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Recognition of projecting objects 
  
  
  
Determination of orthoimage areas 
  
  
affected by projecting objects 
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Replacement of the affected areas 
  
  
  
  
  
  
  
  
Figure 3: Enhancement of orthoimage texture 
fitting the position of 3D objects interactively and fixing the 
shape automatically afterwards). 
4.2. Determination of Areas Affected by 3D Objects 
The areas of the orthoimage affected by the projecting 
objects are geometrically restricted by their polygons lying 
on the DEM surface. The polygons can be calculated using 
ray-tracing. The rays are defined by the center of projection 
in the case of displacements or the sun position in the case 
of shadows and the outlines of the 3D objects. 
As the shape of the 3D objects, especially of trees or hedges, 
is usually not known well enough the polygons of shadows 
or displacements will not be geometrically exact. Therefore 
combining the ray-tracing method with image processing 
methods like region growing or edge detection is suggested. 
The profit of image processing are geometrically exact 
polygons. The problem is the determination of the right 
polygons. One way to solve it for the case of shadows is to 
illuminate a automatically derived DEM with the given 
sun-angle, to segment the shadowed areas by thresholding 
and to use this regions as seeds for region growing in the 
image (Eckstein/Steger, 1996). 
4.3. Replacement of the Affected Areas 
After the derivation of their outlines, shadowed or occluded 
regions are replaced by more or less situation-independent 
image information. For the case of occlusion this informa- 
tion is taken from another image of the stripe or block, 
where the occluded area is visible. To fill formerly shad- 
975 
owed areas, information from adjacent regions of the same 
object is used. 
Orthoimage pixels that are both shadowed and occluded can 
not be replaced using pixels of another image, because the 
shadow will cover the same region considering a short time 
delay between capturing of the two aerial photographs. 
Thus first occlusions are replaced, filling shadowed areas 
afterwards. 
The result of this processing is called an enhanced or 
situation-independent orthoimage. Note that the effects of 
shading also caused by the position of the sun at the time of 
exposure are not taken into account. This enhanced ortho- 
image is used as texture information for all objects lying on 
the DEM surface. The 3D objects have to be modeled 
separately. 
5. PHOTOREALISTIC PRESENTATION 
OF 3D OBJECTS 
A great number of different 3D objects is present in real 
landscapes. Nearly everything is a projecting object, e.g. 
grass, stones etc. For photorealism, the biggest and most 
eyecatching objects have to be handled first. Smaller objects 
are tackled later. In rural landscapes big objects are farms, 
woods, hedges, trees, fields of cereals or corn. 
Experiences have been made modeling farm-buildings and 
trees. Mapping textures from terrestrial photographs to ob- 
jects with simple geometry already results in realistic rep- 
resentations of buildings (e.g. farm buildings consisting of 
six plane surfaces as used for the scene in Fig. 2). In general 
problems arise similar to that using digital orthoimages. For 
example trees or bushes in front of the buildings occlude 
parts of the walls or shadows are caused by projecting roofs 
or details of the buildings like chimneys (Gruber et al, 
1995). The same methods as described in section 4 can be 
used to get rid of these problems. 
For trees sophisticated algorithms like l-trees (Prusinkie- 
wicz,1994) lead to a detailed geometrical description giving 
photorealistic presentations. However, considering the 
available computer graphics facilities, they are not practi- 
cable for landscapes with numerous trees, hedges and 
woods. 
Therefore, the method of texture mapping was improved 
using photographs of the vegetation (photo textures). This 
method is based on the simple geometrical description of a 
vertical rectangle always facing the viewing position. It is 
called the billboard method and generates quite realistic 
trees or avenues (see Fig. 4). The attribute of alpha-transpa- 
rency is used as additional information within the texture. 
Transparent pixels can be defined around the outlines of the 
trees and within the foliage. For its separation from the 
terrestrial photographs of trees it is only necessary to gua- 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996