No abstract

AbstrnctA Fortran routine is presented for clipping a polyline to the inside or outside of a polygon, the boundary of the polygon being treated as part of the inside. There is no limitation on the number of sides of the polygon or vectors of the polyline, and no working storage is required. The polygon may have sides that intersect and vertices or sides that coincide with each other.purpo= There is sometimes a need to clip polylines to an irregular polygonal boundary, for example to ensure that calculated contour lines are terminated at a coastline. A similar problem is to fill a polygon with a repeating pattern.A routine to do this task could be used to clip polylines to ensure that they do not extend beyond the physical l i m i t s of a rectangular device, or to clip them at some other rectangle parallel to the physical edges, but the general routine would be an inefficient way of performing this very common special case.]The routine could also be used to shade a polygon all over with a pattern of parallel lines. Again, this special case can be handled more efficiently than by the general routine. However, the routine presented here is in a form which separates the task of finding intersections between a vector and a polygon into a separate routine, which could be used by a routine for shading. SpecificationA polyline is defined by any sequence of points in a plane: the points will be termed "nodes" and the lines joining adjacent nodes in the sequence are "vectors". A polygon is also defined by any Computer Laboratory, Ldccster LEI 7RH England. 554455 a t . 56ULlivaSity of Leiccster, sequence of planar points, the points here being called "vertices" and the lines "sides". A polygon has as many sides as vertices; a polyline has one fewer vectors than nodes.If every sequence of points is allowed to count as defining a polygon, then polygons may have intersecting sides. In this case the inside of the polygon includes any point from which a line extending infinitely in one direction would cut the polygon an odd number of times. This definition of a polygon allows some awkward cases: if several consecutive nodes are colinear and in the same direction, then all except the first and last are redundant; the point of intersection of two sides need not be listed as a vertex.On the other hand, the definition of a polygon requires that all the sides are directly or indirectly connected, and so excludes polygons with separate internal boundaries (or boundaries that do not include each other or overlap).Nodes and vertices may be numbered in the order in which they are listed. The nth vector or side is the one from the nth node or vertex to the next, so the mth side of a polygon with m vertices joins the mth vertex to the first.

The Conference was held on the three days 24th to 26th March at Glasgow University, the first day devoted to two parallel tutorials, the second day opening the conference proper and including meetings of special interest groups and the final day consisting of submitted and invited papers.