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.