Understanding extrusion bridges

Lately I have been working with XML extrusion bridges for scenery and while doing so I noticed some weird artifacts. Even though I out the end points under the ground, they sometimes still float above it. And I also noticed that two bridges don’t always connect even though you specify the same position and altitude for the connecting point.

So I decided to dive a little deeper and see if I can understand what is going on. And after examining a lot of bridges I have come to the conclusion that the scenery engine adds a weird elevation offset to all bridges. And this offset is equal to the average height of all points of the bridge. So the taller your bridge is, the more the bridge is raised and thus also the more your end points will float.

The good news is now that I understand the logic behind the offset it is easy to compensate for it, to make sure you get the bridge you expect. But I still don’t understand why it was implemented like this. I mean when you type an altitude in the XML file that’s what you expect to get, right?

2 thoughts on “Understanding extrusion bridges

  1. Kevin Firth says:

    Nicely done Arno, I always suffered from anomalies like that and it took ages adjusting each point height til the bridge kind of made sense. I’m afraid I had neither sufficient time or patience to attempt a second bridge 🙁

  2. Gropied says:

    Hi Arno,

    some things in my mind about bridges :

    – last section of extrusion bridges are dependent of mesh altitude
    – there are 3 types of bridge : railway, roadway and bridges for human walking (smaller than others)
    – if a bridge across a freeway is too low upon the freeway, we see that traffic is crossing not under the bridge but upon the bridge
    – the bridge piers are determined in function of sections existing in OSM. Too few and often bad placed. It is necessary to make a new cutting before compilation in order to harmonize piers position

    Many files of extrusion bridges for French territory on my site.

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