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Another condition for point thinning is based on the fact that 
less points are needed on smooth surface regions than in the 
neighborhood of edges. This condition may be introduced at 
this stage of the process or - especially in the case of a subse 
quent creation of a triangular mesh - at a later processing stage. 
3.7 Simple plots derived from point clouds 
Usually, point clouds are not sufficient as a final documentation 
result. They might be considered as a means of archiving the 
geometry of an object, however, much in the same way as 
photogrammetric images can be archived for later processing. If 
this is planned, the point cloud data should be evaluated careful 
ly and standards should be demanded, defining if and how the 
procedures mentioned above (cleaning, filtering, registration, 
thinning) have to be applied before the data are archived. 
If cross-sections are required, those can be derived from the 
point clouds by selecting a thin layer of points near the plane 
where the section is needed. The generation of lines connecting 
the points can be accomplished in 2D if an appropriate tool is 
supplied with the scanning software or after importing the 
selected points into a CAD program. 
4. SOFTWARE FOR FITTING OF PRIMITIVES 
If the scanned object consists of parts of geometric primitives 
(such as planes, cylinders, cones, spheres, ...) or its geometry 
shall be reduced to such a simple representation, these 
primitives must be fitted to the point clouds belonging to the 
respective object parts. CAD programs, which actually should 
provide appropriate tools, are presently not able to handle the 
large numbers of single points supplied by 3D scanners. 
Possibly, it should not be too difficult for the developers to 
improve the performance of these software products to the new 
requirements arising from the 3D scanning techniques. 
Fitting software solutions supplied for 3D scans differ consider 
ably as far as performance is concerned. If there is a foreknown 
collection of primitives (e. g. in the case of an arrangement of 
pipes with standard diameters in an industrial environment), 
automatic fitting methods are available which need only little 
manual input and editing. This applies also to much more com 
plicated structures (such as flanges, pumps, and steel girders) if 
certain types (only) are known to be present as parts of the 
whole object. Some software products even supply links to re 
spective 3D CAD catalogues. If primitives with different para 
meters have to be expected, the software must provide means to 
manually pre-select those parts of the point cloud which belong 
to a primitive to be fitted. Since this selection has to be accomp 
lished in three dimensions on a two-dimensional screen, it is 
important that this task can easily be handled and that it is toler 
ant against points included erroneously into the pre-selection. 
In cultural heritage documentation, primitives are usually not 
sufficient for the representation of the objects encountered. 
Classical architecture could be treated this way, but 3D scann 
ing does not really seem to best method for its documentation. 
5. SOFTWARE FOR THE CREATION OF COMPLEX 
SURFACE MODELS 
5.1 Polygonal Meshing 
2'AD versus 3D. Polygonal meshing is a standard method to 
achieve a geometric description of an irregular surface. The 
method is well known to surveyors and photogrammetrists from 
topographic applications. It has to be realized, however, that for 
topography 2AD solutions are sufficient, i. e. the whole geome 
try can be described attaching height values (vertical distances 
from a reference plane) to 2D vertices defined in this plane. 
This method, included in some 3D scanning software as only 
meshing alternative, can only be used for modeling 2 l AD 
objects such as half reliefs. True 3D polygonal meshing, on the 
other hand, must be applicable for closed objects, like statues, 
and allow the modeling of hollow and open object structures 
(such as the inside and the handle of a jug). Since this is a rather 
complicated process, it is not surprising that the solutions 
offered presently leave much to be desired. Computing time can 
be excessive and the geometry of the results does not always 
represent the wealth of information contained in the original 
point cloud. 
Point thinning. Point thinning has already been discussed in 
sections 3.3 and 3.6 of this article. There, the thinning proce 
dure served to eliminate excess points originating from high 
point density due to overlapping scan areas or short object 
distances from the scanner. Since these thinning conditions are 
based on the scanning geometry (see above), it seems advisable 
to run this process prior to meshing. Nevertheless, all thinning 
operation steps can be combined into one procedure executed 
immediately prior to the mesh triangulation process. It should 
be noted, however, that the conditions for point thinning in 3D 
modeling are different: It is economical to leave a dense point 
pattern near curvatures and edges whereas only few points are 
needed in smooth areas of the same object. This is contradictory 
to the algorithms of some 3D meshing programs which demand 
an even point pattern over the whole object. 
Operation and performance. In theory, the whole 
triangulation process for meshing should run automatically after 
the user introduces a parameter defining in one way or another 
the resolution of the 3D object to create. In reality, many 
software products demand much interaction by the operator, e. 
g. because of the requirement to define sequentially partial 
areas of the object where a local 2AD meshing has to be 
performed. Often, unreasonable triangles are created or holes 
are left in the mesh although enough points were available in 
the region. 
Edges. If the scan resolution was high enough, the software 
should detect edges by itself, making sure that points close to 
edges are not deleted and that the sides of the mesh triangles do 
not run across an edge. Since edges are not always perfectly 
defined, the user should have the opportunity to define edges 
where these conditions have to be enforced. 
Editing. Some problems originating from the scanning process 
(such as wrong points, noisy data, or concealed areas) as well as 
problems arising from the mesh triangulation (holes, unreason 
able triangles) make it necessary to edit the triangular mesh. 
Procedures to detect problematic areas as well as intelligent 
editing tools should be provided in order to come to a satis 
factory result within a reasonable amount of time. 
Expenditure. Several software products are available for 3D 
polygonal meshing. Their flexibility and performances vary 
considerably. The generation of an acceptable result (i. e. 
including editing) may be accomplished in 10% of the time 
with one product as compared to another one, whereas other 
products again may not be able to solve the problem at all.