ATC Part I - Real Physics

This is a repost of a reply I wrote on avsim that sort of grew out of control. In part II I will try to explain what we're doing with the ATC system and how far we will get in X-Plane 10.0. Anyway, post follows.


I will have to write a blog post to go into this in more detail later, but I think a lot of what has been written here is wrong. Y'all started under the assumptions that:

1. The flight model is going to be too expensive to run on AI planes and
2. A table based flight model would be faster.

Those are both questionable assumptions at best...pretty much everything you can conclude from that is, IMO, dubious.

(I should mention at this point that I am unaware of any "dumbing down" features in the FM right now. My understanding is that we will pre-process control inputs in a number of ways, and the frequency of the FM can be set to multiples of the framerate, but even at the lowest setting, we do a full physics integration and the plane is the sum of the physics that are applied to it. I mention this now because I am about to speculate on some optimizations that _could_ dumb down the physics model, and I want to make clear that shipping X-Plane 9 does not do this!!)

Here's the short version:

The most expensive part of the flight model is ground interaction - that is, the flight model doing collision checking with the ground and various parts of the airplane. When an airplane is "clearly in the air" (e.g. an initial test shows it has high altitude) the FM isn't very expensive; when it is near the ground, CPU time cranks up as we make sure to get the touch-down characteristics just right; same with taxiing.

So if we want to make the FM faster, there's really only one place to attack: ground interactions - it's the lions share of CPU time. There are a number of simple things we could do to improve ground interaction. For example, we could stop checking for body scrapes - since X-Plane has to handle physics correctly even if the user lands gear up and scrapes an engine, the sim normally tests the full geometry of the plane against the ground (which is not flat, even at an airport) - that adds up. If we are willing to trust that the AI planes don't screw up a landing* we could cut down ground check to only real landing gear, which would improve performance.

Now what if we did some kind of 'lo-fi' AI, whether it's table based or it simply says "move the plane forward by this much" (E.g. a sort of track-based system)? If we want the airplanes to 'sit' properly on the non-flat airport surfaces, we _still_ need to do the most expensive part of the FM - the wheel-ground collision checks. So the total savings of a 'lo-fi' AI flight model would be very small, because at best we might partly improve the performance of code that doesn't have much impact on the sim.

(To understand why you can only boost performance by attacking the biggest pigs, see here.)

However, there would be a pretty huge cost to a lo-fi flight model: we would have to code a SECOND implementation of pretty much everything we already do in the real flight model! We would have to have new flight model files to support this new alternate flight model. The opportunity cost here is in developer time...the time spent building a separate flight model could have been spent performance-tuning the real flight model...even if we had a second flight model, performance tuning time would now be divided between the two flight models, and neither would reach its optimal performance.

Besides my explanation above of why a lo-fi flight model wouldn't really be a win, two more comments:

In software development, it often pays to try the simplest thing first, see how it works, and go from there, rather than speculate how a system may perform and write a ton of code up front before you have real data. This is what we are doing...the simplest thing we can do is to run the real FM on the AI planes, and so far it looks like it's going to work reasonably. IF we hit data that says "no we have to do something radical", then we will...when the data says so, and no sooner. So far indications are that the real FM is going to be fine, and this makes sense from what we know about its performance characteristics. We also know that we have a lot of tricks we could pull to make the real FM faster for AI planes (e.g. removing engine scrape-checks, per above) before we have to go and write a whole new FM.

And finally, dude, the real FM looks good. With the real FM, the AI planes move the way big heavy airplanes should move. They track the ground perfectly. If the ground has a bump and the airplane's suspension is loose, it sways like it should. The control surfaces deploy with their real time. When you're at an airport performing ground ops, you can get really close to the AI planes, and at that point these things matter! I speculate: once you take follow an AI plane running the real FM on the ground, it'll be hard to go back to a 'synthetic' FM.

* This may not be a safe assumption...what if a microburst hits an AI plane?