In the hierarchical representation of VLSI layout, some pre routed modules or lines may be re designed or re routed if it is subsequently found to be more beneficial to have some paths cross over the area previously occupied by them. More of this approach is described in section 4.5. An obstacle edge is extreme if its two adjacent edges lie on the same side of the line containing the edge. 3rd ACM Symp. 3rd Annual ACM Symp.
Rectilinear paths are the most important and, thus, the most studied.
All horizontal segments of the path are on the H layer and all vertical segments are on the V layer . Since the algorithms reported in Refs. Let V be the set of obstacle vertices. The following is our starting graph which preserves at least a shortest path. The reader is referred to Refs.