to complete the model alignment, and get the complete point cloud model of the bridge body.
3.4 Model Processing
After de-noising, streamlining, segmentation, and feature extraction of the point cloud model of the bridge body obtained after alignment, the surface data are modeled and reconstructed, and the model reconstruction usually adopts the way of using the point cloud to construct the grid for curved cultural relics. This method has high accuracy, good surface fit and high automation. The reconstructed bridge grid model is processed by skinning, hole filling, grid repair, smoothing, sharpening, texture mapping, etc., to obtain the digital three-dimensional electronic model of the bridge and archive it, and save it to the database.
3.5 Model slicing and 3D printing
Using 3D printing technology, the archived high-precision digitized electronic cultural relics model is appropriately adjusted and modified, and the computer is used for model slicing, calculating the print restoration trajectory, and generating G code. According to the material type of cultural relics to select the appropriate 3D printer and raw materials, G code input to the 3D printer, the printer uses the nozzle high temperature melting raw materials to follow the trajectory of the principle of layer-by-layer stacking of the digital model is restored to the real thing. The physical object can be used for real cultural relics damaged part of the restoration, can also be used to visit the exhibition instead of real cultural relics.
Zhaozhou Bridge phased process results are shown below.
3.6 Data Supplementary Restoration
When scanning with the station-carrying 3D laser scanner, it is inevitable that data loopholes will arise, and the lack of data will bring difficulties to the establishment of digital models and 3D printing. Such problems can be solved by two means. One is in addition to the single use of station-carrying scanning, with the handheld 3D laser scanning synchronization, scanning angle problems caused by data gaps can be supplemented by the handheld scanner, flexible and convenient. The other is to supplement and repair the missing data during data processing, including mesh optimization, hole filling, smoothing and sharpening and other means. Among them, hole filling is the most critical, including curvature, tangent, plane, internal holes, boundary holes, bridging and many other methods. Compared with the traditional restoration methods, through the three-dimensional laser scanning to obtain high-precision data, you can establish a more realistic digital three-dimensional model, the use of software for virtual restoration, to provide a variety of restoration effects than the selection, as a basis for improving the real restoration program. Virtual restoration mainly contains cultural relics material body gap filling, painting repair, texture restoration and so on. After repair, the model becomes smooth and smooth and clear contours.
Conclusion and Prospect
4.1 Conclusion
Through the combination of three-dimensional laser scanning and 3D printing technology, the current situation of cultural relics protection and restoration, the working principle, the basic process and its advantages are described. Three-dimensional laser scanning repair of cultural relics is not only restored with high precision, but also can effectively avoid secondary damage to cultural relics in the process of cultural relics digitization. The process has unique advantages in the field of cultural relics protection and restoration, and can solve the traditional cultural relics digital information retention and accurate restoration of the problem. At present, Dunhuang Mogao Grottoes sculpture successive use of laser scanning and 3D printing technology to achieve the digital archiving of cultural relics and cultural relics reproduction, the technology is gradually maturing, the diversity of the process is being gradually improved.
4.2 Outlook
At present, the problems that need to be further solved are the diversification of 3D printing raw materials, the number of 3D printer nozzles and the volume of 3D printing to further improve the efficiency and practicality of 3D printing. At the same time, the laser scanning combined with 3D printing technology can be integrated into hyperspectral technology on the basis of laser scanning, the laser scanning of high-precision models and high-precision texture obtained from close-up photogrammetry integration, you can realize the model of cultural relics and the color of the high-precision double restoration and repair, which is of great practical significance for cultural relics protection and restoration. After scanning the digital archive of the electronic point cloud cultural relics model can be stored in the database, through the Internet and cloud storage technology, combined with virtual reality VR technology, to build an online digital museum, so that tourists can be realized without leaving home to visit the online browsing. 3D printing of cultural relics can also be used for the actual exhibition of cultural relics and the related industries of the theme of education. The popularization and application of this technology is of great significance.
