International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BS. Istanbul 2004
5. VISUALIZATION
5.1 3D model versus photography
Virtual images from meshed 3D models can give a much better
perception of the object than photographic images of the real
object (see fig. 4, left and right image). This is especially true
when there is hardly any texture to the object as is the case with
sculptures from stone or bronze. Since the virtual model is
illuminated by virtual light sources which can be arranged
according to the desired effects, the resulting images show
details much clearer. If the lights (or the object itself) are gently
moved in an animation, the resulting changes and movements
allow a good 3D perception, even though the monitor image
itself is still 2D. If the whole object definition relies on texture
only, photographic images have to be used. The written text in
figure 3 could not be modeled from the scans, for example. In
such a case, photographs have to replace or complement 3D
scanning.
5.2 3D model versus line drawing
It has often been discussed whether metric line drawings are an
adequate means of documentation of sculptural works of art.
Nevertheless, this form of documentation is often asked for and
the considerable cost caused by the line drawing process which
is usually accomplished using stereo photographs is accepted.
Undoubtedly, this form of documentation can result in
remarkable interpretations of the art works (fig. 3 bottom, fig. 4
center). The selection of the lines to be drawn is a subjective
process carried out by the operators on their own ideas.
Complex surfaces are reduced to a set of lines. Metric quality is
present but cannot really be used if a complete reconstruction
should become necessary. The virtual 3D model from scanning
data, on the other hand, is a complete metric representation of
all surfaces involved and has a considerable visualization
potential at the same time (as can be seen in figs. 1, 2, 3 top and
4 right).
5.3 Outlines from 3D models
Outlines describe a 3D object's furthest extent in a specific 2D
projection. In stereo photogrammetry, these silhouette lines are
often very difficult to measure. If the images have been taken at
an angle to the selected projection and the object has no defined
edges (as is often the case with sculptures) it is nearly
impossible to find these lines correctly. Meshed 3D models can
be used to find the correct outlines. After the rotation into the
desired projection is accomplished, a program developed at
i3mainz selects all those lines that are common to a pair of
triangles where one triangle has a normal pointing towards the
observer and the other triangle has a normal pointing away from
the observer. After all hidden lines are removed, the resulting
line pattern will give a good description of the object if it has a
definite 3D structure (as in fig. 5). In the case of relief type
object parts, only few lines have an outline character (as in fig.
6 center).
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Fig. 5: Outlines of the cenotaph, automatically derived from the meshed 3D model.
