A Simple and Efficient Haloed Line Algorithm for Hidden Line Elimination

1. Introduction An efficient algorithm, HALO, is given to compute haloed line drawings of wire frame objects. (Haloed line drawings are described by Appel et al. 1 HALO has two parts: CUT and DRAW. CUT uses an adaptive grid to find all edge intersections. It overlays a square grid, whose fineness is a function of As computer aided design (CAD) deals with more complicated databases, it becomes crucial to display the data effectively so that people can comprehend it. A suitable method must be efficient since users will be interactively manipulating and displaying their data. With special purpose hardware becoming less expen-the number and length of the edges, on the scene. It determines the cells that each edge passes through, sorts sive, and even custom VLSI design becoming as easy as writing software (for those with the appropriate facili-these by cell to obtain the edges in each cell, and then, in each cell, tests each pair of edges in that cell for ties), a suitable algorithm should lend itself to parallel-ism and implementation in silicon. Since a CAD data-intersection. For broad classes of input this takes time linear in the number of edges plus the number of inter-base may contain wire frame models without any surface information, the algorithm should be able to han-sections. CUT writes a file containing all the locations where each edge is crossed in front by another. Given a halo width, DRAW reads this file edge by edge. For each edge, it subtracts and adds the halo width to each intersection to get the locations where the edge becomes invisible and visible. It sorts these along the edge, and then traverses the edge, plotting those portions where the number of " visible " transitions is equal to the number of " invisible " transitions. DRAW takes time linear in the number of edge segments. Dividing HALO into two parts means that redrawing a plot with a different halo width is fast, since only DRAW need to be rerun. dle them. This paper offers an efficient algorithm called HALO to solve this problem via the technique of haloed line elimination. Haloed lines are introduced in Appel et al.1 which cites many reasons for using them and gives good examples. Briefly, we assume that each line has a narrow region, or halo, that runs along it on both sides. If another more distant line intersects this first line, …