Life would be easier if the Earth was a big cube.* I was reminded of this yesterday when I discovered (thanks to a bug report from an author) that X-Plane has been drawing objects with slightly incorrect heading for the last who-knows-how-long amount of time.

The bug (present in 945 and going back who-knows-how-long) is that the object’s heading can be off by up to 1 full degree clockwise or counter-clockwise. (This maximum error occurs at the north pole – average error for real use cases is more likely to be about 0.35 degrees.) The amount of rotation depends on how the scenery system is shifted.

The case the author sent me was a converted scenery pack, where a pair of parallel runways were modeled partly out of draped polygons (which don’t exhibit the bug), but with markings modeled as an OBJ. Depending on the scenery system’s “centering” the markings would be a little bit to the left or right of the draped polygon.

A brief aside: do not model your scenery this way! A bug is a bug, and X-Plane 950 beta 1 fixes this problem, but there are a number of reasons not to use a giant OBJ to model your runways.

  • OBJs do not “hug” the terrain, and the Earth is round. You will never really get clean, artifact-free flat surfaces unless they can follow the terrain contour. Even if you “flatten” the terrain, it’s not flat – it’s constant MSL.
  • The case where I see this more often is with buildings, where authors build every single terminal building in one OBJ. X-Plane only “connects” the OBJ to the ground at one point (0,0,0 in the object). So to get a good ground connection, you need your objects to be smaller, so they can all “sit” on the ground. (Technically they need to be rotated slightly to wrap around the Earth, but for the scale of an airport this error is only about 1/30th of a degree or so – hard to see.)
  • Finally, X-Plane chooses to draw or not draw an object on a whole-OBJ basis. So if you have a huge object, it’s always in view (because at any one time some part of it is in view). In fact, even if the object is not in view, the larger the object, the more the “sloppiness” of the visibility check causes false positives. (The visibility check is optimized for speed, so it really answers “this object is probably in view”, erring on the side of drawing too much by doing less analysis.) So the bigger your objects, the more they are drawn.

Enough ranting – in this case the scenery was the output of FS2XPlane, and the author hadn’t gotten to cleaning the results. Fortunately the interim product showed the rotation bug.

X-Plane 950b1 (in beta now) fixes this problem – rotational alignment of a very large OBJ with a draped polygon should match between X-Plane (with all frames of scenery system reference) vs. WED 1.1 preview 2 pretty much exactly.

*This post is in the long and distinguished tradition of “life would be easier if” musings relating to scenery, e.g. global scenery would be easier if the entire planet was paved in asphalt, cities would be easier if their road grids only ran perfectly north-south and east-west, clouds would be easier if they weren’t translucent, and rendering would be easier if nothing on the planet ever was translucent or cast a shadow. So apparently a programmer’s paradise is somewhere between “Mad Max” and a Dali painting…

About Ben Supnik

Ben is a software engineer who works on X-Plane; he spends most of his days drinking coffee and swearing at the computer -- sometimes at the same time.

One comment on “Life Would Be Easier

Comments are closed.