Apoxyomenos, an ancient bronze
statue of an athlete scraping off oil and dust from his body after
a competition, was found in 1999 in the sea close to Lošinj, a
Croatian island (Fig. 1). Investigations suggest that it might
be an original Greek statue dated back to the 4th century BC,
or a Roman copy of this statue. Since only a few original Greek
bronze statues remained, Apoxyomenos is an extraordinary example
of the world cultural heritage, therefore restoration and determination
of its origin is of great importance. Despite structural damages,
erosion and fused sediments caused by centuries of underwater
stay, specialists in the Croatian Conservation Institute in Zagreb
already succeeded in bringing the shine of the statue to light.
Fig. 1 Statue of Apoxyomenos
lying 40m beneath the surface of the Adriatic Sea
In January 2002, the company "Topomatika"
d.o.o., from Zagreb conducted full 3D-digitizing and measurements
of the statue in the Croatian Conservatory Institute. The sculpture
of Apoxyomenos is full of very fine and realistic details which
cannot be captured by measuring its length, width, height, diameter,
angle, etc. The beauty of the shape and the richness of details
can be recorded by photo documentation and sketches but the result
is only a two-dimensional outline of the sculpture, failing to
give enough information about its shape and measures. Only digitization
provides for an accurate recording of the sculpture's shape by
gathering accurate and dense measuring points on its surface.
This task was successfully carried out using the ATOS and TRITOP
systems from GOM (www.gom.com).
The statue of Apoxyomenos was digitized in individual
measurements using the ATOS II system, with a typical measuring
area of 350 x 280 millimeters. During each of these measurements,
multiple fringe patterns were projected on the object and the
resulting images of the object area were recorded by two high
resolution cameras. The data gathering for one area typically
takes 10 seconds and typically leads to one million data points,
defining the shape of the measured area very accurately. Then,
the individual measurements were registered in a global coordinate
system based on reference points (markers), applied on the fixtures
and a few on flat areas on the object. The exact position in space
(3D coordinates) of these reference points was determined before
scanning using the photogrammetric system TRITOP (Fig. 2).
Fig. 2 Reference points and camera positions used
to define the reference points' exact position with TRITOP
Fig. 3 Three-dimensional digitizing of the statue
of Apoxyomenos with the ATOS II system
Since the complete restoration of the statue
of Apoxyomenos is still in process, scanning had to be made in
the actual position of the sculpture. First, the front side was
scanned (Fig 3), then the sculpture was turned upside-down in
its holder for scanning the backside. As the head of the statue
is separated from the body, it was scanned separately (Fig. 4).
The results of all three scanning sessions were combined in a
global coordinate system using common reference points, giving
the complete shape of the sculpture.
Fig. 4 Digitizing of the Apoxyomenos
head with ATOS II - the projected fringe pattern and the
reference markers are visible
The bronze surface of the sculpture was very
dark and reflective. For standard engineering work the objects
and tools can usually be treated (sprayed or painted) to show
a uniform dull surface, ideal for optical scanning. Here a surface
treatment was not acceptable. The ATOS II system can be set to
gather data with different exposure times in one measurement to
capture data on dark and bright areas. For glossy areas, the shape
data can be calculated based on the images from one camera only.
Using these parameter settings, the statue could be successfully
scanned with ATOS II, capturing very accurate and dense data,
thus proving its great potential for scanning of shiny and dark
surfaces. To digitize the complete statue 114 scans for the body
and 57 scans for the head were needed, some of these scans were
needed only for small areas, to define the complete shape of detailed
parts such as hair or fingers.
The result of the complete digitization is a
file with more than ten million data points connected to a polygonal
mesh (STL data) as shown in Fig. 5-7. Due to the high resolution
scanner (typically 13 measuring points per square millimeter),
all details of the sculpture including damages are visible. Based
on the digitized data, parallel sections can be easily computed
as shown in Fig. 8.
Fig. 5 Scanned data of the head
Fig. 6 Detail of the scanned data
Fig. 7-8 Complete scanned data of Apoxyomenos
Three-dimensional digitizing in this quality
is a digital copy of the real part in the actual moment. It enables
to document the restoration process, enables shape analyses, computer
presentations and monitoring the shape of the statue during time.
But also highly accurate copies of the statue or optimal holders
for its transportation can be produced based on these data. This
highly accurate computer model rich in detail was obtained by
non-contact measurement and without any treatment of the bronze
surface of Apoxyomenos, so, at no time during the scanning process
the sculpture's safety was endangered.
The measuring systems ATOS II and TRITOP once
again proved as a valuable asset in three-dimensional digitizing
of complex and delicate parts.
We would like to thank the Croatian Conservation
Institute for their trust and the permission
to show this report to the public.