A multidisciplinary approach to 3D survey and

reconstruction of historical buildings

Laura Micoli, Gabriele Guidi, Davide Angheleddu

Dept. of Mechanical Engineering Politecnico di Milano

Milan, Italy

laura.micoli, gabriele.guidi, davide.angheleddu @polimi.it

Michele Russo

Dept. Design

Politecnico di Milano

Milan, Italy

michele.russo@polimi.it

Abstract —The aim of this research is to suggest a

methodology based on 3D survey and reconstructive modeling,

suitable to increase the actual knowledge of an historical building

and supporting its historical interpretation. The case study used

for testing the proposed methodology is the huge Chartreuse of

Pavia, with a special focus on a relatively unexplored portion of

the monument. The survey, based on 3D laser scanning and

orthoimages, integrated by historical studies and other

complementary information (thermoluminescence dating, IR

imaging, hystorical analysis), allowed to read all the architectural

aspects hidden in this highly architecturally stratified monument,

improving in this way the comprehension of the building’s

transformations in time. A 3D reconstruction approach was then

suggested, merging several information of different nature, from

the actual geometry of the building to the interpretation of

historical documents, suggesting a sequence of diachronic models

as virtual narration of the historical evolution. On other hand the

3D models were used to obtain a cross-validation of the historical

evolution hypotheses developed by experts in the various

disciplines involved in the project. The data collected were

exploited through a web portal in order to enhance the

readability of tangible and intangible heritage associated to that

Chartreuse portion, nowadays not accessible to common public.

Keywords — Multi-scale approach, 3D survey, point cloud

analysis, virtual reconstruction, diachronic reconstruction,

promotion of Cutural Heritage.

I. INTRODUCTION

The work presented in this paper represents only a part of a larger project called “Integrated technologies and cultural heritage of Lombardy” (TIVAL)1, carried out by University of Bicocca, University degli Studi di Milano, University of Pavia and Politecnico di Milano [1]. The project was focused on a complex case study: the Chartreuse of Pavia, candidate as UNESCO World Heritage. The monastic complex, built from the end of the 14th century up to the mid-16th century, shows the artistic evolution in Lombardy from Gothic to Renaissance.

The structure of the Chartreuse of Pavia presents high levels of complexity for size and articulation of the architectural spaces (Fig. 1). It is composed by many buildings edified near the church and organized around cloisters of different size and structure in relation with the activities

1 Project granted by Regione Lombardia, Italy.

dictated by the Carthusian rule, devoted to total isolation and contemplation. For this reason the chartreuses were built in isolated territory, that represent the “desertum”, and structured by a network of increasing isolated buildings: the external ones present normal doors as filters between the monastic life and the outside community. Here only the lay brothers have access, (i.e. those monks devoted to practical management activity such as cooking meals, doing laundry, undertaking physical repairs, etc.). A second level is represented by the Church, the Sacristy, the Capitular Room, the Refectory and the lay brothers Dormitory. All of them were meeting places for the monk community. At the top of this hierarchy there were the monk cells, distributed around the large cloister, in which the choir monks (i.e. those monks committed to seclusion from society, dedicated to pray and meditation) carried out their contemplative life.

Fig. 1. Aerial view of the complex of the Chartreuse of Pavia, showing the

two main cloisters of the structure.

The project is focused on buildings around the small cloister (Figs. 1 and 2), representing the center of the monastic life. The morphology and the structural scheme of that area has changed many times during the past centuries based on its destination. Despite the relevance of this area, only few historical documents are available, and this is why many historical questions are still open. For this reason this area was identified as an interesting test-field for experimenting an unprecedented integration of 3D technologies (laser scanning and photogrammetry) with complementary technologies such

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as thermoluminescence dating and IR imaging, a stratigraphy based in the architectural traces “readable” on the facades, and a deep historical analysis. The part of the TIVAL project described here, refers to the 3D component of the whole study, which has been strongly influenced by the various contributions coming from the other disciplines, but that influenced also the solution of some historical issues, fueling a positive interdisciplinary feedback.

A complete multi-scale survey was needed in order to suggest hypothesis and sometimes answers. The creation of virtual models from these data has been useful to enhance a part of the Chartreuse that contains hidden material and immaterial masterpieces from the middle of XIV to the end of XIX centuries.

Fig. 2. Areas of interest: the focus of the project was on the Library and the

Capitolum (red), furtherly extended to the annexed portion (yellow).

The virtual representation of the buildings was carried out according with two complementary modeling approaches: “reality-based” and “reconstructive” [2].

The reality-based modeling was mainly implemented with color laser-scanning of the area of interest. The equipment used (FARO Focus3D), thanks to a coaxial alignment of laser and image sensors, implements the collection of RGB values with no alignment distortions between geometry and texture. After an ICP based alignment of raw scans covering the whole area shown in fig. 2, a colored cloud of 3D points was therefore generated. Such result, even without any meshing, became an effective instrument for exploring and measuring the building volumes using a simple PC, allowing non-technical people involved in the project to prove their hypotheses just by examining such dense cloud representation of the areas of interest.

In addition, reconstructive models were generated integrating several information of different nature, going from the true geometry of the building, attainable by the abovementioned survey, the interpretation of historical documents, and the “reading” of architectural signs on the building structure. The different information that could be extracted from the architectural portion (material data, stratigraphic survey etc.) has been conveyed on the reality

based 3D models obtained through colored cloud of 3D points, and through virtual reconstructions, exemplifying the chronological sequence of the building evolution. The individuation and comprehension of the building constructive phases made also possible to understand which were the different uses of each space for a complementary conservation project.

Each piece of information collected for walls, rooms, and other architectural structures in the studied area, independently of his nature (geometrical, historical, physical, chemical, etc.) has been stored into a database capable to accommodate the different kind of data, associating them to the various locations of the building. Such integrated information system represents now a useful instrument to facilitate the comprehension of this monument and its transformations during the last six centuries, providing an important tool for improving knowledge, analysis and communication of the cultural heritage content hidden inside the Chartreuse.

II. HISTORICAL BACKGROUND

An accurate bibliographical and iconographical analysis of the huge amount of historical documentation related to the whole Chartreuse of Pavia represented an essential step for understanding the historical transformation on the analyzed portion of the monument [3, 4, 5, 6, 7, 8].

In the 1393 Gian Galeazzo Visconti, the first Duke of Milan, decided to build a new Carthusian monastery. Different architects and engineers participated at this huge work, like Bernardo da Venezia, Giacomo da Campione e Cristoforo di Beltramo, cooperating with the Carthusian Priors to create an architecture suitable with the rules of the life order and a symbol of Visconti’s family prestige.

In the early years of 1400 the big cloister and the monk cells were completed, while the other buildings (Sacristy, Capitular Room, Library, Barbary, Hospital and Refectory) and the small cloister, were finished before the 1430. In that period the Refectory was used as church while the Hospital and the Barbary were temporary used as Refectory. Only after the church consecration in 1497 all those buildings resumed the role for which they were initially designed. The colonnade of the small cloister was concluded at the end of the XV century, when the wooden pillars were substituted with stone columns.

In the period between 1490 and 1500 most of the works on the church facade were completed (Fig. 3), the interior decorations were finished and the Church was finally consecrated. But due to the start of a series of wars in northern Italy, for the next three decades the whole complex became a settlement for troops.

Around the half of 1600 the final portions of the Chartreuse were finished, with the introduction of the huge iron railings to close the lateral chapels and the transept. In the same period the Chartreuse of Pavia became an important destination for both religious and tourists.

The guardianship of the whole complex changed a few times, from the original Carthusian monks to Cistercians and finally to the Carmelites, until the orders suppression in 1810 due to a Napoleon decision. During these changes many artistic

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artifacts were stolen or moved to other locations. In 1843 an ordinance by the Emperor Ferdinand I allowed the return of the Carthusian monks, while in the 1866 the new suppression of religious orders forced the local government to take in charge the Chartreuse of Pavia. At the end of 1881 the Carthusian monks abandoned definitely the Monastery.

Fig. 3. Construction of the Chartreuse façade (from a table of Amborogio

Fossano, Musei Civici di Pavia)

In the late XIX century very important maintenance works were carried out and the Museum of the Chartreuse was inaugurated inside the ducal palace beside the church. Nowadays the complex is under the responsibility of the local superintendence and a small community of Cistercian monks takes care of the building and welcomes tourists.

III. SURVEY

The survey of the architectonical structure was organized in two different steps: an initial historical and morphological analysis at architectonic scale; the architectonic survey and the creation of a reality-based digital model according to well the known 3D processing pipeline [9]. This was used as reference to create a set of diachronic 3D reconstructive models with different reconstructive hypothesis [10].

The first geometric survey of the architectonic buildings, carried out by one of the TIVAL partners with a strong expertise in stratigraphy, was based on a topographic approach integrated with detailed direct surveys [11]. During the data acquisition a photographic campaign was carried out to collect visual information needed for the stratigraphic processing. The final result was obtained through the interpretation of orthoimages, mapping on them the various architectural additions and modifications [12]. This methodology allowed us to gain the first key information regarding the evolution in time of the architectural structure, at least as regards the external areas, rich of easily readable signs. Furthermore, as a byproduct of the stratigraphic interpretation process, the orthoimages were used as textures for the subsequent 3D processing.

Due to its large size and the complexity of the whole area, the 3D acquisition campaign required the definition of a detailed survey project, in order to choose the best solutions in terms of survey techniques and data acquisition steps (Fig. 4). In this phase a 3D laser scanning multi-resolution approach was chosen, evaluating different parameters like the relevance

of the acquired building and the role of the range map inside the scan plan [13]. The possibility of scanning common areas from the different rooms of the buildings around the main cloister, allowed us to avoid the creation of a topographic network with artificial targets for the following 3D data alignment. In addition, the historic and stratigraphic connections highlighted by the other partners during the project were taken into account, widening in progress the survey area where needed, to better understand the building transformations. An example is given by the west side of the small cloister, which represents the first part of the structure built earlier than 1402, containing some key elements to clarify the building evolution in time.

The 3D acquisition of the wide architectonic area was covered with a big number of points of view, using the Focus 3D laser scanner by Faro, whose light weight and working distance (2÷80 meters) features were suitable to carry out the task. The 3D digitization campaign was organized in several days, in order to acquire each portion of the whole area with 146 scans, 111 related to the single rooms and 35 for the creation of the connection network. Different scanner set-ups were used in relation with the level of details, the working distance and the role of each scan position, producing a final resolution from 2 to 20 mm.

Fig. 4. Network of scan positions on the ground floor during the 3D

acquisition campaign.

In the same time a photographic survey was carried out, acquiring nearly one thousand digital images with a Canon 5D mark II digital camera, equipped with 20 and 50 mm lenses, mounted on a tripod with normal or panoramic head.

Fig. 5. Orthoimage of the Library ceiling, obtained through SFM processing.

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These images were captured in order to create a digital database of planar or spherical images, supporting both the texturing process of the digital models and the creation of multimedia products like image presentations or QTVRs. Some images were also used to create high resolution dense clouds of colored 3D point with the Structure from Motion (SFM) technique. Each cloud was metrically scaled thanks to the measures extracted from 3D point cloud and used to obtain metrical orthophotos of some room ceilings. Such orthophotos were used to texturize for example the ceilings of the New Sacristy and the Library (Fig. 5), both of them decorated with frescoes and characterized by a complex shape.

IV. DATA PROCESSING

The huge amount of 3D data required the definition of a strict and ordered data recording in a database for making as smooth as possible the registration step. In order to minimize the alignment errors a long cleaning process was completed for each scan for deleting the typical geometrical artifacts generated by the Focus 3D with a laser beam-to-surface angle too far from right angles (approximately below 20°), all the artifacts generated on abrupt range variations (e.g. in correspondence of corner edges), and all those data unrelated with the object, as for example the points acquired on people standing up in the survey location.

Each cleaned point cloud was aligned to the others in the same area (internal room or external parts), in order to create 14 different models. These were then uniformly re-sampled with a spatial frequency of 1 point every 2 or 5 cm, depending on the size of the represented building (Fig. 6). A second alignment step was then carried out to orient all these partial 3D clouds in a common reference system, obtaining a colored point clouds representation of the whole complex.

Fig. 6. Point clouds of the Library aligned in one single model. The yellow

icons indicate the different scanner positions during the survey.

The general colored 3D cloud was the first operating outcome sharable with the other project partners as a tool for verifying their historical/stratigraphic analyses, providing in this way a useful multidisciplinary interaction. The aligned raw point clouds of each room, stored on the project repository, could be visualized and navigated through the web by each partner, using the Trueview software tool (Leica). It allows to show the 360° panoramic view seen by the scanner from his location, and use metric tools for measuring details on the image, using the accurate 3D laser data “behind” the image.

These virtual artifacts allow to metrically explore each room, allowing a better understanding of the environment also by non-expert users. For example it is possible to extract a length, measure an angle between two walls or the 3D coordinates of a specific point, sharing these data with other partners.

Based on the global colored point cloud a unique reference system was positioned correspondingly to the internal corner of the little cloister, in the junction between Southern and Eastern arm, centrally located in the studied area. To define this exact point, three fitting planes were created from the point clouds, selecting different areas of the cloister pavement and the two walls intersecting in the location chosen as origin. Such planes defined the new reference system, from which some sections were created: two horizontal sections at 2 and 6 meters height; nine vertical sections, three for each arm of the small cloister (South, East and West). The vertical sections, as the example shown in fig. 7, were particularly useful for the architectonic and historical interpretation. For example the longitudinal sections of the New Sacristy, Library and Refectory allowed to clearly visualize the connections between different rooms at different levels of the building, keeping some important measurements that could be hardly obtained in the reality for complexity and size of this the huge building. These measurements were also used for verifying the correct interpretation of historical building descriptions, were old measurement units were mentioned.

Moreover, The images extracted from the longitudinal sections were both useful for dimensional feedback and for the integration with ortho-rectified images used in the stratigraphic analysis (Fig. 8). This non-destructive analysis method, carried out thanks to another group of Politecnico, supports the reading of direct sources of information contained on the walls in full respect for the stratified and preserved “material culture” [12] allowing to guess the time sequence of every construction activity with high accuracy.

V. 3D RECONSTRUCTION

In the modeling phase a semantic approach was followed, optimizing and simplifying the management of the models. Starting from the global oriented point cloud, a subdivision process for each room was carried out, sub-sampling and creating the related polygonal models. This time-consuming task allowed to obtain more manageable polygonal models, which were afterwards used as suitable reference for the reality-based CAD redrawing of the building.

The decision to employ mathematical surfaces, rather than meshes coming directly from scanned 3D data, was mainly due to the possibility to generate semantic subdivision and modeling with different level of details, highlighting the relevant items for determining the historical evolution. This approach was applied for example to some portion of the Library, like the lateritious remainings belonging to a not anymore existing third vault (Fig. 9). Besides, the surface reconstruction leaded to a lighter modeling, easy to manage and share with the project partners. The whole area around the small cloister, excepted the church, was remodeled and it was paid special attention to the South-East corner, including the Library and the New Sacristy, which was texturized with a specific set of digital images.

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Fig. 7. Longitudinal section of the East side arm of the small cloister, extracted from the aligned clouds of points. From right to left: the apse of the south church

transept, the exernal walls of the New Sacristy (dashed rectangle), the trasversal section of the Library and a little portion of the large cloister portico.

Fig. 8. Stratigraphic analysis of the dashed area in figure 7.

These reality-based 3D models allowed scholars to geometrically analyze the building structure on their desk, validating their historical hypothesis and configuring their virtual historical representation for each scientific purpose. For example during the project one of the historian partners verified the correspondence between the “braccia viscontee” (visconti’s arm) and the actual measures of the building, working out some novel interpretative hypothesis.

Once the reality-based modeling was completed, a cross-analysis started, integrating 3D models, historical texts and stratigraphic analyses in order to outline the geometrical models of seven historical phases of the building, starting from the 1402 till now (fig. 10). Such data integration allowed to create a first version of that models and it was used to highlight some incoherencies between historical and stratigraphic analysis. For example the first hypothesis regarding the roofs shape and the position of some windows at the first floor of the second evolution phase, corresponding to the period before 1426, had to be changed because the 3D model clearly showed that they could be as supposed.

a)

b)

Fig. 9. CAD models generated from real data: a) 3D surfaces reconstruction

of the Library based on the acquired mesh: b) final texturized model.

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Those critical aspects were analyzed by several professional figures (modelers, architects, historians etc.) and an iterative process of modeling/verification was started, in order to obtain an interdisciplinary validation of modeling hypotheses, converging to a “best guess” version of the historical phases. Some iconographic sources were also used just to visually confirm historical elements and stratigraphic analyses, even if no reverse modeling was made directly on their perspective views [14].

Fig. 10. A sequence of images related to the different historical phases, with

the global evolution of the buildings through longitudinal sections of the

internal shape of the rooms. Left to right and top to bottom: before 1402,

1402, 1426-27, after 1460, the end of XVI century

VI. DATA VISUALIZATION

The aforementioned 3D digital models can represent a gateway for accessing different historical, architectonical, conservative and stratigraphic data associated to the Chartreuse. Regarding the area of interest, a tridimensional informative system was experimented in order to visualize an heterogeneous set of interdisciplinary information acquired and synthesized in the 3D models.

The tool adopted is based on a videogame rendering engine (Unity 3D), which allows a real-time navigation of 3D scenarios [15], connected with a data retrieval query updated in real-time, dependent on the user position inside the model and the viewing direction in the virtual space. The result is a web application for the access and management of data in a tridimensional context, based on the ISEE platform [16]. It supplies an interactive visit of the virtual environment: here information can be obtained just “looking around” through an

intuitive interface, producing automatically a relevance viewing order depending on the line of sight orientation. In addition a 2D map in the navigation window allows the visitor to understand his position/orientation in the model (Fig. 11).

As any point-and-shot videogame, the application allows to walk into the 3D model showing the real-time rendering stream in a “Viewing Area”. The related data, previously attached to the 3D model in a data mapping stage, is collected in real-time identifying the center of the screen as the point of interest targeted by the user, and searching information around that point. Simultaneously, in a window located on the right of the Viewing Area, the “Information Area” shows a set of data records associated to that point, searched in the surroundings of the point of interest, and ordered according to the distance to it. The result of this continuous query might be an heterogeneous set of multimedia information, represented in this case by texts, images and maps.

Besides, the system fruition is possible in the opposite sense, from the information to the model: selecting one particular information from an available list, it’s possible to see the connected area of the model.

Fig. 11. 3D model and data on the ISEE platform.

Despite this data system actually connected to the 3D model doesn’t represent the whole information produced by the project (stratigraphic dating, ortho-rectified photos, historic tables etc.), the applied instrument only demonstrates the possibility to create a much wider information system, completely based on tridimensional navigation of virtual models, and vice-versa, associating areas of the model with textual data. Such integration can assume the double role of analysis and spreading of the cultural heritage contained in the Certosa di Pavia for touristic purposes.

The researchers can use this system as a 3D scrapbook, to acquire and share data of different nature, mapping, in 3D annotations, tables or images directly on the model. In particular, the stratigraphic images can be shown on the model surfaces, thanks to a specific function to visualize chronologically coherent areas in sequence.

Any user can also navigate the model remotely via web, taking advantage of pieces of information that may enhance his future visit, suitably preparing the related touristic experience.

VIEWING AREA INFORMATION AREA

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Fig. 12. Pipeline of the integrated survey and 3D reconstruction methodology

VII. DISCUSSION AND CONCLUSIONS

In this paper a multi-scale and interdisciplinary cultural heritage inspection approach has been described, in order to suggest a methodology aimed at discovering historical transformations on a building, non directly readable from a single source. Starting from different points of view and indirect information sources (text, drawings and historical images) and direct 2D and 3D sources (ortho-images, 3D laser scanning surveys), an iterative process to set up the evolution hypothesis of an historical building with many architectural stratification, has been shown.

The block diagram shown in Fig. 12 represents a synthesis of the whole process that is re-applicable in any situation where an architectural stratification has to be discovered and represented. Here the dense color point cloud of the building under study and of part of the surrounding area has a pivotal role. Once shared with other experts it can be used both as metric reference for the ortho-images needed in the stratigraphic analysis. Then it can be used for verifying historical reconstruction hypotheses originated by written descriptions. Finally if is the metrological base for generating CAD representations of the ancient building based on real data. Of course this is not a model as close to the real as the

polygonal mesh directly generated by the acquired 3D points. However, specially if coupled with the original cloud of colored 3D points originated by the survey, it can be used as a synthetic representation of the physical object, such as a first (nicely readable) level of detail of a more geometrically complex structure, faithfully represented by the 3C cloud. From such model, from a deep stratigraphic analysis and from a study of historical texts, a first hypothesis of diachronic reconstruction can be made by the experts in 3D modeling. This gives the possibility to the experts in Historic and Architectonic disciplines to check the hypotheses intrinsically contained in the proposed shape, refining and amending it according to a their view and sensitivity. This geometry-history feedback loop can be iterated as many times as needed up to reaching a reasonable result under different disciplinary points of view. In the specific case study shown in this paper this looping was iterated just two times, while in other previous cases where a similar approach has been experimented [10], this needed three or four loops up to reach the best guess.

The need of iterations for building up progressively a believable 3D reconstruction of an ancient building not anymore in its original state is due to an apparently obvious reason, actually much less trivial than expected: the possibility to see what the theoretical reasoning allows to just to imagine,

Photographic+survey+ 3D+Scanner+survey+ Indirect+sources+analysis+(texts,'drawings,'historical'

images)'Global'3D'point'cloud'

Ortho<rec=fied'image''Sec=ons'extracted'from'

point'cloud'

Stra=graphic'analysis,'direct'sources'

Historical'evalua=on'3D'reality<based'model'

Preliminary'hypothesis'of'building'evolu=on'

3D'geometrical'models'of'the'historical'phases'

Final'hypothesis'of'building'evolu=on'

3D'geometrical'models'of'the'

historical'phases'

Coherence(check(

Coherence(check(

Metric(resize(

Dimensional(comparison(with(historical(sources(

Navigable'3D'informa=on'system'

(ISEE)'

247

is part of a progressive focusing of a reconstructive hypothesis, whose revelation gives to the historian or to the architect the possibility to refine his starting vague hypothesis for reaching an higher level of detail of his initial idea.

The main difference with the previous application of this idea [10] - that makes this case particularly significant - is the presence of multiple feedbacks generated by different disciplines, acting simultaneously on the same shape, pushing towards a multidisciplinary best guess.

In addition, all the 3D outputs generated in this way can be used as navigable spatial representation annotated with links to pieces of information of various types through a multimedia 3D information system. This possibility enriches the 3D representations with information both useful for enhancing the touristic visit to reconstructed monument but also for improving the study of it with the possibility to annotate various kind of information in 3D, both on the reconstructions or on the reality based models (in this example only the latter possibility was experimented).

The strong interdisciplinary character of this project forced the partners to find common languages in order to converge toward the best possible final results. In this sense the reality-based 3D models played a pivotal role, helping to synthesize the knowledge about a monumental buildings under investigation. The availability of those resources through the web allowed a group of scholars from different disciplines, working in different physical places, to concurrently benefit of such synthesis, helping the study and development of different hypotheses about the architectural evolution of this site.

Such 3D-based interaction was also fundamental for implementing - in a relatively straightforward fashion - the multi-disciplinary feedback needed to reach a step-by-step definition of the reconstructive 3D models related to each historical phase taken into consideration. Any 3D proposal was easily submitted to a coherency check from experts in different disciplines till to obtain the most probable guess according to the different points of view. This latter result represents both the output of the described project but also a new starting point for a more advanced historical discussion about the Cultural Heritage object approached in this way.

ACKNOWLEDGMENT

The authors would like to thank all the persons in charge of each research unit participating to the TIVAL project, Carla Simone as responsible of the whole project and M.T. Mazzilli for the collaboration in all the historical work. In particular the authors would like to thank S. Bortolotto, A. Garzulino, R. Simonelli, E. Ciocchini and F. Zangheri for their precious work on the stratigraphic analysis of the building shown in figure 8. The authors are also grateful to Laura Pecchioli for implementing Certosa’s models in the visualization system ISEE. A special thank is finally dedicated to Father Giorgio and the other monks of the Chartreuse of Pavia for their

availability, hospitality and collaboration during the hard survey work carried out within this project.

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