REPLICAS IN CULTURAL HERITAGE: 3D PRINTING AND THE MUSEUM EXPERIENCE

Abstract

3D printing has seen a recent massive diffusion for several applications, not least the field of Cultural Heritage. Being used for different purposes, such as study, analysis, conservation or access in museum exhibitions, 3D printed replicas need to undergo a process of validation also in terms of metrical precision and accuracy.
The Laboratory of Photogrammetry of Iuav University of Venice has started several collaborations with Italian museum institutions firstly for the digital acquisition and then for the physical reproduction of objects of historical and artistic interest. The aim of the research is to analyse the metric characteristics of the printed model in relation to the original data, and to optimize the process that from the survey leads to the physical representation of an object. In fact, this could be acquired through different methodologies that have different precisions (multi-image photogrammetry, TOF laser scanner, triangulation based laser scanner), and it always involves a long processing phase. It should not be forgotten that the digital data have to undergo a series of simplifications, which, on one hand, eliminate the noise introduced by the acquisition process, but on the other one, they can lead to discrepancies between the physical copy and the original geometry. In this paper we will show the results obtained on a small archaeological find that was acquired and reproduced for a museum exhibition intended for blind and partially sighted people.

Full text

REPLICAS IN CULTURAL HERITAGE: 3D PRINTING AND THE MUSEUM
EXPERIENCE
M. Ballarin 1*, C. Balletti 1, P. Vernier1
1 Laboratorio di Fotogrammetria, Università Iuav di Venezia, Santa Croce 191, 30135 Venezia (martinab, balletti, vernier)@iuav.it
Commission II, WG II/8
KEY WORDS: 3D printing, Museum, Cultural Heritage, Replicas, Modelling, 3D acquisition
ABSTRACT:
3D printing has seen a recent massive diffusion for several applications, not least the field of Cultural Heritage. Being used for
different purposes, such as study, analysis, conservation or access in museum exhibitions, 3D printed replicas need to undergo a
process of validation also in terms of metrical precision and accuracy.
The Laboratory of Photogrammetry of Iuav University of Venice has started several collaborations with Italian museum institutions
firstly for the digital acquisition and then for the physical reproduction of objects of historical and artistic interest. The aim of the
research is to analyse the metric characteristics of the printed model in relation to the original data, and to optimize the process that
from the survey leads to the physical representation of an object. In fact, this could be acquired through different methodologies that
have different precisions (multi-image photogrammetry, TOF laser scanner, triangulation based laser scanner), and it always involves
a long processing phase. It should not be forgotten that the digital data have to undergo a series of simplifications, which, on one
hand, eliminate the noise introduced by the acquisition process, but on the other one, they can lead to discrepancies between the
physical copy and the original geometry. In this paper we will show the results obtained on a small archaeological find that was
acquired and reproduced for a museum exhibition intended for blind and partially sighted people.
* Corresponding author
1. INTRODUCTION
1.1 The diffusion of 3D printing in the Geomatic world
The recent technological evolution has seen a massive diffusion
on the global market of solid printing. In recent years, the
advent of 3D printing has opened new scenarios and new
possibilities in the production of commonly used objects,
especially since the costs of these machines have significantly
lowered, making these tools available to a wider audience. At a
professional level, it has had a great impact for example in the
design field. Here, this technology has opened the way for
numerous designers and artists who have started taking
advantage of 3D printers in order to create products to be placed
directly on the market. In fact, this type of technology has an
enormous creative and technological potential.
In the Geomatics world, the physical representation of an object
often starts from a point cloud and passes through a digital
model. This path contains three steps that imply a different way
of representing an object. These three representations have
different purposes and consequently different characteristics.
The point cloud model is part of the surveying world. It is
acquired through photogrammetric, laser scanning and
topographic techniques, which are strictly connected to a way of
representing reality linked to the concepts of precision and
accuracy. Through these methodologies we obtain numerical
data that imitate the shape of an object and that always
guarantee a metric control on the reliability of the result.
The purpose of digital and physical models is different, as they
both are traditionally linked to the concept of usability of an
object. They allow the user to view reality in a clear and
immediate way, in particular where it is no longer directly
accessible. Some examples may be the virtual anastylosis of a
collapsed building, or applications designed to allow a direct
contact with the object, which is often prohibited, especially in
the field of Cultural Heritage (see, for example, Arbace et al.,
2013; Santopuli et al., 2010).
In architecture, these models are usually the result of two
different processes. The first one involves the creation of closed
surfaces by joining together the vertices of the point cloud, with
different methodologies. The second one is a process of
interpretation, which passes through CAD software products,
where the quasi-continuity of the detected data is discretized
into a series of lines that represent just the elements necessary to
characterize its architectural structure. This transition from a
type of representation by points (which we can call a numerical
model) to a type of representation by lines or surfaces (for
example a mathematical CAD model) up to a physical
representation often implies very long data processing.
The numerical data is often redundant compared to the purposes
of the final model, both in terms of the amount of data acquired
and in terms of precision and accuracy (Bitelli et al., 2017). On
the other hand, we often deal with numerical models in which
some significant parts for the description of the monument are
missing. Protrusions or undercuts on the objects could cause
this phenomenon on the point cloud model.
Nonetheless, surveying is an essential requirement for knowing
and representing the shape and geometry of an object, in terms
of dimensions and proportions of its parts. Therefore, it is
necessary for a faithful reproduction of an object, regardless of
its shape.
1.2 3D printing in museums
This rapid diffusion has also had consequences in the field of
Cultural Heritage, and in particular in museums. The most
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
This contribution has been peer-reviewed.
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55

avant-garde institutions are beginning to recognize in modern
technologies a means to add new “reading” methods to the most
traditional visit paths (Wilson et al., 2017, Petrelli 2013, Dudley
2010). In fact, the focus is shifting from the museum visit as we
understand it, towards a multi-layered and multi-sensorial
experience: more and more, modern museums place supports
that allow new ways of interaction next to the works of art
exhibited in traditional display cases. Monitors, projections,
physical replicas add information, but also they modify the way
in which the user approaches the artwork itself.
Our way of experiencing reality passes through all the senses
we have at our disposal. The museum as a vehicle of knowledge
cannot fail to take this aspect into consideration (Mc Ginnis
2014; Sportun 2014).
Particular attention has been given to haptic senses, through the
creation of touchable exhibition, and handling sessions that
enable the user to personally interact with the object they are
looking at.
While recent studies have demonstrated that this is an easier
way for people to learn and experience reality (Neumüller et al.
2014a), it becomes compulsory when objects are used for
didactic purposes (i.e. to involve children in the learning
process) or for allowing access to blind and partially sighted
people.
And it is not just about handling and touching objects. The
association between printed copies and other kind of media
enable us to overcome the traditional “static nature” of the
physical model, which confines the objects to a specific
historical moment.
The Laboratory of Photogrammetry itself, together with the
research group "Visualizing Venice”, has worked for an
exhibition at Palazzo Ducale, using, among others, projections
on three dimensional printed models to tell the stories of the
lagoon and some of its islands (Galeazzo 2017; Balletti et al.,
2016; Calabi, Galeazzo, 2015).
In this way, we managed to show non-expert people the
historical transformations of the city, using 3D printed models
and 3D video mapping. In fact, combining these two means, we
were able to overcome the concept of the 3D model as a “frozen
representation” that shows a precise moment in time and to
provide the experience of witnessing the continuous flow of
history. This was particularly true in the case of the island of
San Secondo, for which we created three 3D models showing
three different moments in time, with a project of video
projection mapping that turned “the surfaces into a dynamic
video display” (Balletti et al., 2016).
Moreover, nowadays we are witnessing an attempt to add
information to the physical object, creating “sensorized”
models. Among a great number of 3D copies produced just for
display purposes (Scopigno et al., 2017; Scopigno et al., 2014;
Allard et al., 2005; Neumüller et al. 2014b), literature shows us
many examples of integration between 3D printed copies and
touch sensors or buttons to better explore objects, visualize
them in monitors and tell their stories (Balletti et al., 2017;
D’Agnano et al. 2015; Capurro et al., 2014).
In this context, solid printing acquires added value compared to
the simple mass production objects, which have made their way
into the global market.
However, another distinction has to be done. On one side there
are printed models whose final purpose is to provide a new and
more in-depth modality to obtain a more accurate knowledge of
the world, especially in a context – the museum – in which the
tactile exploration is often precluded. On the other side there are
printed models that are used for scientific purposes, such as
study, analysis and conservation of archaeological findings or
works of art.
The first ones are used to better access and enjoy exhibitions.
Therefore, the most important aspects to be considered are
realism and likelihood, that involve the material used for 3D
printing, its colour, but also the weight of the object and its
texture and touch feeling.
Otherwise, if the object is reproduced for scientific purposes,
then it has to be the perfect copy of the original in every single
part. If the aim is to replace an existing object with a strong
historical-artistic value, one must inevitably consider the
problem of the construction of digital models to be printed on
one side and the conformity of the copy to the original on the
other.
2. THE CASE STUDY
2.1 Purpose of the survey
The object analysed is an archaeological finding of the upper
Palaeolithic (11.600 years B.P.) named “Uomo barbuto di Vado
all’Arancio” (Bearded mad of Vado all’Arancio). It is an
engraved limestone slab found near Massa Marittima (GR)
(Martini 2016). On the main face we can still see his nose, his
eye, his long moustache, his straight and thin mouth, his beard
and what could be his hair or a headgear. On the opposite side,
there are still traces of what could have been another human
face, but the drawing looks unfinished (figg. 1-2). The size of
the object is very small: its dimensions are 8.2x4.1x1.1 cm.
Figure 1: Uomo barbuto di Vado all’arancio
Figure 2: Drawings of the limestone slab (after Martini 2016)
The project was carried out in collaboration with the
Archaeological Museum of Massa Marittima, where the object
is held, and its purpose was the production of a path accessible
to blind or partially sighted people through audio-tactile works.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
This contribution has been peer-reviewed.
https://doi.org/10.5194/isprs-archives-XLII-2-55-2018 | © Authors 2018. CC BY 4.0 License.
56

2.2 Data acquisition and processing
Because of the small size of the object, we decided to use a
triangulation based laser scanner with projection of a laser light
blade (Range 7, Konika Minolta) that allows us reaching the
best precision among the ones we have at the Laboratory of
Photogrammetry. This instrument guarantees a sub-millimetric
precision (up to 40 µm), it can be used with two different
lenses, tele and wide-angle, and it allows the acquisition of
small object from a distance of between 450 and 800 mm.
According to the type of lens mounted on the instrument, the
size of the acquisition area can vary from 79x99 mm to
267x334 mm, on the XY plane.
For this case study, we used a tele lens: as already stated, the
object was very small and the engravings on its surface were so
light they could barely be perceived by touching it. In order to
acquire these small deformations of the surface, we had to
obtain the highest resolution.
Before starting the acquisition phase, the instrument was
calibrated, reaching a precision on calibration of 9 µm. This was
necessary also to avoid possible consequences caused by travel
shocks and vibration or deformations (such as thermal
expansion) caused by temperature changes.
The object was acquired through 23 scans, taken at an
approximate distance of 700-800 mm. For each scan, a manual
focus was used, clicking on a central point of the object. The
high number of scans was due to the necessity of guaranteeing a
high overlapping among data for their subsequent orientation,
and to the limit of the tele lens, which has a small depth of field
(fig. 3).
Figure 3: Acquisition field
A first rough alignment performed still at the museum (with the
software Range Viewer) allowed us to check the coverage of the
entire object and to be sure no important data were missing.
Subsequently, the 23 scans underwent the standard processing
pipeline, using the software Geomagic Studio (Geomagic):
 first, each scan was cleaned and we deleted the marginal
parts, which are mostly affected by noise;
 then, we refined the registration through a global alignment
(average distance: 0.251 mm; standard deviation: 0.474
mm);
 we merged the scans into a single file, removing the
redundant parts;
 in the end, we filled the small holes that were left.
The operations of hole closure were made only for the smaller
holes and it was not particularly time consuming, as the number
of scans and the object geometry (that do not have undercuts)
allowed us the acquisition of an almost complete digital model.
At the end of data processing, the model was composed by
approximately 1 million triangles, therefore we did not need to
decimate it. In fact, the 3D printer management software
products usually do not allow to work with models made by a
high number of triangles (often working with over a million
triangles is a problem). The small dimensions of the object
allowed the acquisition of a low number of triangles: therefore,
we did not need to simplify its geometry by applying
decimation algorithms. In our opinion, the choice of this type of
object constituted an added value to the analyzes that were
carried out.
2.3 3D printing
The digital models realised were then printed using an online
printing service: Sculpteo (Sculpteo). This is one of many
websites that give the possibility to upload a 3D model designed
by the user, get it printed by a wide range of 3D printers,
techniques and materials, and get it delivered at home by
express delivery in any part of the world. The website also
offers a consulting service that helps non-expert users
producing optimal models for printing.
The presence and growth of these kinds of services demonstrate
the diffusion of solid printing in the market. Even if 3D printers
are by now quite cheap and affordable, nowadays, users do not
even need to buy one. They just need to upload their models on
the internet and wait for the delivery.
Using this website, we decided to test different printers and
materials, as we wanted to see which one gave the best results.
In fact, as stated before, if a replica has to be used in a museum
context, it is clear that it has to feel similar to the real objects, in
terms of weight, texture and general appearance. Checking the
results of different printers and materials is therefore
compulsory.
We printed four test copies: three with the same machine but
using three different materials; the fourth one with another
printer that allows the creation of fully coloured replicas.
The technique chosen is Selective Laser Sintering (SLS): it is a
technique which uses a laser as a sintering source of a
thermoplastic powder: thin layers of dust of different materials,
such as polycarbonate, nylon, ABS, are laid down progressively
and consolidated, where necessary, by the laser. The succession
of layers is guaranteed by the descent of the plate on which the
object lies (usually they are displacements in the order of tenths
of millimetres). This type of printing does not need supports,
because the object and its protrusions are supported by the same
powder that has not been consolidated. At the end of the
process, the prototype is freed from excess dust using
compressed air guns and subsequent sandblasting (Balletti et
al., 2017; Gibson et al., 2015; Scopigno et al., 2014).
The two white copies were printed with a EOS Formiga P395
using a plastic material (Nylon).
The first one (fig. 4) is Nylon PA 12, created from a fine
polyamide powder and available in different colour. Depending
on the wall thickness, this material can be both solid and
flexible: if the walls are 0.8 mm (minimum), the final object
will be flexible; it they are 2 mm (minimum) it will be rigid.
We chose a layer thickness, among the ones available (100/150
– 60 µm), that allowed us to obtain a good resolution also for
the smallest details (100 µm).
The second one (fig. 5) was printed in Nylon 3200 Glass-filled
(glass-filled nylon), which is made of a mix of polyamide
powder and glass beads. The surface of the material is white and
slightly polished. For this reason, the replica could not be used
for further analyses, as the laser beam of Range 7 passed
through the surface and could not acquire any data.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
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57

Figure 4: Nylon PA 12
Figure 5: Nylon Glass-filled
Figure 6: Alumide
Figure 7: Coloured replica
The grey copy (fig. 6) is made of alumide: a mix of polyamide
powder and fine aluminium particles, which gives the final
product a shiny look. There is only one resolution available for
this material as the smallest layer thickness is 150 µm.
The fourth replica (fig. 7) was realized with a ZPrinter 650s by
3D Systems, that uses a fine powder, similar to sandstone,
which is painted during the printing process. The original file
uploaded must then be composed by both geometrical shape of
the object and texture and colour information.
This machine can print in 390,000 different colours and it is
easier and cheaper than other methods, such as the application
of a coloured film on the replica or direct brush painting on the
surface, even if it often does not guarantee the same quality on
the results (Rivola et al., 2016).
The website itself guarantees a good result, but warn the users
the product could have small differences between the colours of
the real object and the ones of the replicas, especially for blacks
and fresh colours.
Maximum resolution available with this printer is 100 µm,
while they guarantee the correct reproduction of details that
have a minimum size of 0.4 mm.
2.4 “Augmented” 3D printing
As the final aim was the access of the object also for blind
people inside a museum exhibition and as the surface shows
some engravings that are just visible to human eyes but cannot
be felt by touching it, we decided to create a sort of
“augmented” printed model.
The digital .stl file was imported into Mudbox (Mudbox), a
software by Autodesk for digital sculpting and painting. Here,
we used a texturized model (obtained by applying one of the
photos that were captured during surveying) as a guideline to
increase the depth of the model where we could see the signs of
engraving. In this way, we were able to produce a 3D replica
also adequate for handling sessions (fig. 8).
This replica was not used for further analyses as its metrical
accuracy was already compromised by the realization process
itself.
Figure 8: Comparison between the coloured 3D replica and the
“augmented” one.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
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58

3. ANALYSES
As already stated, purpose of this research was the analysis of
the printed replicas not just from a “figurative” point of view, in
terms of aesthetic quality of the result, but also from a metric
point of view, in terms of precision of the final product.
We decided to analyse all the replicas produced, also those
created by the same printer, in order to check the reliability of
the test itself and the behaviours of different materials.
3.1 Data acquisition
The acquisition process followed the same pipeline used for the
original object.
The instrument used was the triangulation based laser scanner
Range 7, with the tele lens described above. In order to
guarantee the same precision on the acquisition and to control
the thermal expansion, the instrument was calibrated again
(precision: 9 µm).
What changed was the number of scans and the use of a rotating
stage, that speeded up the acquisition time also performing an
automatic first rough alignment. This system uses an auto focus,
but we believed it would not have had any significant effects on
the final model. In fact, already during the acquisition of the
original object, we saw that both methods guaranteed the
recording of suitable data for the purposes, considering both the
depth of field and the number of scans acquired.
The acquisition and processing phases were kept unchanged for
all three objects. Therefore, for sake of clarity, we will describe
the process carried out on a single object, specifying instead
from time to time the statistical indices obtained for the
different alignments.
For every object, we acquired three sets of data:
1) 12 scans (setting the movement of the rotating stage to a 30°
angular step) placing the object leaning on its side,
2) 6 scans (angular step of 60°) with its main side facing up;
3) 6 scans (angular step of 60°) with its main side facing down.
In this way, we were able to acquire data with the right focus on
the entire object.
Given the complex data acquisition, the processing phase was
more difficult than the one carried out on the original object,
but we tried to keep the workflow as unaltered as possible.
Therefore, we decided to first process the three datasets
separately and subsequently align and merge them.
For every dataset, each single scan was first cleaned, then
realigned. The precision on the alignment of the single scans for
each one of the three datasets was good for all the replicas: we
obtained an average distance of approximately 0.03 mm with a
standard deviation of 0.04 mm. After the alignment, the scans
were merged into a single mesh, obtaining three data for each
replica.
The three meshes obtained were aligned together into the same
coordinate system. The noisiest parts were then deleted in order
to keep just the best data for each part of the object. The
precision in this second alignment had different results: the
monochromatic replicas showed an average distance of 0.03
mm with a standard deviation of 0.07 mm; the coloured one
showed an average distance of 0.07 mm with a standard
deviation of 0.10 mm.
As with the original object, because of its geometry and the
high number of scans, we did not need to perform a long post
processing phase. There were just a few small holes we had to
fill and there was no need to decimate the meshes.
3.2 Comparisons
The digital data of three printed objects acquired were then
compared to the one that was used for 3D printing.
The module used for the analysis of the two data sets is
contained within the alignment menu of Geomagic Studio
(Geomagic), which uses the same ICP algorithm used for the
orientation of the scans, and it allows displaying a series of
basic information for analysis: the maximum distance between
two comparable points in the two meshes, the average distance
and the standard deviation.
The two models obtained by the acquisition of the
monochromatic printings show comparable results (table 1):
Grey Nylon
White Nylon
Maximum distance (mm)
0.276
0.352
Average distance (mm)
0.035
0.046
Standard deviation (mm)
0.036
0.044
Table 1: Statistical indices obtained from the comparisons of
the first two replicas
A part from some small differences between the two
comparisons, probably caused by global alignments, the data
are concordant. As we can see from figures 9 and 10, the flat
areas are more precise compared to the most complex ones. An
interesting thing to note is the difference on the engraved parts:
in the printed models they appear less deep, which is a clear
sign of the decreasing of precision in 3D printing.
Figure 9: Results on the Nylon PA 12
Figure 10 : Results on the alumide
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
This contribution has been peer-reviewed.
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59

On the other hand, the one printed with the multicolour printer
shows an average distance nearly one order of magnitude higher
than the other ones (table 2):
Grey Nylon
Coloured
Maximum distance (mm)
0.276
0.400
Average distance (mm)
0.035
0.096
Standard deviation (mm)
0.036
0.065
Table 2: Statistical indices obtained from the comparisons of
the grey nylon replica and the coloured one
Figure 11: Results on the coloured replica. Above: the first test;
below: the second one.
In order to be certain of the results, we performed another
acquisition of the coloured printed object. In this way were able
to discard possible causes of imprecision, such as the variation
of the instrument temperature and a non-correct alignment of
the single scans.
Although with small differences, this second test confirmed the
over-all results of the first one (fig. 11). The printed model
appears generally bigger than the original one, probably caused
by the printing process itself. In fact, the printer first creates a
uniform layer of powder and then it colours it with another
head. This two processes could easily lead to the creation of a
slightly bigger model. Quoting the website: “The print itself is
carried out layer by layer. A rolling batch leaves a uniform layer
of the sandstone-like powder. From there two printing heads
pass over the batch, coloring and adhering the object at the
same time. The batch of powder then makes another pass, until
the object is completed” (Sculpteo Multicolor Material).
4. CONCLUSIONS
In Cultural Heritage, 3D printing technologies have opened a
wide range of new possibilities both in terms of museum access,
and in terms of cataloguing and study, providing the basis for
visualization on the one hand and the analysis of shape and
geometry on the other. One of the advantages of 3D printing is a
high flexibility compared to the traditional process that leads to
physical reproductions. For example, before producing the
physical copy, the digital representation can be edited, scaled,
modified: its geometry could be altered in any possible way.
Replicas also have the advantage of being touchable without
damaging the conservative state of the original object, thus
providing a new way of interacting with the objects of art and
therefore learning from them.
Precisely because of this dual nature of the copy, which is used
for informational as well as scientific purposes, it must be
realized taking into account a series of factors: on the one hand
the realism and the verisimilitude of the reproduced object,
which must have texture, weight and appearance coherent with
the original, on the other the adherence of the shape, which is
expressed, in the Geomatics world, with the terms of precision
and accuracy of the printed model. Moreover, if solid printing
has become one of the possible products of a metric survey -
alongside the traditional representations in orthogonal
projection - the analysis of the process that from the acquisition
of the metric data leads to the creation of the digital model that
has to be printed becomes compulsory.
Within this process, techniques and methods of data acquisition
and processing can lead to a loss of adherence of the copy to the
original. Firstly, there are different precisions of the acquisition
systems, which, depending on the technique used, lead to the
formation of a point cloud that moves away from the real object
already in the acquisition phase.
Subsequently, data processing uses filters that, while leading to
the optimization of the model for printing or display, also lead
to a change in the geometry of the copy. The main ones are
decimation and smoothing, which are often inevitable
processes: the metric data acquired through photogrammetry or
laser scanning is frequently redundant with respect to the
purposes of physical reproduction and usually very noisy. From
time to time we must make choices based on the evaluation of
the number of points (and consequently the size of polygons)
and on the filters to be applied for the elimination of noise, in
relation to the quality of the final model.
This question is linked to another aspect related to the
adherence of the model to reality: what is the form of modelling
that allows a greater mimesis of the real object? Surface models
created by triangulation allow to obtain meshes based entirely
on the points clouds acquired. However, the processes
described above lead the triangular mesh to never maintain the
accuracy of the initial model. On the other hand, even if these
operations remove the digital model from the real one, they still
are inevitable steps to obtain a qualitatively good copy. On the
contrary, solid models constructed using graphic primitives are
also very far from the real object, because they are the result of
an interpretation.
In this paper, we chose to focus on the last step of the process:
the one related to the precision of printers. In fact, the
comparisons were made between the digital model of the
printed object and the original one after the optimization for
printing. However, beyond the precision of the instrument used
and the alignment of the scans, no post processing was carried
out through the application of filters. For this reason, the object
was considered an ideal application case for these analyses.
Moreover, its dimensions allowed to work on a 1:1 scale.
In our opinion, from the tests described above the discrepancy
between verisimilitude and metric precision is evident: the
coloured object is certainly much closer to reality from a
qualitative and descriptive point of view, but from a metric
point of view it is the one that presents greater deviations from
the original geometry.
The precisions obtained are comparable with other tests
previously developed by the Photogrammetry Laboratory (see
for example Balletti et al., 2017). Further analyses are currently
under way on objects of different typologies and scales (e.g.
statues or architectural monuments) always in a museum
perspective. In order to analyse not only the accuracy of the
printing, but also to validate the process that leads to the
formation of the digital model, different acquisition techniques
and different elaborations methods will be compared.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
This contribution has been peer-reviewed.
https://doi.org/10.5194/isprs-archives-XLII-2-55-2018 | © Authors 2018. CC BY 4.0 License.
60

ACKNOWLEDGEMENTS
This work has been carried out under the GAMHer project:
Geomatics Data Acquisition and Management for Landscape
and Built Heritage in a European Perspective, PRIN: Progetti di
Ricerca di Rilevante Interesse Nazionale – Bando 2015, Prot.
2015HJLS7E.
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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
This contribution has been peer-reviewed.
https://doi.org/10.5194/isprs-archives-XLII-2-55-2018 | © Authors 2018. CC BY 4.0 License.
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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy
This contribution has been peer-reviewed.
https://doi.org/10.5194/isprs-archives-XLII-2-55-2018 | © Authors 2018. CC BY 4.0 License.
62