Wednesday, November 05, 2008

Fact or Fiction

More ranting on the question of whether a file format is based on factual information or not. For the sake of taxonomy, let's call this:
  • Factual. The file format aims to capture "real world" information. The file spec is thus written against real world norms. Example: a runway is described by the location of its centerline at its threshholds, the type of aproach lighting fixtures, and the material it is built out of. This is all fact that can be verified by going to the runway and measuring it (while trying to avoid 747s).
  • Artistic. The file format gives authors a creative platform to create "stuff", e.g. an image, a model; the file format dictates how client applications might interpret that "stuff". Example: OBJs are artistic - it describes what affect on drawing the various bits of the OBJ file have.
Apt.dat is actually a hybrid format - most of it is factual, with one glaring exception: pavement surface areas.

Pavement surface areas are simply an overlapping pile of bezier polygons with holes. There are multiple ways to create a given layout, and you couldn't make an argument that one is "more factually correct" than the other.

Artistic file formats give us a way to be open-ended, and so they are particularly useful for problems that we cannot solve in a practical manner using factual file formats. When we worked on the apt.dat 850 format, I clung to a 100% factual approach for as long as I could, hoping to be able to truly describe "ground truth" about airport pavement. What I found in the end was that real world instances of airport pavement are so varying and weird in real life that almost any factual approach would fail to correctly model important real-world airports. So we punted and simply said "put pavement wherever you want, make it look nice."

The result of going artistic instead of factual is two-fold:
  1. The taxiway data in the apt.dat file is less broadly useful to a wide range of client applications; you might be able to infer some aspects of the real taxiways from the data, but the taxiway shape has very little structure to it.
  2. You can model just about anything you can dream of - there really aren't any limits.
That taxiways are "artistic" will probably always bug me a little bit from a theoretical viewpoint, but I think there is no question that this was the only practical standpoint.

Final thought: factual file formats are usually not precomputed - that is, if we have a list of runways described by their real-world properties (and not modeled as a collection of textured triangles) then there is probably work that still needs to be done to make the file useful for X-Plane. (That work is done by X-Plane's file loading code.)

Okay - I'm OOTO for a while - see you before thanksgiving!


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