Every now and then someone tries to set a taxiway in WED to “transparent”, and it pretty much never does what the author expects. Here’s a
brief explanation of what’s going on.
“Transparent” is one of the many built-in surface types that runways can take on in X-Plane; more commonly you would pick asphalt, concrete, or grass. So what is a transparent runway?
The answer is: it is a runway with:
- No texture. That means you see nothing where the runway is. (This is fast by the way; we are not drawing the runway with a 100% clear texture, we actually don’t even place the polygons.)
- No physics. The runway does not change the physics from the underlying ground.
At this point a sane author is thinking: then what does a transparent runway actually do? Why have a no-op?
The answer is: user interface and lights.
- A transparent runway is still a runway; and thus X-Plane can know “hey, there is a runway 3L at KXYZ airport.” X-Plane even knows where the runway is (since the transparent runway has ends and a width) and can thus start your aircraft ont hat runway.
- A transparent runway has approach lights and all other types of runway lights. A few of the common approach light fixtures with “rabbit” strobes are incredibly annoying to build by hand (you can do it, but you basically need a plugin, a gajillion objects, and super-human patience).
So the transparent runway lets you do the graphics and physics with draped polygons and leave the hard things (user interface and lights) to us.
The primary thing to note: the physics are up to you too, and the expectation is that you’ll do the physics with the same tool you’ll use for the graphics. So if you put a draped .pol file down, you can set its surface type (with the SURFACE directive) to match the visuals of the texture you are using.
Taxiways follow the same logic, and thus they are really quite silly.
- Physics and graphics are up to you – the taxiway does nothing.
- There really isn’t anything else to a taxiway; it isn’t part of the UI, and you can place taxi lights directly using light/line strings in WED. You don’t actually need the taxiway polygon.
The fact that you can make a transparent taxiway in WED is actually a bug – the UI simply knows all surface types and does not have special code to say “hey, for a taxiway this is silly!”
What Transparent Taxiways Are Not
Transparent taxiways and runways are not a way to get the physics without the graphics. Instead, get the physics by putting a surface directive on your draped polygons.*
* There is one inefficiency here: if you have a huge draped orthophoto that covers a wide area, it will contain imagery that spans multiple surfaces: grass, concrete, etc.
Here is my suggestion: overlay a second polygon (with a repeating texture at very high res) with some kind of “grit” overlay. Place this only on the areas with concrete (or asphalt, depending on the kind of grit you use) and set the overlay’s surface type to match the overlay’s appearance. This way the polygons you must place for physics correctness at least add visual value too.