  Based on the concepts of isovists and medial axes, we developed a set of
algorithms that can automatically generate axial lines for representing
individual linearly stretched parts of open space of an urban environment. Open
space is the space between buildings, where people can freely move around. The
generation of the axial lines has been a key aspect of space syntax research,
conventionally relying on hand-drawn axial lines of an urban environment, often
called axial map, for urban morphological analysis. Although various attempts
have been made towards an automatic solution, few of them can produce the axial
map that consists of the least number of longest visibility lines, and none of
them really works for different urban environments. Our algorithms provide a
better solution than existing ones. Throughout this paper, we have also argued
and demonstrated that the axial lines constitute a true skeleton, superior to
medial axes, in capturing what we perceive about the urban environment.
  Keywords: Visibility, space syntax, topological analysis, medial axes, axial
lines, isovists
