Tuesday, August 15, 2006

Taxiway Layouts - positive or negative modeling

X-Plane 850 allows you to model taxiways by cutting holes in pavement, as well as by building up pavement around a hole. In other words, you can make a letter-'O'-like shape by making a circle and then cutting out the center, or by making two semicircles that touch.

Which is better? Here are a few guidelines:
  • Making an area out of smaller pieces is slower in that you must introduce individual lines only to "cut" the area into smaller parts. So generally subdividing your pavement is harmless as long as the cuts can be a few simple straight lines. Using bezier curves or lots of straight segments just to partition an area into smaller parts can slow down frame-rate. So if you're going to partition, keep it simple.
  • If you keep your partitioning smple, there's no harm and it does let you control the "grain" of the taxiways textures.
  • It takes X-Plane longer to build layouts that have more complex polygons. So subdivision can make airport build-up faster. So for example, modeling an entire class B airport with a single polygon is probably too complex - consider breaking up the polygon a bit.
  • It's better to have two adjacent polygon taxiways with more complex boundaries than one polygon on top of the other. X-Plane has to further subdivide polygons based on the topography of the local terrain, and that subdivision happens for every layer of pavement that's overlapped.
  • One exception: two edges of polygons that touch and are bezier curves may not line up properly, due to rounding errors. So if you want to have one curved surface over another, you may have to use overlaps.

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