CONCLUSION
In our study we
compared manual and automated photogrammetry and micro-CT scanning in terms of
accuracy, reproducibility, and efficiency in 3D surface generation of a typical
vertebrate fossil. The following summary presents our conclusions and
recommendations that are in general accordance with the recommendations made by
Mallison and Wings (2014) and Sutton et al. (2014).
3D surface comparisons and documentation of processing and training times
revealed that manual photogrammetry has a high degree of reproducibility and is
the most efficient and least costly method of those tested, given that
appropriate lighting and camera settings are chosen in order to ensure accuracy.
Manual photogrammetry is generally recommended if only the external surface of a
specimen is of interest to the researcher, and especially in case of a small
budget. However, one needs to account for training and practicing time prior to
the application of this method.
Automated photogrammetry yielded results similar to manual photogrammetry but
larger data volumes. If one is interested in an online object panorama
(presentation and curation purposes) or in a 3D surface, but does not have the
time for practicing or data processing, MDS Witikon offers a convenient
digitization method ranging in costs between manual photogram-metry and micro-CT
scanning.
Generally, when using photogrammetry, back-ground color should contrast the
color of the specimen, allowing for correct distinction between the two during
surface generation.
CT scanning (including micro-CT) has its strength in the reconstruction of
internal structures. While it is possible to extract external surfaces from CT
scans, extremely long post-processing times, lack of photo-realistic surface
colors, and high costs make this method less suitable for external 3D surface
generation than the other methods tested. (Micro-)CT scanning should only be
used for external surface generation if no other means is available. We would
also recommend the additional use of manual photogrammetry, even if CT scans are
available, if both internal and external surfaces are to be digitized.
ACKNOWLEDGMENTS
First, we thank Oliver Hampe and Thomas Schossleitner (Museum für Naturkunde
Berlin) for access to the specimen, and valuable background information and
discussion. We also thank Irina Ruf (Senckenberg Naturmuseum Frankfurt am Main,
formerly Steinmann-Institut Bonn) and Georg Oleschinski (Steinmann-Institut
Bonn) for micro-CT scanning, and Daniel Baum (Zuse Institute Berlin) for
discussing surface registration techniques in ZIB-Amira. Heinrich Mallison
(Museum für Naturkunde Berlin) is thanked for taking one set of photos used for
photogrammetry and for helpful suggestions. We furthermore thank René Guráň,
Róbert Sičak, Paul Safko and Mira Silanova (EDICO SK, Bratislava) for digitizing
our specimen with MDS Witikon, and Daniel Girardeau-Montaut (Grenoble, France)
for constructive comments regarding the use of CloudCompare. Funding was
provided by the Deutsche Forschungsgemeinschaft (DFG FA 889/2-1) and the
Alexander von Humboldt Foundation (Feodor Lynen Return Fellowship for JMF in
2012).
Publication costs were covered thanks to a grant from Transmitting Science to
the Journal of Paleontological Techniques.
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