How Many Cessnas can fit on the head of a pin?

When is up really up? X-Plane has two choices when you try to build something that goes "up" in the scenery system:

• Up can mean straight up against gravity. For scenery far away, up is no longer parallel to up nearby because the Earth is round.
• Up can also mean up in the XYZ coordinate system. This is the same as up against gravity at the center of loaded scenery, but wrong for everywhere else.

So what does X-plane use? Well, it depends on what we're doing. Usually up against gravity is the "right thing to do", but often XYZ up is close enough and a lot faster, so we sometimes take that shortcut for frame-rate. Here's a rough breakdown:

• For DSF terrain meshes and objects, up means against gravity.
• For roads, forests and facades, up means XYZ up.

Now in X-Plane 850, we try to move scenery around the world rather than rebuild it. But 3-d stuff like roads and facades are only built very near the plane (which is very near the center of XYZ space), so generally speaking when a road, forest or facade is built, XYZ up is really going "XYZ" up. (If that didn't make sense, don't worry about it - my point is only that scenery shifts don't affect this discussion.)

X-Plane 850 will allow a number of 3-d definition-types to interact physically with the plane: objects can specify the surface type for their hard surfaces on a per-triangle basis (e.g. grass, concrete, asphalt). Facades can have hard walls or hard roofs, and segments of roads can be hard. (This will make it possible to land on the roads again IF we can apply these attributes to our definition files.)

So how many Cessnas can sit on the edge of a fence? Well, if that fence is straight up (and I mean "XYZ" up) then none. X-Plane checks for collisions and ground contact by looking straight down (XYZ down, not against gravity) to find what is below the plane. A truly vertical triangle has no width when seen from straight above, so the plane can never sit on it.

(Lest you worry about the imprecision of this, please note that when you are on top of something, the distance from your wheels to the geometry is 0 meters, so the error between the two "ups" is 0.)

When you are making an object, remember that the object is not facing XYZ up, but rather it is slanted based on the curvature of the earth. So a perfectly vertical wall might actually not be vertical and might thus work as a hard surface . Similarly an almost vertical wall might become vertical and not work. The safe thing to do: make sure that all hard surfaces are not even close to vertical. 45-degree slopes, etc. are okay though!

For a facade, the walls cannot be hard because they will always be perfectly vertical. This presents a problem if you want to make a fence that the plane can't pass through. One solution: use the angled roof feature of facades to curl the top of the fence in at a 45 degree angle. That segment of the fence won't be vertical and will cause the hard walls to work properly. (In fact, hard walls in facades will be provided in 850 specifically for this purpose. For a building, the walls are vertical and the roof can be hard.)