Cake and Pie

On this week's FSBreak, Holger Sandmann described MSFS's scenery system as a "pie" made of layers. This made me think: well, in that case X-Plane must be a cake. It turns out X-Plane has been a cake for a while.

If this discussion seems tasty but confusing, let me clarify. The issue is: which parts of a scenery system are combined when the simulator runs, and which parts must be combined ("baked") in advance.

With MSFS, you can separately install a new mesh, new land class data, new coastlines, and new orthophotos. In X-Plane, all four of those elements must be pre-combined into a single "base mesh". In X-Plane, you have to bake those elements.

This means that you can't make "just" an add-on mesh for X-Plane. You have to create an add-on that addresses elevation and land class and orthophotos and water. Third party authors are quite often not very happy about this - I don't know how many times I've seen "can I replace just the elevation and not the water" posted in forums.

But the Orbx team brings up the exact reason why I thought (five years ago when designing DSF) that requiring all of the elements would be okay: often if you replace one part of the scenery and not another, the results are inconsistent. If you move a coastline but don't adjust the mesh, you may have water climbing up a mountain. If you move a mountain but don't adjust the landuse, you may have a farm at a 45 degree angle.

Cake and Pie

Not everything in X-Plane is pre-baked. As of X-Plane 940, you must pre-bake:

• Land class
• Wide-scale orthophotos
• Terrain Mesh
• Coastlines

You can separately replace:

• All art assets (E.g. change or add types of buildings, replace the look of a given land class for an area).
• All 3-d (forests, buildings, airports, roads*).
• Small area orthophotos
  

Orthophotos are on the list twice. This is because X-Plane has two orthophoto technologies. Draped polygons (.pol files) can be overlaid anywhere, but they degrade performance if used over too large of an area. They are really meant for airport surface areas and other "detail" work. Orthophotos terrain (.ter files) can be used over huge areas with no performance problem, but must be baked.

* Roads are sort of a special case in X-Plane 9: you can replace them with an overlay, but the replacement roads must be "shaped" to the underlying terrain mesh, which means they won't work well if a custom mesh is installed. This limitation in the road code dates back to X-Plane 8, when most of us had only one CPU - pre-conforming the road mesh to the terrain shape saved load time.