USE OF 3D LASER TECHNOLOGIES
IN THE RESTORATION WORK OF
THE HALL OF THE KINGS
UTILIZACIÓN DE LAS TECNOLOGÍAS LÁSER 3D EN LOS
TRABAJOS DE RESTAURACIÓN DE LA SALA DE LOS REYES
ABSTRACT Counterforms placed under the vaults were used to protect the leather vaults during the res-
toration process. To build these counterforms, the vaults have been scanned using a laser scanner.
The data generated by the scanner has been cleaned to remove non-vault structures. The resulting model
has been simplied and its noise has been reduced. The surface of the vault in this model has been moved
to make room for a thin layer of polyurethane foam that cushions the contact between the counterform
and the vault.
In order to insert the support into the vault and ensure correct contact, it has also been necessary to break
down the counterforms into eleven pieces.
The 3D models of the pieces have been used to manufacture the expanded polystyrene counterforms
using numerical control machining systems. These counterforms have served to support the vaults during
the restoration process.
This article describes the process followed from data collection to the placement of the counterforms,
emphasizing the digitalization and processing methodology of the data generated by the scanner.
KEYWORDS 3D scanner, Restoration, 3D printing, Alhambra, Hall of the Kings
RESUMEN Para proteger las bóvedas de piel durante el proceso de restauración se utilizaron contra-
formas colocadas bajo las bóvedas. Para construir estas contraformas se han escaneado las bóvedas
usando un escáner láser.
El resultado del proceso de escaneado se ha limpiado para eliminar las estructuras ajenas a la bóveda. El
modelo resultante se ha simplicado y se le ha reducido el ruido. La supercie de la bóveda en este mo-
PEDRO CANO OLIVARES
pcano@ugr.es
JUAN CARLOS TORRES CANTERO
jctorres@ugr.es
RESEARCH GROUP IN COMPUTER GRAPHICS AND
VIRTUAL REALITY OF THE UNIVERSITY OF GRANADA
468
PEDRO CANO OLIVARES
|
JUAN CARLOS TORRES CANTERO
delo se ha desplazado para dejar espacio para una na capa de espuma de poliuretano que amortigua
el contacto entre la contraforma y la bóveda.
Para insertar el soporte en la bóveda y asegurar un contacto correcto además ha sido necesario des-
componer las contraformas en once piezas.
Los modelos 3D de las piezas se han utilizado para fabricar las contraformas en poliestireno expandido,
utilizando mecanizados con sistemas de control numérico, que han servido para sustentar las bóvedas
durante el proceso de restauración.
Este artículo describe el proceso seguido desde la toma de datos hasta la colocación de las contrafor-
mas, haciendo énfasis en la metodología de digitalización y procesamiento de los datos generados por
el escáner.
PALABRAS CLAVE Escáner 3D, Restauración, Impresión 3D, Alhambra, Sala de los Reyes
COMO CITAR/ HOW TO CITE: CANO OLIVARES, P., TORRES CANTERO,J.C., Utilización de las tecno-
logías láser 3D en los trabajos de restauración de la Sala de los Reyes, Cuadernos de la Alhambra, 2021,
50,pp. ISSN 0590-1987
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
Recibido: octubre 2021 | Revisado: noviembre 2021 | Aceptado: noviembre 2021 | Publicado: diciembre 2021
ISSN: 0590 - 1987 I eISSN: 2695-379X I Depósito legal: GR 70-1965
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
469
USE OF 3D LASER TECHNOLOGIES IN THE RESTORATION
WORK OF THE HALL OF THE KINGS
T
he “Hall of the Kings” is a group of three naves next to
come from the motifs of the paintings that decorate
place was used for receptions and as a rest area.
process must be carried out by accessing the roof and removing
the external covers. In order to prevent possible detachment of the
vaults and accidental blows to the vaults that could damage the
paintings, it was proposed to build counterforms to support the
vaults during the time when they were not protected by their roofs.
Initially, these counterforms were to be built completely
by hand, manufacturing a series of wooden pieces which, like
form a supporting framework. In addition to being costly, this
alternative was not very accurate
1
.
-
ble at the time, it was decided to build expanded polystyrene
counterforms that would adapt exactly to the vaults in order to
-
rate documentation of the shape of the element, which is parti-
technology was used to record the previous state of the lion
sculptures in the Palace of the Lions prior to their restoration
2
.
- Dissemination of heritage, generating images, animation or
virtual visits. As an example, the digitisation of the Fountain of
the Lions has been used to create a virtual reality application
that makes it possible to compare the lions and study the evo-
lution of the fountain over time
3
.
1. GONZALEZ, María José; MONTERO, Araceli; BAGLIONI, Raniero. The
paintings in the Hall of the Kings of the Alhambra in Granada: a project, a
method, an intervention. PH Magazine. Instituto Andaluz del Patrimonio
Histórico. N. 83. October 2012. pp. 74-89
2. CANO, Pedro; LAMOLDA, Francisco; TORRES, Juan Carlos; VILLAFRANCA,
María del Mar. Use of 3D laser scanner to record the pre-intervention state
of the Fountain of the Lions in the Alhambra. Virtual Archaeology Review.
Vol 1 N. 2. pp. 89-94. May 2010.
3. CANO, Pedro; GARCÍA, Manuel; TORRES, Juan Carlos; LAMOLDA, Francis-
co; PEREZ, Silvia. Interactive 3D Application for the multimedia valorization
of the restoration process of the Fountain of the Lions of the Alhambra
based on 3D laser scanner registration. In Digital Heritage 2015. Interna-
tional Congress on Digital Heritage - EXPO. IEEE.
- P
can generate scale or life-size replicas of the elements.
Analysis. It is possible to calculate physical properties, or to
- Information support. Most of the information relating to he-
ritage elements is associated with locations on their surface.
system
4
, which can be used to record information on the inter-
ventions carried out on the element
5
.
In this work, the digitisation served on the one hand as
a graphic documentation of the state of the vaults and their
supports, and on the other hand to make a physical model,
although not of the vaults, but of the space underneath them.
In order to build the counterforms, it was necessary to
generate a three-dimensional model of each vault so that the
surface of the counterforms could be built in the exact shape
the vault to be occupied by the counterform.
carried out as part of the contract “Project for the restora-
tion of the paintings on leather in the Hall of the Kings in
the Palace of the Lions of the Alhambra. 3D modelling of the
vaults”, signed between the Alhambra and Generalife Board
of the University of Granada, directed by the professor of the
University of Granada, Mr Pedro Cano Olivares, and coordi-
nated by the head architect of the conservation service of the
-
molda Álvarez.
printing technology used. Section 3 discusses the methodolo-
gy used in the process. Finally, the fourth section shows the
results obtained and the conclusions of this work.
4. LÓPEZ, Luis; TORRES, Juan Carlos; ARROYO, Germán; CANO, Pedro;
MARTÍN, Domingo: An Ecient GPU Approach for Designing 3D Cultural
Heritage Information Systems. Journal of Cultural Heritage. 2020. 41, pp.
142-151.
5. TORRES, Juan Carlos; LÓPEZ, Luis; ROMO, Celia; ARROYO, Germán;
CANO, Pedro; LAMOLDA, Francisco; VILLAFRANCA. Using a cultural herita-
ge information system for the documentation of the restoration process.
In: 2013 Digital Heritage International Congress (DigitalHeritage). IEEE,
2013. pp. 249-256.
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
470
PEDRO CANO OLIVARES
|
JUAN CARLOS TORRES CANTERO
TECHNOLOGY
with laser scanners and 3D printing. Both were emerging te-
chnologies at the time.
3D scanning
A laser is a device that generates a coherent beam of light (the
phase relationship in the beam is constant). If the medium
in which it is transmitted is non-dispersive, the beam travels
in a straight line, making it possible to match the position at
which the beam strikes the object with the direction in which
it was emitted.
Laser scanners are devices that send a low-intensity laser
Two laser scanner technologies can be distinguished according
-
be used to measure the time it takes for the laser to travel,
which is proportional to the distance, or the angle at which
exceed one kilometre. In order to capture a large area of the
using mirrors. Usually the scanner head is rotated to change
the angle to the vertical and mirrors are used to change the
tilt. Some scanners are capable of scanning in all directions
(except the area underneath it), capturing the entire space
surrounding the position in which they are located. As an
example, the Callidus CP3200 scanner used in this work can
capture 360° horizontally and 140° vertically.
-
ners, and are used to digitise objects over long distances, from
a few metres to kilometres.
Triangulation scanners place the sensor at a position dis-
tant from the emitter, which allows them to measure the angle
digitise objects in close proximity, from centimetres to a few
In either case, the device returns the coordinates of points on
the surface of the object, using the position of the scanner as the
-
phs of the object and can return colour information in addition
to the coordinates of the points. In some cases it is also possible
One aspect to consider when using laser devices is the le-
standard establishes seven classes of laser devices according to
their hazardousness, of which only class 1 lasers are safe un-
der all foreseeable conditions of use, including when the beam
allows them to be used in public spaces without the need to
set up a security cordon.
taking data from the object (which can be done either with
a laser scanner or with other technologies) and processing it
-
sentially the measurement of the positions of a dense set of
points on the surface of the object, usually referred to as a
point cloud. Two of the most important characteristics of a
digitisation system are the error in the measurement of the-
measurement error indicates the margin of uncertainty in the
distance between samples (related to the sampling density,
and usually referred to as resolution) indicates the size of the
smallest surface detail that can be captured, depending on the
In the capture process it is necessary to record the entire sur-
face of the object. In one scanner shot, we will only have those
parts of the element that are visible from one of the two sides of
thepositions in which the scanner has been placed. It will usually
be necessary to take several shots, either by moving the object
or by moving the scanner. In each shot the scanner will measure
positions of the object surface visible from its position, these posi-
tions will be referenced to the position of the scanner in the shot.
Computer processing generates a 3D model of the object
structures containing the surface representation of the object.
the object) and colour information.
depend on the type of processing we want to do with it, and
will determine the complexity of the processing to be done to
create it. We need simpler models to visualise the object than
to calculate its volume.
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
471
USE OF 3D LASER TECHNOLOGIES IN THE RESTORATION
WORK OF THE HALL OF THE KINGS
In some cases the model may simply be a set of points. In
shots taken of the object.
visualisation can try to be as similar as possible to a photograph
(photo-realistic visualisation) or try to be similar to a drawing
(expressive or non-photo-realistic visualisation).
anastylosis or to plan restoration processes.
-
ciated with the element surface.
-
ce area, moment of inertia or centre of gravity.
make life-size or scaled copies.
Both for the 3D printing of the model and for property
calculations, the model must be a closed polygon mesh that
does not contain self-intersections. A polygon mesh is a set of
connected polygons covering the outer surface of the object.
Closed implies that there are no cracks in the mesh, such that
there is no path from the inside to the outside without cros-
no two triangles of the mesh intersect. Together, these proper-
ties are necessary to be able to orient the mesh, i.e. to be able
to determine which is the inner and which is the outer face of
as it is used to describe a physical solid.
Meshes generated from laser scanner point clouds con-
cloud is done by generating triangles and joining neighbouring
points. In order to check that the mesh is closed and to calcu-
late properties, the adjacency relations between the triangles
must be stored in addition to the coordinates of the vertices.
Once the mesh has been created, it is necessary to check
that it is closed, detecting and covering any cracks.
to the generation of the valid polygon mesh, is usually
carried out as follows
6
6. BERNARDINI, Fausto; RUSHMEIER, Holly. The 3D model acquisition pi-
peline. In Computer Graphics Forum. Oxford, UK: Blackwell Publishers Ltd,
2002. pp. 149-172.
-
the surface of the object must appear in at least one shot, and
Data collection A measurement is made at each of the posi-
tions set for the scanner, resulting in a point cloud for each one.
Merging of point clouds. One single point cloud is created
-
-
-
this process, duplicate points can be removed, points that are
process can be automated by using markers in the shots.
threes to form triangles that cover the surface of the object.
Some of the process can be done automatically, but most of
Here we are omitting the capture of colour, as it is not
relevant in this work.
3D printing
3D printing creates a physical object from a computer model.
In a way, it is the reverse of digitisation. 3D printing has now
where material is added layer by layer to create the object.
-
ting with a block of material from which the excess material is
cut to create the object. To do this, computer-controlled cut-
to create objects of large size and high precision using virtually
any type of material.
METHODOLOGY
-
pan moulds, printed from the digitisation of the vaults, and
continuous contact.
product commonly used as protection in the packaging of deli-
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
472
PEDRO CANO OLIVARES
|
JUAN CARLOS TORRES CANTERO
Il. 1. The scanner was set up without a tripod, placed directly on the sca-
olding in the vaults. Pedro Cano and Juan Carlos Torres. Year: 2007. ©
Alhambra and Generalife Board of Trustees.
cate products or as a building material. It is light, highly resis-
tant to shocks and impacts, and does not allow bacteria to grow.
Porexpan is also rigid, so its contact surface is hard. For
smooth contact with the vaults, the counterforms are coated
with a thin layer of polyurethane foam. Polyurethane foam is
-
terform, absorbs vibrations. For this foam layer to be added,
the surface of the counterforms have to shrink and leave a
small gap between them and the surface of the vaults.
the shape of the vault, is obtained by the digitising process. To
must be added, which must be placed horizontally (regardless
of the deformation of the counterform) in order to facilitate
its positioning, and lateral planes that must not collide with
the walls of the room.
On the other hand, the dimensions of the room are smaller
than those of the vault itself, so it is necessary to divide the coun-
bottom surface so that they can be placed in the correct positions.
below.
Digitisation
A Callidus CP 3200 laser scanner was used to digitise the
-
tegrated digital camera for colour capture, controlled from a
that allows it to rotate 360° on the horizontal plane in steps of
between 0.0625° and 1.0°. With the help of a rotating mirror,
the laser beam is dispersed in a fan shape, covering an area
rotate and collect the coordinates of surrounding objects at
measurements are recorded on the Callidus LMS protected
In order to record the counterforms, the scanner was pla-
as shown in Figure 1. To eliminate vibrations and occlusions
scanner in the centre of the vault and the other two by pla-
cing the scanner in the centres of the circles described by
the ends of the vaults. In all three cases with the scanner on
the fact that the surface was theoretically simple, due to the
irregularities and deformations of the counterforms, which
meant that in each shot there were areas that were not captu-
process. Colour is not captured in these shots because, on the
one hand, it is not relevant to the process, and on the other,
the vaults were already covered with a layer of protection,
which would have made it impossible to capture the colour
of the paintings. In addition, capturing colour would have re-
of the scanner’s camera.
-
Besides the surface of the vaults, each shot includes all the
elements present in the scene, such as walls, cornice, canvases
Point cloud processing
-
pling of dots on the surfaces visible from the scanner in each
mesh of triangles representing the surfaces of all the objects in
the scanned scene.
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
473
USE OF 3D LASER TECHNOLOGIES IN THE RESTORATION
WORK OF THE HALL OF THE KINGS
Il. 2. Section of the 3D model of a vault before the removal of structures
outside the vault, in which the scaolding can be seen. Pedro Cano and
Juan Carlos Torres. Year: 2007. © Alhambra and Generalife Board of Trus-
tees.
used, together with MeshLab
7
and programs developed speci-
Alignment of the three shots, making the three point
clouds in the same coordinate system. For this purpose, the
two side shots are transformed into the coordinate system of
in shot pairs, and then performing an iterative adjustment by
minimising the distance between the three clouds.
mesh of the entire scene.
-
fabrics, etc.), leaving in the working model only the surface
of the vault, the cornice that supports it and the walls of the
room that are necessary to build the desired model of the
Modelling of the counterforms
cornice that supports it (which is the area of interest) and the
rest of the model used to adjust the measurements of the lower
support that will be added to the model of the counterform to
enable placement on the installed supports. Il. 3 shows the re-
-
step is a mesh representing the surface of the vault.
To generate the counterforms, it is necessary to construct
a closed mesh, for which a surface section must be added to
delimit the lower part of the counterform. To ensure perfect
adaptation to the surface of the vault, we cut the model below
7. CIGNONI, Paolo; CALLIERI, Marco; CORSINI, M.assimiliano DELLEPIANE,
Matteo; GANOVELLI, Fabio; RANZUGLIA, Guido. MeshLab: an Open-Sour-
ce Mesh Processing Tool. In: SCARANO, Vitorio; DE CHIARA, Rosario; ERRA,
Ugo.Sixth Eurographics Italian Chapter Conference. (Salerno, Italy, 2008),
pp. 129-136.
the cornice, guaranteeing a safety distance from the cornice
supporting the vault. For this purpose, a horizontal plane is
calculated that adapts to the deformations of the cornices,
which is taken as a reference to create the cutting plane. As a
result, we obtain the model of the part of the vault we are inte-
rested in for the construction of the supporting counterform.
From the resulting mesh, a new mesh is generated by mo-
-
kness of the polyurethane foam (2 cm), so that when the foam
-
responsible for the assembly process to ensure that there are
-
cilitate the design of the supports, and given that the vaults
In the vertical extension of the lower area, a larger dis-
placement than that used for the vault is used so that it can
be adapted without colliding with the cornice that supports
each vault. In some cases this ledge is completely missing, but
adjustment in this area.
At this point, we have a solid model of the counterform.
constructed counterform and the original model obtained in
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
474
PEDRO CANO OLIVARES
|
JUAN CARLOS TORRES CANTERO
Il. 3. Area of interest of the model to build the counterform. In this model,
all structures outside the vault have been removed. Pedro Cano and Juan
Carlos Torres. Year: 2007. © Alhambra and Generalife Board of Trustees.
Il .4. Comparison of the model of the counterform (in blue) and the vault
surface (in grey). Pedro Cano and Juan Carlos Torres. Year: 2007. © Alham-
bra and Generalife Board of Trustees.
the scan, where we can see the perfect adaptation of the surfa-
Since the surface of the vault is larger than that of the
room, it is necessary to divide the counterform into smaller
pieces to allow for its placement. In addition, this breakdown
will enable adjustment of the counterform in the vault.
made so that all the pieces were inserted in the centre and
adjusted towards the sides, ensuring a correct positioning wi-
the vault. Figure 5 shows the pieces designed by Pedro Salme-
was marked as an extrusion of the part number on the inside
of each part.
To divide the counterform into its eleven parts, the cu-
shall be used to cut the model of each counterform. For the
labelling, a model is created with the numbering subtracted
from the model of the counterforms, generating the numbe-
shows an infographic of the parts of the counterform (in blue)
placed on the scanned model of the vault (in grey).
In order to check the accuracy of the constructed mo-
del, the distance between the solid model of the counterform
and the digitised surface of the vault was calculated. Figure 9
shows an image of the calculation of the distance between the
vault and the counterform, with a colour scale showing the
distance in metres. It can be seen that for the counterform
area it is between 1.5 and 2.2 cm. By calculating these distances,
it was possible to verify that the distance between the coun-
terform model and the vault was always within the established
distance range.
Creation of the counterforms
the solid model of the eleven pieces of the counterforms of
-
ring process was carried out by the company Tragacantos, S.L.
Figure 10 shows the Porexpan pieces of one of the counter-
forms. A layer of polyurethane foam was added to these parts
in the areas where they were in contact with the vault. Figure
11 shows the placement of the polyurethane foam layer on one
of the counterforms.
Placement
outer parts (one to eight) and closing with the interior parts
-
rried out without displacement of the counterforms on the
vault surface.
Digitisation of the backs of the vaults
Once the counterforms had been placed and their covers
removed, the supports of the vaults were digitised from the
outside. A Minolta Vivid 910 scanner, which is a triangulation
be used over short distances (no more than two metres), but
allows measurements with high resolution and accuracy (in
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
475
USE OF 3D LASER TECHNOLOGIES IN THE RESTORATION
WORK OF THE HALL OF THE KINGS
Il. 5. Division of the counterforms into parts. Pedro Salmerón Escobar. 2007. Alhambra and Generalife Board of Trustees. Pedro Cano and Juan Carlos
Torres. Year: 2007. © Alhambra and Generalife Board of Trustees
Il 6. Drawings used to cut the counterforms into parts. Pedro Cano and Juan
Carlos Torres. Year: 2007. © Alhambra and Generalife Board of Trustees.
Il 7. Final model of the counterform, divided into parts and labelled. Pe-
dro Cano and Juan Carlos Torres. Year: 2007. © Alhambra and Generalife
Board of Trustees.
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
476
PEDRO CANO OLIVARES
|
JUAN CARLOS TORRES CANTERO
Il 8. Simulation of the placement of the counterform (in blue) on the origi-
nal model of the vault (in grey). Pedro Cano and Juan Carlos Torres. Year:
2007. © Alhambra and Generalife Board of Trustees.
Il 9. Verication of distances vis-à-vis the original model. It can be seen
that the distance in the area of the leather vault is between 1.5 and 2.2
cm. Pedro Cano and Juan Carlos Torres. Year: 2007. © Alhambra and Ge-
neralife Board of Trustees.
models were used as documentation of the pre-intervention
state. Figure 13 shows the digitisation of the reverse side.
RESULTS AND CONCLUSIONS
-
ber three. Once the assembly of the counterform was completed
satisfactorily, the procedure was validated and applied in vaults
were correctly and durably held in place during the entire res-
In their work on the intervention on the paintings in the
Hall of the Kings in the Alhambra in Granada, Gonzalez,
Montero and Baglioni, evaluate positively the results obtained
with this methodology
-
tion on the front of the paintings of two counterforms made of
wood covered with various layers of shock-absorbing material.
scanning of the three paintings, this was replaced by counter-
forms in expanded polystyrene, obtained by numerical control
like a second skin to each pictorial surface that formed a solid
-
ments demanded of this part, considered a key element in the
moments of the project in which the vaults would be devoid
of their natural support and supported exclusively by them.”
ACKNOWLEDGEMENTS:
-
TIC-401 project.
8. Op. cit. Las pinturas de la Sala de los Reyes de la Alhambra de Granada (1)
CUADERNOS DE LA ALHAMBRA I núm. 50 I 2021 I págs. 467-477
477
USE OF 3D LASER TECHNOLOGIES IN THE RESTORATION
WORK OF THE HALL OF THE KINGS
Il 10. Porexpan parts of one of the counterforms. Pedro Cano and Juan Carlos Torres. Year:
2007. © Alhambra and Generalife Board of Trustees.
Il 11. Placement of the polyurethane foam layer on one
of the counterforms. Pedro Cano and Juan Carlos To-
rres. Year: 2007. © Alhambra and Generalife Board of
Trustees.
Il 12. Placement of the counterform. Pedro Cano and Juan Carlos Torres. Year: 2007. © Al-
hambra and Generalife Board of Trustees.
Il 13. Digitisation of the backs of the vaults. Pedro Cano
and Juan Carlos Torres. Year: 2007. © Alhambra and Ge-
neralife Board of Trustees.
