International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
5.4 Complete line drawings from 3D models
Additional lines are needed to complement the outlines if a line
representation is needed. If a drawing similar to a stereo plot is
desired, the rules a stereo plotter operator uses to select and
draw lines have to be analyzed. If these rules are known, a
mathematical model can be created which selects the proper
lines from the 3D surface model accordingly. Trying to find
those rules is possible only to a certain extent since every
operator will select different lines intuitively and without a
fixed set of rules. In addition to the outlines, all sharp edges are
significant for the object and will undoubtedly be selected.
Other features (e.g. eyes and other parts of faces) are important
to understand the drawing. Therefore, an operator will add lines
to show such elements although there is not enough geometrical
evidence in the 3D model to justify those lines.
If a procedure is to be developed to select meaningful lines
from a meshed 3D model, it has to detect edges (in addition to
the outlines). Edges in meshed models can be described as local
curvature which can be quantified by the angles included
between neighboring triangles. It cannot be expected that the
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resulting plan will have the quality of a plan produced by an
operator with artistic skills, but if this product can be derived
automatically, it can serve as a template which can be used as a
background for drawing a meaningful and expressive 2D line
drawing with metric qualities.
The software developed at i3mainz computes the normals for all
triangles of a mesh. For every triangle, the spatial angles to the
three neighboring triangle normals are identified and a weighted
mean value is computed. Positive (convex) and negative (con-
cave) values can occur. Then, a histogram of all values is
produced. Based on this, the user can define a number of classes
for local curvature and select intensity values (green for convex,
red for concave) which are used to shade the triangles
concerned. This can easily be accomplished if the OBJ format
is used for the mesh. Since shaded triangles are used, the result
is not a true vector plan. This is not a real problem since — for
the reasons mentioned above - the result cannot be used as a
final drawing anyhow. Instead, it is used together with the plan
of the outlines as a background for the creation of the final
vector product.
Fig. 6: Left: Part of a virtual relief model. Center: Automatically detected outlines. Right: Automatically detected edges;
unfortunately, the red and green colors (for concave and convex edges) cannot be shown in this black-and-white print.
6. ACKNOWLEDEMENTS
We want to thank Westcam Datentechnik GmbH for the fruitful
cooperation during the measurement process. Linsinger
Vermessung, a private surveying company, did an excellent job
performing the photogrammetric documentation of the
monument. Monica Bordas Vicent from i3mainz
conscientiously created the 3D models from the point clouds.
Mirko Siebold and Stefan Tschópe, also from i3mainz,
developed the software for the line drawings. Our
acknowledgement goes - last not least - to the local authorities
of Tyrol (Land Tirol, Landesbaudirektion) for the financial
support and the continuous help solving the administrative
problems.
7. REFERENCES
Boehler W., Bordas Vicent M., Hanke K., Marbs A., 2003:
Documentation of German Emperor Maximilian I’s Tomb.
CIPA Symposium 2003, Antalya, Turkey. The ISPRS
International Archives of Photogrammetry and Remote Sensing
Vol. XXXIV - 5/C15 (ISSN 1682 - 1750) and The CIPA
International Archives for Documentation of Cultural Heritage
Vol. XIX - 2003 (ISSN 0256 - 1840), pp. 474-479.
Hanke, K. 2003: Dokumentation des Grabmals Kaiser
Maximilian I. in der Innsbrucker Hofkirche. In: Chesi, Weinold
(Hrsg.) "12. Internationale Geodátische Woche Obergurgl
2003". Wichmann- Verlag. ISBN3-87907-401-1
Marbs, Andreas, 2002: Experiences with Laser Scanning at
i3mainz. Proc. of the CIPA WG6 Int. Workshop on Scanning
for Cultural Heritage Recording.
http://www.isprs.org/commission5/workshop/
i3mainz, 2004: 3D scanning web site:
http://scanning.fh-mainz.de
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