The curved earth

 Ever since FsX has been released, there have been a lot of discussion about the display problems with the old style ground polygons that are used a lot in addon sceneries. In FsX they have added an additional curvation of the earth, but these older polygons remained as flat as they ever where, so that meant they did no longer follow the terrain correctly over greater distances. With SP1 some changes has be made, to quote Pill Taylor:

Made
significant progress to try to better map large, legacy add-on scenery to
correct for the new round Earth modeling in FSX. NOTE:  There will still be some
problems seen with this, but we’ve done our best to minimize the visual
artifacts.

So what does that mean for the addon developer? Yesterday I have been testing the ground polygons I had used on a general aviation I made a few months ago. For this airfield I had used GMax (with the Fs2002 gamepack) to make different layers of ground polygons. First there is a layer that contains an aerial image, this layer consists of two polygons of 1×1 km with each a 1024×1024 texture mapped on that. On top of this layer I put a noise texture to make the grass of the airport look less blurry when you taxi on it (a bit like the noise you see in the mesh scenery as well) and finally a layer that contains all the aprons and taxiways as polygons.

When I first loaded this airport in FsX it did not really show up like it should. There was a flatten problem, so I first made a new flatten with SBuilderX to be sure that the ground was flat underneath my airport (flat, but still curved of course). After that I noticed that all the aprons show up fine, but the layer with the aerial images did not.

Remembering that there was some discussion about the fixes that MS has added in SP1 only working on small polygons (about 100 meter in size), I decided to see what happened when I would split my polygon that contains the aerial images into more slices. So I used to slice tool of GMax to turn my 1×1 km polygon into a nice grid of 100×100 meter tiles. And guess what, when using this grid everything shows up perfectly in FsX. These smaller size polygons are nicely bended by the scenery engine to follow the earth curvation.

So what does that mean for when you are making ground polygons for FsX? The first conclusion is that not so many has actually changed. You can still export your polygons with a single reference, there is no need to give each 100×100 meter tile its own reference point. But you need to remember to add some extra vertices to your polygon if it is big. I have not checked until which tile size it still displays fine, so maybe 200×200 meter tiles also work. But I can tell you that 100×100 meter works for sure.

I think you can compare how the scenery engine does this with the bend tool we have in GMax itself. When you try to use it on simple 4 vertex polygon it does not work. But if you then slice your polygon into more segments, you will see that you can give it a bend with the modifier. This much be exactly the same the scenery engine now does to our ground polygon, all we need to do is give it a few more vertices to do its calculations with.

Does this now mean that we can continue developing addon scenery like we “always” did? Well maybe, but I would still encourage everybody to take a good look at the new features FsX offers us. In quite a few cases I think that the new increased mesh resolution can be a good replacement for the ground polygons we have used in the past. But of course there are also cases where these polygons are still the best option, markings on an airfield is one of these cases that comes to my mind right away.

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