The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008
1007
Figure.6 Linear Feature Extraction results
2.5 Extract the main direction of buildings
Extracted the main direction with main direction and its vertical
line segment, excluded some segments that have nothing to do
with the building edge, which can rule out some features of the
surrounding buildings interference; extracted the main direction
of buildings that is easy for searching the edge lines in this
direction, improve the computing speed and enhance the
stability of extraction.
Assuming the total number of line segments is N, static the
length of each line segment, as well as the corresponding
azimuth. In the range of image dealing with, the building edge
lines is connected much longer, so we should focus on the long
segments and give much greater weight. In this paper, adopt the
square of the length acting on the azimuth. Establish cumulative
matrix, which will be cumulated to the corresponding angle,
then the corresponding angle of maximum in the accumulator is
the main direction of buildings. Figure 7 shows the effects of
line planning.
Figure.7 The effects of planning line
Deletes line segments when Satisfy the following conditions:
a € [0,90], |a-6?| > T and |a+90°-£?| > T (1)
a € [90 180), [x-0|> T and \<x-90°-&\>T (2)
Reserve line when Satisfy the following conditions:
a € [0,90 ], |a-9\< T or \a +90 °-0\ < T (3)
a € [90 180), \x-8\< T or| a -9O°-0|< T (4)
2.6 Judge of segments relation with model.
Because of noise or other buildings, the straight edge line of
building may be cut off, there are many interferential segments.
Which have an impact on the accuracy of building structures
search. In this paper, with the method of model to judge the
relationship of segments, connect the broken border line, and
remove some interference surrounding the buildings.
2.7 Handling the relation between lines.
The long segment should be the main edge after connected the
straight line, and there are some short line segments in the
vicinity which can be considered as illusive edge line, they will
be removed. First of all, sort all segments in accordance with
the length, starting with the shortest straight line. Only proceed
between the straight lines in the parallel. Figure 8 shows the
effect of treatment.
Figure 8 the effect of treating line relations
2.8 Region segmentation and growth.
Calculate the average gradient of each segment, sort the
segments from the big to small according to the gradient, then
extend the segments and segment the image to different region
and mark each region.
Region growth is a process that pixels or sub-regions are
converged to bigger region in accordance with definitional
criteria. If direct making use of region growth in the buildings
within the seed growth, easy access the brink of noise, the edge
of extraction is not linear. However, on the basis of extracting
the edge of building, and processing segmentation, which
relatively easy solve the noise problem and non-linear margin.
2.9 Region merger.
Calculating the average of grey U { and variance of each
region, if the seed selection region meet — w o | < Au o and
|<5^- — S Q | < AS q ,combine the growth of region. Au q and
are u o S Q linear functions, the selection of parameters of
function is due to the real case of buildings.
After merging the top of buildings, extract the outer edge and
track it, and now the result of tracing out at the edge is regular
closed line, due to the restriction of straight line above, so that
we have the rectangle between the adjacent edges, then act on
straight line approximation, finally, we obtain the line on the
top of buildings, which is the result of building extraction.
Figure.9 The drawing of buildings edge extraction