You Can't Unbake a Cake

First, I just want to be clear: I am not announcing any future features for the scenery tools. I am not saying when they will be released, and I am not saying what they will do, because honestly I do not know. There have been too many cases when users have emailed me and said either "I thought you guys were going to do X" or "you guys promised you were going to do X", so now I am officially paranoid.

So this blog post is not about what the future scenery tools will do - it is simply a discussion of the difference between editing source data and compiled DSFs.

When we make the global scenery, we start with a bunch of source data that roughly consists of: road maps, coastlines, elevation, landuse, climate data, airport locations, etc. When we build a DSF out of it, we "bake" these items together into a single file. During this baking process, our tools apply some "integration effects". Here are a few of the more obvious examples:

• Terrain under airports is forced to be an airport grass texture, appropriate for local climate data.
• Roads are removed from airport areas.
• Intersections are computed for highways - that is, a highway and city street form an overpass, but two city streets become a real intersection.
• Generated Buildings are put around the roads, not under them.
• Buildings are oriented to "face" the slope of the ground they are under, based on their shape.

That's not a complete list, but it gives you an idea of some of what goes into making a DSF. All of this information is precomputed and represented in the final DSF. But consider this last point: the DSF contains the actual orientation (north/south/east/west) of each building. It does not contain the slope underneath the building and it contains no information about which buildings need reorientation. In other words, the results of the process, not the inputs to the process, are present.

So consider what would happen if you could simply edit the data in the DSF:

• If you moved an airport, the airport grass would stay in its own location.
• If you moved an airport over a road, the road would still be there.
  
• If you changed a city street to a highway, it would not form an overpass.
  
• If you moved a road on top of a generated building, the building would remain in place.
  
• If you changed the ground elevation, buildings would not change their orientation to face the new slope.

If you changed the source data and re-ran the DSF building process, these effects would occur. But remember, we need elevation, land use, etc. to build a DSF from scratch, and the DSF itself doesn't contain its source data.

So we have two possible strategies for editing DSFs:

1. We could build a DSF "retouching" tool that let us make very small changes to the existing DSF without having any source data. None of these effects would "work" so authors would have to make very small changes and then hand-fix any problems that appeared.
2. We could build a DSF "rebuilding" tool that let authors make new DSFs from source data. All of the effects would look good, but authors would have to get some of the source data. (We could post our source data, or provide links to places where it can be downloaded.)

Note that we can't have our cake and eat it...we cannot get the "integration effects" listed above unless we go back to the source data. If I can stretch my cake metaphor to the breaking point, once we make our cake, we can't easily remove the flour and add another egg - we need to start over with the raw ingredients.

Which strategy will the scenery tools use? I don't know yet. I am focusing on airport and overlay editing, which sidestep this issue a bit (we can easily edit an apt.dat 850 or overlay from the final product). We may do a bit of both strategies - it depends on what users want and what we can code efficiently.

Why don't the finished DSFs contain everything we need to edit them? The answer is size. The finished global scenery was about 56 GB of DSF. When I last checked, the raw data that forms the DSFs was at least 100 GB or more. So for each DVD we shipped of scenery, we'd have to ship two more DVDs of source data, for a 21 DVD set, most of which wouldn't be useful to most users.