@TechReport{TR042,
  author =	 "Walter G. Kropatsch",
  institution =	 "PRIP, TU Wien",
  number =	 "PRIP-TR-042",
  title =	 "Equivalent {C}ontraction {K}ernels and {T}he
                  {D}omain of {D}ual {I}rregular {P}yramids",
  year =	 "1995",
  url =		 "https://www.prip.tuwien.ac.at/pripfiles/trs/tr42.pdf",
  abstract =	 "Dual graph contraction reduces the number of
                  vertices and of edges of a pair of dual image graphs
                  while, at the same time, the topological relations
                  among the 'surviving' components are
                  preserved. Repeated application produces a stack of
                  successively smaller graphs: a pair of dual
                  irregular pyramids.  The process is controlled by
                  selected decimation parameters which consist of a
                  subset of surviving vertices and associated
                  contraction kernels. Equivalent contraction kernels
                  (ECKs) combine two or more contraction kernels into
                  one single contraction kernel which generates the
                  same result in one single dual contraction. 
                  Decimation parameters of any individual pyramid
                  level can be reconstructed from the ECK of the
                  pyramid's apex if both vertices and edges of this
                  ECK receive labels indicating their annihilation
                  level in the pyramid.  This is a labeled spanning
                  tree (LST) of the base graph which allows efficient
                  design and control of different types of dual
                  irregular pyramids. Since the LST determines the
                  pyramid, primitive modifications of the LST
                  transform also pyramids into other pyramids on the
                  same base graph. They open a large variety of
                  possibilities to explore the domain of 'all'
                  pyramids.",
}