Monday, February 08, 2010

Why Does the Global Scenery Take 6 DVDs?

Periodically I see the question asked: why does the global scenery take up so many DVDs? Or more typically the question is: if it is so big, why is X missing, where X is the detail the user expected to see but did not find.

The size of X-Plane's global scenery is due to two factors:
  1. Some aspects of the scenery are very detailed, and this takes up disk space.
  2. Some aspects of the scenery are computed ahead of time, not by X-Plane, and this takes up disk space.
Big Data

A few data sources make the scenery pretty big:
  • The scenery is generated from 90m SRTM data for the entire world.
  • US scenery includes the entire US road grid.
  • Coastline data is fairly detailed in the US and for most oceanic coasts.
In other words, part of the size of the scenery comes from having reasonably accurate topography and coastlines everywhere.

Precomputation

The other source of file size is that X-Plane precomputes aspects of the scenery to reduce load on X-Plane while you fly. (We do this to improve frame rate.) In particular, the scenery uses a class of algorithms that are expensive to compute, and thus we compute them ahead of time and save the result (using up file space).
  • Land class changes not on a regular grid, but along iregular polygonal boundaries following the shape of the terrain. This produces much more natural terrain transitions in the mountain, but requires more storage.
  • "Auto-Gen" forests and buildings don't follow a grid - rather, they fit into the irregular spaces made by vector roads and water. This fitting process is too slow to run inside X-Plane, so every auto-gen building and forest area must be pre-computed and saved.
Room To Grow

One interesting side effect of saving the end results of building locations ("pre-placement", what X-Plane does) rather than the formula for how to place them ("auto-gen", what FS X does) is that we can make the placement algorithm for buildings significantly smarter without any increase in file size or decrease in frame-rate. Having already eaten the cost of putting every building into the DSFs, we can make those locations better without hurting performance.

That algorithm to pre-place buildings also can have access to source data that isn't available in the final DSF, allowing for effects that might not be possible in an auto-gen system.

2 comments:

vined said...

As for me, I'm ready to buy 10 DVD if you'll make the best terrain ever.

gthomas said...

Make that 15-20 if the caribbean could be made viable!

Three of the 10 most dangerous airports in the world (TNCM, TFFJ, and TNCS) are less than 30 nm apart. St Maarten (TNCM) is reasonable. St Barth (TFFJ) and Saba (TNCS) bare little resemblance to what is needed.