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Figure 2: Parameterization of the building primitive with 
gable roof 
  
2. Projection of the wire frame model of the primitive 
into the image section after hidden line removal. This 
projection is based on approximate values for the 
height and default values for the parameters of the 
primitive. If it is not the first primitive the height of 
the base level of the previous primitive is used. If the 
primitive has already been selected, the old parame- 
ters are used as default. 
3. Interactive adaptation of the form of the wire frame 
model to the image content (cf. below). During this 
action the pose and form parameters are determined 
and the primitive is visualized in real time using an ap- 
proximate height of the primitive. The operator may 
choose that resolution of the image which is suited 
best for this interaction. 
During this process a snap-function can be activated. 
Based on some user specified snap-radius relations to 
already extracted primitives are automatically estab- 
lished and in case they are found shown to the oper- 
ator and visualized at the screen. This enables gluing 
of points, edges and especially faces of neighboring 
primitives. 
The accuracy of this adaptation can be reduced if a 
subsequent fine tuning takes place. 
4. Height determination using either an automatic tool 
(cf. below) or interactively a second image if it is the 
first primitive within the CSG-model of the building. 
Optional, the parameters can be fine tuned using a 
least squares adjustment (cf. below) after matching 
the wire frame model to the edges extracted in all 
available images. 
5. (optional) Visualization of the parts extracted so far 
for checking the result. 
Only the automatic fine tunig in step 4 requires digital im- 
ages. If a sufficiently accurate adaptation of the model to 
the images is available the procedure can also be realized in 
an Analytical Plotter with Stereosuperimposition. 
Extracting Prisms. The extraction of prisms relies on auto- 
matically extracted image edges. They have to be provided by 
some edge extraction procedure. The operator interactively 
selects a sequence of image edges representing the ground 
plan at eave level, thus assuming all the selected image edges 
to lie in a horizontal plane. Neighboring image edges are 
linked by intersection. In case of strong shadows where no 
image edges are available, the operator' also can define break 
points of the polygonal ground plan. The geometry also may 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
  
  
|select operation and model iei model data base 
A Y 
  
  
  
  
  
  
  
  
  
| project | 
Y 
| adapt pose and form | 
A Y 
| determ. height and tune fine | 
Y 
L operator interaction | | visualize | 
  
Table 1: Steps of semiautomatic building extraction 
Figure 3: Extracted building block consisting of two saddle 
roof primitives, a box and a pyramid 
  
be changed interactively if necessary. Using an automatic tool 
or by interaction the height of the eave level is determined. 
Using the same procedure as for interactively adapting the 
parameters of CSG-primitives the height of the prism, i. e. 
the ground level is found. 
2.3 Parameter Adaptation 
The interactive adaptation of the form and pose parameters 
is performed in monocular mode. Stereo display is not nec- 
essary. An extension of the interactive steps for the case of a 
stereo display is simple. 
The adaptation of the parameters is done in a sequence of 
steps, each specifying one or two parameters of the model. 
The parameters are changed in dependence of the sequence of 
two points specified by mouse clicks and in dependence of the 
type of primitive. Therefore the number of interaction steps 
is between 50 % and 100 % of the number of parameters. 
Which parameter actually is to be adapted is stored in a so 
called association table setup to support intuitive interaction. 
The association table is build up according to the following 
rules: 
e The first of the two points chosen is the reference 
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