Polygons Part 2 - Real World vs Custom

In a previous post I discussed polygons, how they can be used, and a little bit about how they relate to X-Plane 850 and the new apt.dat system. I have been working on some demo scenery that will make this all clear, but the great is the enemy of the good, so rather than wait I'll post more on this now and get the demos done as soon as I can.

There are essentially two ways to get at the new polygon code: via the apt.dat system or via an overlay DSF. When should you use apt.dat and when should you use an overlay DSF?

• If you are trying to model something that is directly in the apt.dat spec, use an apt.dat file. For example, use apt.dat if you are making blue taxiway lights.
• Use a custom overlay DSF if you are modeling outside an airport. (Do not make "fake" airports to use apt.dat features.)
• If you need a custom look not supported by apt.dat, use an overlay DSF - it's the only way.
  
• Use a custom overlay DSF if you are modeling something that isn't found in an airport, even if it looks similar. (For example, if you want blue lights to model some unique architecture in an airport, do not use apt.dat taxiway lights - your lights may look the same, but they are not the same!)

This last point is important: do not "abuse" the definitions of apt.dat files just because they look similar.

There are essentially two kinds of features in the scenery system:

1. Features that do not change how they look, ever. For example, we do not change the way a textured triangle looks in an OBJ file.
2. Features that are designed to model the real world. Over time, we change them to look more like the real world. For example, approach lights have changed a lot in 850 to look more like the real world ones.

This second type of feature is the one where I issue caution: if a feature in apt.dat or the sim is meant to emulate how the real world looks, we will change how it looks to improve rendering! This is why it is important not to use apt.dat feature for purposes other than they were intended for. It might be that in X-Plane 8.50 the blue taxi lights look just like some other feature you want to code. But in a future version, we might make them more realistic for an airport and they will look worse for your other application. By using apt.dat features only for their true purpose, you help us ensure that our changes to the base artwork make scenery better, not worse.

(This division of all scenery features between ones that are "stable" and ones that are "based on the real world" can also be seen in most parts of the sim. In particular, the flight model is designed with a "based on the real world" philosophy, a very controversial decision I'll have to blog about some other time.)

Next: fixing airport terrain with polygons.