Sunday, June 17, 2007

Crash Bang Goes the Airplane

Ari asked a good question regarding sloped runways and the new apt.dat 850 format:
Am I understanding it right that airport taxiways, ramps and runways are from now on going to be merged into one big mesh, instead of bunch of rectangle pieces overlapping each other? If yes, will this finally allow us turning on sloped runways option in X-Plane without any of the current side effects?
This brings up some interesting questions. First the most basic answer:
  • apt.dat 850 prrovides curves, irregular taxiway shapes, which will allow you to create complex taxiway shapes with only one piece of pavement, rather than many overlapping ones.
  • X-Plane still honors the order of apt.dat 850 for drawing, so you can also overlap and get visually consistent results.
  • We recommend using a smaller number of curved taxiways rather than many overlapping rectangular ones because X-Plane can handles this case more efficiently. It is not necessary to build the entire airport out of one taxiway though.
Now the second part of the second of this question is a little more complex, because the cause of bumps in the scenery changed.

Bumpy Runways in the Good Old Days

Back in X-Plane 806 there was a fundamental problem with the way we did sloped runways that made them virtually unusable: while the corners of each rectangular piece of pavement would sit directly on the terrain (no matter what the terrain's slope), the area of the taixway was formed by a flat plane. This means that the middle of the taxiway might be above or below the terrain.

Now the real problem comes when we have two taxiways that overlap. Because they are only aligned to the terrain at their corners and not centers, there may be differences in their height when one taxiway's corner hits another taxiway's center (which happens a lot). As the airplane travels from one taxiway to another, the elevation of the ground changes instantly, inducing a major jolt to the suspension. At high speeds these damage the airplane's suspension.

Bumpy Runways Now

In X-Plane 850, we break all runways and taxiways (new and old) into multiple pieces each tiem the terrain underneath them has an edge. The resulting taxiways are then aligned to the mesh at their corners. But since no taxiway center goes over a mesh corner, the taxiway "hugs" the mesh perfectly. And since all taxiways hug the mesh in the same way, there is never a height gap between taxiways.

It's the Mesh, Stupid

So why do we still have bumps in X-Plane 850 if we so carefully make sure the taxiways exactly reflect the mesh height? Well, you're effectively driving on the terrain, so any bumps are ones from the terrain. Simply put, even with the new system the usability of sloped runways is only as good as the underlying terrain.

Now our meshes come from SRTM data, which is radar data - it naturally has a certain level of noise and "speckle" which makes it pretty unusable for airports...airplanes are very sensitive to even small bumps during takeoff.

We attempt to "condition" the elevation data for airport use, smoothing out hills and bumps. Unfortunately our algorithm doesn't always work right. The X-Plane 8 US scenery was way too bumpy to be usable. The 7-DVD set is better, but still makes bumpy airports in a few cases:
  • If there was no airport in the apt.dat file at the time of scenery creation, no conditioning was applied, and the underlying terrain is probably inappropriate for draping.
  • The scenery creator has a bug that causes airport flattening to fail when it's very close to water. For example, the big bump in the runway at KLGA is due to a water-airport interaction.
  • I think that flattening across DSF tiles can have problems too.
If you've been thinking -- wow, the diagram for 850 has a lot more triangles (10 vs 4) than the one for 806, you are right. Fortunately, the number of triangles, all part of one taxiway, in an airport layout, doesn't really affect frame-rate, since this is handled by the GPU.

But this is also a case where a few curved polygons can be much more efficient than several overlapping ones - when we cut up the taxiway based on the mesh, if there is ovelapping pavement, each overlapping taxiway must be cut, multiplying the effects of the mesh on triangle count.

It also turns out that in the real case this is somewhat moot: because X-Plane smooths the airports and then induces triangle borders around the edge of the airport. Since the interior area is so flat, it doesn't require a lot of triangles, and therefore the trianglse inside an airport tend to be big, so the number of times we have to cut an actual layout is quite small.

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