What is the limit to how precisely you can build scenery in X-Plane using WorldEditor?
In this picture, I created a runway “OBJ” exactly the same reported size of the runway, in the same location and orientation in WED. The windsocks are also placed on the exact corners of the runway in WED. This is a typical “MRI” scenery pack – that is, a test scenery package designed to show how well/poorly the sim is performing.
So what do we see?
- The alignment of the windsocks with the runways is pretty damned good. It might not be perfect, but it’s awfully close.
- The alignment of the OBJ runway and the real runway below it is not particularly good – the heading isn’t quite perfect enough to stay aligned over a 10,000 foot runway.
This lack of precision is a design limitation – that is, X-Plane does not try to provide unlimited precision in scenery authoring. (At some point, increasing precision increases file size and processing time, which would have a detrimental effect on overall system performance.)
In particular, X-Plane tries to provide high quality point-to-point matching. That is, given two locations that are both specified by latitude and longitude, X-Plane tries to get them to be as close together as possible.
What doesn’t work well is trying to correlate a specific location (specified by latitude and longitude) with another location specified by relative location (e.g. from this location, go 50 meters to the west).
Generally, the longer the “arm” of relative location, the worse the precision. in this example, the runway is specified by the center of the ends, and the windsock locations are specified directly. Thus the runway corner is on a 25 meter “arm” from the runway end center. We may pick up a little bit of imprecision on a 25 meter arm, but the correlation is good.
By comparison, the object end is on a 5 km “arm” because it is specified by its center location and heading, while the runway end center is specified directly. As you can see, over a 5 km arm, a fair amount of error is accumulated.
Thus we can come up with a working strategy for scenery placement:
- When placement precision is important, keep objects small.
- When two scenery elements must be locked together, be sure to use scenery elements that can be specified by exact location and not by a metric distance from another location.
- “Looks close” in WED only means “pretty close.”
The hardest part of this work-flow is using art assets other than objects for large scale features. But (for example), a draped polygon is better both for precise positioning over a large area and for polygon offset/Z-thrash, so you probably need to use draped polygons anyway.
Hmmm. What is actually happening that causes this misalignment? Something to do with the map projection?
P.S. In case you miss the beatings you get for showing in-development stuff, shadows aren’t that dark in real life. 😛
precision loss: any number of things, including but not limited to…
– Precision limits inherent to DSF.
– Precision limits due to the decision on point pooling in the DSF.
– Decision limits due to the internal representations in memory in the sim.
– Precision limits and precision loss during transform in the sim.
– Gnomes inside the code.
Re: shadows: that’s the “fake” decal shadow that you get if you turn real shadows off. I have no idea why it’s so dark..I may lose sleep over this.
Ah. Well, I’ve had some fun with projections myself. My own code naturally used great circles, but the map naturally used loxodromes (straight lines on a Mercator projection); and I didn’t bother to work around that. Fortunately the lines it was meant to deal with are very small, but if you do enter points that are artificially far apart, the discrepancy looks truly bizarre.
Re: shadows: I noticed there was no self-shadowing. Maybe this is just what happens naturally when you flatten a shadow volume? The high shadow concentration would give it its dark color? On the ‘bright’ side, such darkness may improve your sleep. 🙂