I don't have an in-depth knowledge of how is the world rendering implemented in XPlane, but I guess it's more or less comparable to FSX rendering.
I think the fundamental difference between Outerra and FSX/Xplane is the procedural approach vs. the pre-made scenery. From what I've read, XPlane comes on 6 DVDs containing the meshes and scenery data, but still the resolution is relatively low (probably most visible in mountains).
Outerra uses elevation and climate data to generate terrain that can be continuously refined. Commenting on corona's post:
- World = Flat World + Heightmap?
Heightmaps data are processed into LOD layers, and below the finest heightmap level the fractal refiner kicks in and starts to generate the more detailed heightfields by fractal refinement process.
- Custom terrain shaping layer - not sure what's this
- Landclass - this will consist of rough (500m) raster based land class data processed in a way that allows for fractal refinement. Landclass data is used just as a kind of probability seed value to drive the refinement process, to achieve natural transitions. Also, this landclass will only include natural land types; civilization effect will be in a separate layer as vector data. This will also allow for continuous degradation - the return to the natural state.
- Procedural ground textures & vegetation placement maps - generated from terrain attributes - landclass, elevation, slope, curvature and auxiliary fractal channels. Using probability rules to place the vegetation and determine the appropriate vegetation types.
- Vector data are actually applied after heightfield data are computed, before the ground materials are computed
- Model placement is manual, persistent, there's no autogen for this. It's more probable that it will work with specialized generators for vector defined areas - you'll specify area, assign a type (urban type x, pasture, field etc) so that a generator can create separate fields etc. Using OSM data where these regions are defined will be the priority. These vector overlays also modify/remove existing natural land type and vegetation maps.
With OSM data, or any vector data for that matter, we have to provide a city block generator that uses some parameters to create buildings and all objects there. For example, if there's an area designated as residential, separate plots should be cut, houses placed, with pools and gardens and everything - that's up to the generator. Originally there could have been a swamp land, but the overlay and generators will mod the land to a specific type.
As with the natural land generator, the generated detail level should depend on what detail is required, which mostly depends on the distance. Finer stages of the generators are invoked on more detailed tiles.
Ultimately it should allow to generate insanely detailed procedural world, but always the separate generator stages could be switched off to override the procedural output. For example, the part that changes swampland to residential area can be on, but a particular parcel can be excluded from the later stages of the generator, so one can build a custom house there.
