"ΕΝΗΜΕΡΩΤΗΣ"
ΔΗΜΟΣΙΕΥΜΑ Νο
[
02165
]
[
2012.01.12 00:00
]
Jean-Luc
Martinez, an archaeologist with the
Musée du Louvre,
asked the INSIGHT team to help document and reconstruct a fragmentary
column from the Temple of Apollo at Delphi, Greece. The goal
of the reconstruction was to validate Martinez' hypothesis that
the Omphalos (a
well-known sculpture
meant to mark the 'center' of the Hellenic world) once rested
atop the dancers' column. As the column is now in pieces, the
INSIGHT team was faced with the monumental task of capturing
dozens of individual fragments while maintaining exceptionally
high detail in the resulting data.
Overview
The
INSIGHT data presented here is detailed in a technical paper
presented at the 5th International Symposium on Virtual Reality,
Archaeology and Cultural Heritage (K. Cain, Y. Chrysanthou,
F. Niccolucci, N. Silberman Editors):
A Point-Based Approach for Capture, Display and Illustration
of Very Complex Archeological Artefacts
(Florent Duguet, George Drettakis, Daniel Girardeau-Montaut,
Jean-Luc Martinez, Francis Schmitt).
Guillaume
Thibault and Jean-Luc Martinez also contributed a chapter titled
La reconstitution de la colonne des danseuses de Delphes
to the book
Actes du Colloque Virtual Retrospect 2007
(Vergnieux, R. and Delevoie, C. Editors), which makes extensive
use of the INSIGHT data.
3D Scanning and Post Processing
3D capture for archaeological objects has become a routine
research tool for many groups around the world. While the process
remains relatively expensive, it is possible for well-trained
teams to produce good quality results for modestly sized objects
using close-range scanners. It is equally possible to do the
same for whole sites, creating solid results (at slightly diminished
resolution and accuracy) with long-range scanning hardwar. This
equipment has become robust, user-firiendly, and even (depending
on your location) ubiquitous.
Artifacts
such as the Delphi Acanthus column, however, fall in a no-man's
land between close-range and long-range capture. As with similarly
scaled sculpture documentation projects (notably, Dr. Marc Levoy's
Digital Michelangelo Project),
we decided to use close-range optical triangulation scanning.
This necessarily led to the production of many thousands of
individual scans, as the quality of optical triangulation techniques
diminishes with distance.
Guillaume
Thibault wished to maintain high accuracy in the data, so that
archaeologists could clearly see potential connections between
pieces. In particular, the 'unwrapped' point visualizations
Thibault wished to create (see above, right) are highly sensitive
to outliers and noise. (The dark gray area seen in the center
of the image is one such artifact, where sub-millimetric disagreements
between point sets are easily seen.) Typically, a single isosurface
is extracted from a group of point clouds, resolving any conflict
between onion skin layers of scan data; but in this case Thibault
was developing a point cloud viewer without familiar front surface
culling. Therefore, high accuracy was an absolute requirement
and was defined in the following way: each point in the data
set was required to fall within the implicit surface defined
by the set of all points, to an accuracy of less than one millimeter.
This is not the same thing as imposing a requirement that all
scans have one millimeter accuracy -- it's a much stronger criteria.
Each of the thousands of individual scans was required to align
with all its neighbors to within single millimeter accuracy.
One strategy to accommodate this kind of requirement is simply
to
bend each scan as needed
during the alignment phase. Non-rigid alignment techniques guarantee
alignment locally, but since they do not increase accuracy,
we decided against that appproach.
Instead,
we worked with Youda He of
Geometry
Systems
to create a new technique for aligning point clouds. In most
approaches to
iterative closest point alignment,
the goal is to minimize the distance between representative
points in each point cloud being aligned. When iterating over
each cloud in a large group, the global result is that well-aligned
clouds are drawn together. However, this necessarily causes
gaps between those clouds with poor alignment. As closing these
'clamshell' gaps is required in order to maintain high quality
alignment, we implemented a variant of ICP where the the routine
maximizes, rather than minimizes, the error between pairs. Provided
that the initial alignment was relatively good, we found that
the maximum error ICP approach will efficiently close 'clamshell'
regions of poor alignment and eventually converge to a highly
coherent alignment. This key innovation made it possible for
us to register thousands of scans in the data set while maintaining
the extremely high quality noted above. Importantly, the maximum
error ICP was not all that was required. Hundreds of hours of
manual point cleaning and carefully choreographed grouping of
aligned regions were required, as well as section traversal
tools to quickly identify alignment errors in point clouds.
Once the point clouds from all the constituent objects had
been successfully aligned, it was now possible to test Jean-Luc
Martinez' theory. Moving the Omphalos into place above the dancers
(see below, the placement and proportion of the two sculptures
was greeted favorably by Martinez and his team. In addition
to the value of testing this theory, the project also provided
the data needed to visualize and notate the objects themselves.
Archaeologists had been attempting an accurate drawing of the
complex Omphalos for many years; by unwrapping the 3D scan data
into a plane, an accurate drawing was at last possible (see
above).
A range of visualizations were made from the final aligned
data sets. Florent Duguet and George Drettakis rendered the
images above and below. Many more examples are contained in
the papers noted at the top of this page.
ΠΗΓΗ:
ΣΧΕΤΙΚΑ ΔΗΜΟΣΙΕΥΜΑΤΑ ΤΟΥ [ΠΑ.ΣΥ.Α.]
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Νο
[
00001
]
[
2009.11.07 12:00
]
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[
00002
]
[
2009.11.09 00:00
]
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00001
] [ 2009.11.28 00:00 ]
"ΠΕΡΣΕΥΣ"
ΔΗΜΟΣΙΕΥΜΑ Νο [
00010
] [ 2011.02.18 00:00 ]
