| Abstract: | Ray casting may be applied to the terrain rendering from DTMs (digital terrain models). In this paper we propose a ray casting method which exploits exact vertical ray coherence. This method not only achieves a good performance, but also allows any set of viewing parameters. To analyse the vertical ray coherence property, we define the following terms: a ray plane which is perpendicular to the sea-level plane and passes through the COP (centre of projection), and the corresponding ray line which is the line of intersection of the ray plane and the projection plane. Vertical ray coherence can be stated as: if two rays pass through the same ray line, they pass over the identical set of points on the DTM base plane. After a ray is cast on the DTM, we can get a good starting point to cast another ray using this property. Our method tries to cover the projection plane with ray lines. Since ray lines are not parallel to each other, it is a difficult task to cover the entire plane. In fact all ray lines intersect at a point (COP′). We first generate a set of ray lines sufficient to cover the projection plane near COP′. As pixels get further from COP′, more ray lines are generated to fill most of the gaps between pixels covered by the previous set of ray lines. Rays passing through the same ray line are effectively cast using vertical ray coherence. To fill remaining pixels not covered by ray lines, pixel values of vertical/horizontal neighbours are interpolated. To cast rays efficiently, successive points generated by a line algorithm are followed along the projection of the rays on the DTM base plane. © 1997 by John Wiley & Sons, Ltd. |
