On empty rooms in floorplan graphics: comments on a deficiency in two papers

Floorplan graphs are used in many placement and routing systems for VLSI building-block layout. S. Kimura et al.(ibid., vol. CAD-2, p.285-92, Oct. 1983) and W.M. Dai (ibid., vol.CAD-4, p.189-97, July 1985) proposed algorithms to construct such a graph from a geometrical placement of the building blocks and to define a channel structure from the floorplan graph. In the absence of empty rooms in the floorplan graph the algorithms can correctly derive a channel definition and ordering from the graph. However, in the case that there are empty rooms present in the floorplan graph, neither algorithm can guarantee a correct solution. A counterexample is given to point out the deficiency. A correction to the RRDO algorithm given by Dai et al. is presented