In the case a), Li and L5; are introduced with their 
end points determined using their lengths and 
their parallelity to L; and L,. 
In the case b), if there exist line segments L, parallel 
to and belonging to the opposite segment as L;, 
and L, parallel to and belonging to the opposite 
segment as L, (i.e. crosswise parallelity), both of 
which have a common point on the continuation 
of L5 at the other end point, the line segments L, 
and L; are introduced into object space using their 
lengths and parallelity to L; and L,. 
9) Do 6)-8) until no more line segments are 
introduced. 
10) Colinear lines. Look for a line segment L4 which is 
colinear to an already classified horizontal line 
segment L,. Assuming that L; and L, are colinear 
also in object space, the altitude of the end points 
of L4 is known. If they adhere to at least one of the 
following properties, L4 is introduced into object 
space. The properties to be tested are 
a) Another set of colinear line segments L; and 
L,can be found, where L,is already classified, 
and where Ljand L» are orthogonal to L; and L;. 
If L; connects to L; while L, and L, have the 
same altitude, L; and L; are both introduced 
into object space. 
b) Another set of colinear line segments L; and 
L, where at least L,is already classified (not 
necessary the same altitude as L,), and parallel 
to L; and L, can be found. L; and L; should at 
least partly project on each other. If L, is 
unclassified, it can be introduced into object 
space using the same altitude as L5. 
c) A line segment L4 can be found having the 
same altitude as as L,. Also, a line cutting L, 
and Lj through their end points cuts L; at right 
angels and has L; and L; on the same side. 
11) Do 6)-10) until no more line segments are 
introduced. 
12) Miscellaneous. Check the remaining line 
segments. 
a) Windows and doors. If they among themselves 
generate closed polygons completely within 
anyone of the building elements already found, 
and if some of the line segments of such a 
closed polygon belong to group 1, while some 
belong to groups containing horizontal line 
segments, they are classified as windows or 
doors. If they can not be classified this way, they 
are rejected. 
b) Doubly connected line segments. Introduce 
into object space all line segments connected at 
both ends to line segments already introduced. 
13) Do 12) until no more line segments are 
introduced. 
14) Do 6)-13) until no more line segments are 
introduced. 
15) Consistency check. Check the building 
reconstructed in object space for consistency. If it is 
consistent (complete or incomplete), the parsing is 
terminated, if it is inconsistent, restart from 4). 
54 The parser for region segments 
When buildings lie on or close to the nadir point of 
an image, their walls are generally invisible. In this 
case it is natural to build the parsing procedure on sets 
of region segments, aspects, describing composite parts 
of the roof. The possible combinations of a low 
number of geometrical objects describing a part of a 
roof are not too many, while they at the same time 
define specific properties of the building to be 
described. It is in no way necessary to describe the 
complete roof in an aspect, rather only so much that 
the identified part is sure to be a roof part. It is, of 
course, possible also to include wall parts in these 
aspects. 
The segments contained in the input window of the 
segmentation are tested for matching against the set of 
aspect prototypes shown in figure 2. Note that only 
the structure of the aspects is important, i.e. lines that 
are parallel or orthogonal in the prototypes should be 
parallel or orthogonal in the segments to be tested. 
Lengths of line segments are not important, nor are 
symmetry properties in the prototypes. In order to use 
as much information as possible in an aspect proto- 
type, the test for matching is performed starting with 
the most complex prototypes (those containing most 
line segments) and continued with matching on 
simpler ones only as long as no match has been 
found. 
When a matching prototype has been encountered, 
the corresponding line segments in the segmentation 
are introduced into object space. The interpretation of 
all line segments in the prototype is then assumed to 
be known (this is chosen in an obvious way once and 
for all). Line segments placed on the ground are 
assumed to be horizontal, given the altitude zero and 
introduced using relation (1). Vertical line segments 
are tested for passing through the nadir point and 
introduced into object space using relation (2). 
Horizontal line segments connected to points already 
nu uum En CO. 
UO OOo mag uu 
0 I bp 
Figure 2. Prototypes for building parts to be identified 
in segmentations. 
introduced into object space are introduced at the 
appropriate altitude using relation (1). Finally, 
obvious missing lines are introduced into object space. 
If the prototype contains no vertical line at all, the 
horizontal lines are interpreted as roof lines and 
introduced into object space at a preassigned altitude. 
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