Cl PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
460
These initial problems were rather easily solved using larger
overlaps, which strongly reduce sliding and provide much better
positional accuracy. Better selection of photographs depending
on the angle of view for each side overcomes the problems of
small deformations. Similar work with stereo pairs and a stereo
plotter would probably had better results in terms of accuracy,
but time for triangulation, on site photography and control
measurements, along with manual collection of points would
exceed 70 hours (10 pairs). Hence time and cost savings are
obvious.
Figure 5. Ygeia's head, scanned twice. Small deformations and
a significant change of size and shape are noticeable.
3.4 The special case of Kouros
Kouros was a special case considering the volume of data, the
size of the object (1.86 m) and the reproduction scale of 1:1. A
plaster copy was provided by ARF (Archaeological Receipt
Fund). Photography took place in ARF's laboratories.
It must be noted that more complex geometries were tested (two
statues from Bremen Museum, sized 1.4 and 1.6 meters
respectively). Photography took place in site, but during
processing undercuts and extending arms made modelling
almost impossible. That’s the reason Kouros was finally
selected for testing.
3.4.1 Photography
The body of Kouros is of rather simple geometry (fig. 6) but it
is necessary to maintain characteristics in detailed parts, such as
head and feet. Hence two different densities were used. 441
digital photographs were acquired in two days. Ten different set
ups of camera and projector were required in order to cover
every part and aspect of the object. Setting up the projector so
that the grid is dense enough and well focused, in conjunction
with the well focused camera covering as mush of the area and
keeping imaged grid crispy, was the most time consuming
procedure. Since a replica of the original was used, it was quite
easy to handle and rotate it (fig. 2).
3.4.2 Computer Processing
In the beginning of the processing a clear problem has risen.
Lens distortion was not mathematically modelled within the
software and therefore the digital model appeared curved (fig.
6). In order to overcome this problem, distortion correction has
to be taken into consideration prior entering the images into the
software.
Lens distortion forces straight lines in real world to be imaged
as curves in the photograph. Therefore if a number of straight
lines are photographed, then is it possible by measuring points
on them over the image to calculate the lens distortion
parameters (Karras et al., 2001). The straight lines of the grid,
when projected over a flat surface should remain straight. That's
the case with the calibration box, which has two flat panels, and
therefore the projected grid should remain straight over each
panel. This information can be used for a pre calibration of the
images (Sechidis et al., 1999), which could be applied in order
to produce new “calibrated” images.
It must be noted that the photographs record the result of two
lens distortion effects; one from the projector over the object
and a second one from the camera itself. In the general case the
combination of these two lens distortions cannot be combined
under the single lens distortion mathematical model. Since
development of new lens distortion models was not the purpose
of this project, the simplified model used managed to improve
the 3d model (fig. 6).
The lens distortion was being calculated by the straight lines in
the calibration image and then applied in all photographs of the
particular set-up.
Figure 6. Kouro's digital model prior (left) and after (centre)
lens distortion correction, along with the
reconstructed model from resin (right).
For the final model 98 images were used for an equal number of
independent surfaces. Processing the full model with one
million points was not an easy task for the software, and
therefore the final manipulation of small gap filling, refinement
and stitching has been made externally. Since sliding along the
independent parts was a clear danger, the digital model was
measured in height, to ensure that there will be not essential
difference. The four millimetre difference measured from the
original (measured with tape) is negligible and cannot be
observed even by experts.
One million points correspond in an average density of 1.3 mm.
Density in the head and toes was 0.7 mm, while on the body
was up to 1.5 mm. The final file in stl format has been send for
reconstruction. Since a number of problems have been
confronted during this project, it is impossible to have exact
time data. A crude estimation for a complete re-built of the
model is about 50 workdays.
3.4.3 Physical reconstruction
Physical reconstruction needed extra post processing in order to
translate data for the rapid prototyping machine (approximately