Modern technologies in 3D scanning allow us to reconstruct 3D digital representations of real objects in a semi-automatic way, with high precision and wealth of details.

An accurate digital model of Michelangelo’s David has been created during the Digital Michelangelo project  (1999-2000), coordinated by Professor Marc Levoy from Stanford University.

This model has been made using a custom Cyberware laser scanner and post processing software developed by the Stanford’s Computer Graphics Lab. Acquisition and reconstruction required a long time because of dimensions/complexity of the modelling and also due to the pioneering status of the technology.

The availability of an accurate digital representation opens several possibilities of utilization to the experts (restorers, archivists, students), the students or the museum visitors. Virtual presentation and interactive visualization are in general the first uses of these data, but we think that the use of 3D models should go beyond the simple possibility to create synthetic images.

An important application of 3D models should be in the restoration of artworks. The integration between 3D graphic and restoration represents an open research field and the David restoration project has given several starting points and guidelines to the definition and development of innovative solutions.

Our activity is based on problems and specific requests suggested by restorers and briefly described as follows. The 3D digital models are used in two different but not subsidiary modes: as an instrument for the execution of specific investigations and as a supporting media for the archival and integration of multimedia data, produced by the different scientific studies planned during the David restoration.

Surface exposure characterization. We simulate the fall of contaminants (e.g. fall of rain, mist or dust) on the David’s surface, which depends on surface slopes, self occlusion and accessibility. A simulation has allowed us to come to several qualitative and quantitative results, useful to characterize the David’s surface.

The falling directions of the agents is modelled with a random fall direction, distributed around the vertical axe of the statue according to an angle “alpha” which defines the maximum fall inclination

The images below show some results (5 degrees of amplitude on the left, and 15 degrees on the right; different exposure are visualized using a false-colour ramp, with red=absence of fall, blue=high density of fall).

Numeric data were also produced (table and graphs).

 

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Simulation with alpha<= 5 degrees	Simulation with alpha<= 15 degrees

 

Digital measures. Some important data for the evaluation of the static conditions of the artwork were measured or computed directly on the digital model. Among them: the barycentre of the statue, the vertical falling line of the barycentre, the volume (2.098 cubic meters) and mass, the global surface area (19.47 squared meters).

These measures have been taken directly on the digital model, after performing some necessary elaborations (“plumb line” registration of the digital model; removal of open holes in the digital surface, associated to many small surface sections not sampled by the 3D scanner; geometry simplification).

The statue’s barycentre is placed in the interior of the groin, in the pelvis (see images below). The vertical projection of the barycentre on the base (plinth) of the statue is the blue line which exits from the marble on  the high posterior part of the left thigh and enters again in the marble on the right foot.

 

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A second important use for the 3D models is to consider them as instruments of documentation, organisation and presentation of the restoration data. During the restoration campaign, a number of scientific investigations will be performed (which will be in part repeated again with the passing of time, in order to have control over the status of preservation of the artwork). These include chemical analysis, stone characterization, UV imaging, precise characterization of the marble color, etc.

Moreover, the restorer (Agnese Parronchi) is performing a precise survey on the status of the David. She is drawing annotations on a set of high resolution photos which cover all the surface of the David. These annotations describe in a very detailed manner the presence of: imperfections in the marble (small holes or veins), deposits (e.g. brown spots), surface consumption and traces of the workmanship.

All these data will be organized and made accessible through a system implemented with web technology, easy to consult and compare. The 3D digital model will be used both to build a spatial index to those data (pointing out their location on the surface of the statue) or to map some correlated information on the statue surface.

As a first preliminary result, the images below show a mapping on the 3D model of images taken under ultra-violet (UV) illumination (left) and standard colour images (right).