EuroSDR Projects - Host Ireland

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Figure 3-15: The roof segments.
Having segmented the roof faces of a building, the relationship between the faces is defined. By
following the outlines of roof faces, so called ‘topological points’ are first added. Topological points
are defined as vertices where a roof face’s neighbour changes. In Figure 3-16, the topological points
are marked with triangles. Next, each section of an outline between any two topological points is
defined either as an intersection line, height jump section or both. For sections defined as both an
intersection line and jump section, a new topological point is added to split the section. In Figure 3-16,
the intersection lines (thicker) and the height jump sections are shown.
Figure 3-16: Topological points (triangles), intersection lines (thicker) and height jump sections
of the building.
As shown in Figure 3-16, the jump sections are often noisy and difficult to model. Therefore, for each
jump section, lines are estimated using the 2D Hough transform (Figure 3-17 (left)). The slopes of the
estimated lines are then adjusted according to the orientation of the plane (Figure 3-17 (right)).
Figure 3-17: Left: Estimated lines for a jump section. Right: Lines adjusted according the
orientation of the plane.
The intersection points of the estimated lines are used as vertices along the jump sections (Figure
3-18). These vertices along the jump sections are used together with the topological points to define
the roof polygons.