In our paper we will present results of an on-going project connected with monitoring the condition of 12 fragments of the Museum of King Jan III’s Palace at Wilanów (Warsaw, Poland) building façade by 3D structured-light scanning method. During 30 months, 7 series of three-dimensional measurements have been planned. In each of the twelve elevation fragments, chosen by the conservation and architecture departments as particularly interesting, an area of 120 mm x 120 mm was scanned with 2500 points/mm2 resolution. This article describes the methodology of the measurement process, the hardware setup developed especially for this purpose, as well as the data processing path and analysis algorithms. In addition to having such accurate measurement data, we must still be able to match the measurements carried out in the same place at intervals of several months. For this purpose the areas of interest were marked with special aluminum targets, embedded with three intersecting planes. The algorithm of their detection, analysis and use for aligning data from subsequent measurement series is discussed. A portable SLS 3D-measurement head with two cameras, integrated with linear drive has been developed for scanning purposes and adopted to use in outdoor condition. The 3D scanner has a measurement volume limited to 45 mm x 50 mm x 10 mm for a single scan, due to high-resolution requirements. In less than 25 minutes, 40 measurements are acquired at various positions, covering the entire area, with the support of a controlled linear stage stand. Individual scans are pre-aligned with limited accuracy and then fitted using the Iterative Closest Point algorithm. The final representation of each fragment is a cloud of points with color containing more than 200 million 3D measurement points. We present the results of 3D measurements and a proposition of a monitoring procedure for assessing the change in 3D surfaces over time.
The pace of development of information systems nowadays demonstrates the magnitude of the demand for digitization of all aspects of our lives, such as medicine, industry, and documentation of cultural heritage. Digitization is the process of converting objects from the real world into their digital representations. In order to acquire complete and detailed information about the whole surface of an object, several 3D scans have to be taken from different perspectives. The resulting 3D object can be acquired in a form of a numerous amount of 3D point clouds overlaying each other. Sometimes, depending on a quality of a 3D scanner and surface properties, the point clouds can represent a noisy geometrical surface and an incorrect colour. Moreover, the directional point clouds are not perfectly aligned and a registration between them must be applied. The registration of the point clouds is a complex task which is not always possible to automate. Usually, the entire process of registration has to be supervised by a skilled operator. The registration is usually divided into two parts: initial and final matching. Initial matching is a more complex one and in this scenario, it is supported by the known system calibration, which includes, e.g., robotic arm, head of the scanner, sources of lights. Using ICP based algorithms afterward is usually enough to get appropriate final matching. The difficulty of point cloud registration increase accordingly to the number of directional clouds of points to integrate. The aim of this paper is to propose a methodology to decrease or even fully eliminate some of the presented registration issues encountered during the reconstruction of Museum of King Jan III’s Palace at Wilanów.
Graphic prints and manuscripts constitute main part of the cultural heritage objects created by the most of the known
civilizations. Their presentation was always a problem due to their high sensitivity to light and changes of external
conditions (temperature, humidity). Today it is possible to use an advanced digitalization techniques for documentation
and visualization of mentioned objects. In the situation when presentation of the original heritage object is impossible,
there is a need to develop a method allowing documentation and then presentation to the audience of all the aesthetical
features of the object. During the course of the project scans of several pages of one of the most valuable books in
collection of Museum of Warsaw Archdiocese were performed. The book known as "Great Dürer Trilogy" consists of
three series of woodcuts by the Albrecht Dürer.
The measurement system used consists of a custom designed, structured light-based, high-resolution measurement head
with automated digitization system mounted on the industrial robot. This device was custom built to meet conservators'
requirements, especially the lack of ultraviolet or infrared radiation emission in the direction of measured object.
Documentation of one page from the book requires about 380 directional measurements which constitute about 3 billion
sample points. The distance between the points in the cloud is 20 μm. Provided that the measurement with MSD
(measurement sampling density) of 2500 points makes it possible to show to the publicity the spatial structure of this
graphics print.
An important aspect is the complexity of the software environment created for data processing, in which massive data
sets can be automatically processed and visualized. Very important advantage of the software which is using directly
clouds of points is the possibility to manipulate freely virtual light source.
In this paper, a fully automated 3D digitization system for documentation of paintings is presented. It consists of a specially designed frame system for secure fixing of painting, a custom designed, structured light-based, high-resolution measurement head with no IR and UV emission. This device is automatically positioned in two axes (parallel to the surface of digitized painting) with additional manual positioning in third, perpendicular axis. Manual change of observation angle is also possible around two axes to re-measure even partially shadowed areas. The whole system is built in a way which provides full protection of digitized object (moving elements cannot reach its vicinity) and is driven by computer-controlled, highly precise servomechanisms. It can be used for automatic (without any user attention) and fast measurement of the paintings with some limitation to their properties: maximum size of the picture is 2000mm x 2000mm (with deviation of flatness smaller than 20mm) Measurement head is automatically calibrated by the system and its possible working volume starts from 50mm x 50mm x 20mm (10000 points per square mm) and ends at 120mm x 80mm x 60mm (2500 points per square mm). The directional measurements obtained with this system are automatically initially aligned due to the measurement head’s position coordinates known from servomechanisms. After the whole painting is digitized, the measurements are fine-aligned with color-based ICP algorithm to remove any influence of possible inaccuracy of positioning devices.
We present exemplary digitization results along with the discussion about the opportunities of analysis which appear for such high-resolution, 3D computer models of paintings.
Currently, a lot of different 3D scanning devices are used for 3D acquisition of art artifact surface shape and color.
Each of them has different technical parameters starting from measurement principle (structured light, laser
triangulation, interferometry, holography) and ending on parameters like measurement volume size, spatial resolution
and precision of output data and color information. Some of the 3D scanners can grab additional information like
surface normal vectors, BRDF distribution, multispectral color. In this paper, we plan to present results of the
measurements with selected sampling densities together with discussion of the problem of recognition and assessment
of the aging process. We focus our interest on features that are important for the art conservators to define state of
preservation of the object as well as to assess changes on the surface from last and previous measurement. Also
different materials and finishing techniques requires different algorithms for detection and localization of aging
changes. In this paper we consider exemplary stone samples to visualize what object features can be detected and
tracked during aging process. The changes in sandstone surface shape, affected by salt weathering, will be presented as
well as possibilities of identification of surface degradation on real object (garden relief made in sandstone).
Currently, a lot of different 3D scanning devices are used for 3D acquisition of art artifact surface shape and color. Each
of them has different technical parameters starting from measurement principle (structured light, laser triangulation,
interferometry, holography) and ending on parameters like measurement volume size, spatial resolution and precision of
output data and color information. Some of the 3D scanners can grab additional information like surface normal vectors,
BRDF distribution, multispectral color. In this paper, the problem of establishing of threshold for technical parameters of
3D scanning process as a function of required information about the object is discussed. Only two main technical
parameters are under consideration, in order to cover as many different 3D scanning devices as possible - measurement
sampling density (MSD - represented by number of points per square millimeter) and measurement uncertainty (MU - directly influencing final data accuracy). Also different materials and finishing techniques require different thresholds of MSD and MU parameters to collect similar documentation (for example documentation of object state for art conservation department) of different objects. In this paper we consider exemplary painting on canvas, wallpainting, graphics prints and stone samples to visualize what object features can be observed within different values of MSD and MU parameters.
Recent advancements in 3D scanning technology open a window of opportunity in works of art documentation's
possibilities. Contrary to classic techniques of visual documentation (a drawing or photograph), 3D scanning may
become the first technique offering objective and dispassionate recording of reality because the subjective stage of
analysis takes place only during final data processing by end users such as art conservators, historians, archeologists and
epigraphers. The general assumption is made that the best representation of digitized work of art is rough measurement
data (in many modern cases it is a cloud of points - a set of geometric (x, y, z) data along with additional parameters like
color values, surface reflectance etc.). The concept of 3D scanning and data processing has to be designed by an
interdisciplinary team, combining technical competency with knowledge of end users' requirements and demands. The
basic points of this elaboration are: what additional measurement parameters, beside shape, are needed for full
digitization of an object, as well as what accuracy of geometry measurement is high enough for registration of objects
made from different materials. This last question is to be answered within a recently started three-year research program,
whose methodological assumptions are stated in the presented paper. Some preliminary results are also shown together
with discussion of achieved sampling density and accuracy.
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