Thursday, October 18, 2007

Draped Roads - A Can of Worms

I don't like it when overlay scenery depends on the underlying base mesh...I've been telling authors for a while now on the lists, "don't assume the base mesh is the global scenery", and while this actually has been a safe assumption for version 8, I suspect that once we get v9 out the door I'll have some time to write mesh-editing tools, at which point the user will have a choice of base meshes.

Roads are currently built in MSL space (that is, absolute elevation on the planet) and thus don't change when the base mesh changes. In a previous post I discussed the idea of having them be AGL based (draped) so that they would "hug" whatever base mesh you have.

First I should say: this isn't coming any time soon and won't be in 9.0. But also, upon closer examination, there are a number of ugly issues just below the surface. Consider all of the things that the global-scenery generator does with roads:
  • Road type is selected not only from source road data, but from surrounding conditions, some of which are not obviously determined by the final DSF.
  • Road bridging is determined based on nearby roads, water, gradient constraints, and available bridge templates.
  • Road type is further influenced by bridging considerations.
  • Roads are smoothed to try to build the most even access, "hiding" rapid changes in underlying terrain from a high-speed roadway.
  • Powerline tower locations are picked from terrain peak to peak with constrained maximum lengths. (The individual segments also controls the span of wires, so introducing new nodes in the draping process might be undesirable.)
The problem therefore with a draped road is that the decision to bridge (both how high and what kind) are dependent on the underlying terrain.

The eventual solution would be to do all of the road processing in X-Plane while draping - but this implies that the author of a custom road grid doesn't actually have control over how the roads come out. One advantage of the current system is that total control over the roads is provided in the DSF - given appropriate tools, a modeler could very carefully hand-craft the roads with a lot of detail.

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