Category: Scenery

Polygons Part 3 – Airport Grass Terrain

In my previous post I discussed airports using the apt.dat and polygons and when to use custom artwork in a DSF.

One problem with the X-Plane scenery system (and a topic for another blog) is that, because scenery is fully pre-processed*, introducing a new airport won’t put grass underneath the airport surface area. (Only airports that existed at the time the global scenery was created have grass underneath them.)

To make an airport there are two types of processing you need to do to the underlying area:

  1. Flatten it, if it has bumps. Since our mesh comes from SRTM, which is “first return” (meaning whatever the radar bounces off of first, that’s the height of the terrain, even if it’s a tree top), it often is quite bumpy.
  2. Change the underlying texture to something appropriate for an airport.

With X-Plane 8.60 (in RC as of this writing) you can do both! I’ve blogged about airport flattening (and why it isn’t very good) before; today I’ll try to describe how you can fix the base terrain.

You can cover the base mesh with a draped polygon in a DSF overlay. All you need to do is:

  • Create a .pol file that references a PNG for the texture and describes the resolution it will be applied at (i.e. how many pixels per meter) and
  • Use that .pol file in a DSF with a polygon in the overlay to describe where to put that texture down.

In X-Plane 8.60 we provide .pol files for you for all of the basic airport terrain textures. If you open the file Resources/default scenery/820 world terrain/library.txt you will find the library listing for the global scenery terrain. Toward the bottom you will see a section like this:

EXPORT lib/g8/pol/apt_sdry_cold.pol pol/apt_sdry_cold.pol

What this means is: the library path lib/g8/pol/apt_sdry_cold.pol is apped to the file pol/apt_sdry_cold.pol in this scenery package. You will see a fairly large number of .pol files on the left. Basically these represent the basic “environments” that an airport can exist in.

So to make an overlay DSF that has wet cold grass underneath it, make an overlay DSF with a .pol file using the path “lib/g8/pol/apt_wet_cold.pol”.

EDIT: Important — there are actually four .pol files in this library that do not work!! They are:


You will know they don’t work because your scenery will not load with them. Basically, they are accidental hold-overs from a previous naming scheme for airport terrain, but were never deleted. Note how they name only an amount of rainfall (except for apt_grass, which tells us nothing). The rest of the files are named things like apt_sdry_cool, which gives us both temperature and rainfall. This 2-dimensional climate grid (with rainfall and temperature) is how most of the X-Plane terrains are organized.

Why use the library system to do this? Three reasons: copywrite and VRAM, and compatibility.

One advantage of using our library is that it gives you a way to use our artwork without copying our artwork. Remember that you cannot copy the X-Plane PNG files into a custom package and then sell it – it’s a violation of X-Plane’s EULA. But you don’t need to! You can simply use the .pol file and X-Plane will load up the appropriate PNG files.

This is also a win for VRAM. If the wet grass texture is being used in the base terrain (for an airport that has wet grass underneath it from the global scenery) and in your overlay DSF (via the library .pol file) X-Plane will only load one copy of the texture, saving VRAM, speeding up load time, etc. If you copied the PNG file, besides being a EULA violation under some circumstances, you’d also be wasting VRAM. So the library system is more efficient!

Finally, by using our polygons, you ensure that your base grass will match the rest of the scenery system. If we update the look of our grass, because the library .pol files reference our artwork, you will see the same thing in your airport that we have in the default ones.

(Of course my warning from last week applies: if you use a wet airport grass polygon for something other than wet airport grass, then in the future our improvements on the artwork may make your scenery look strange. With these airport polygons we are aiming for “as much like an airport as possible”, not “as much like 850 used to look as possible”!!)

* By this I mean, all the decisions about how scenery will look are made ahead of time, so the look of scenery does not respond to the combining of multiple packages by changing the underlying packages – only by superimposing the new ones on top. Thus a new apt.dat layout does not change the underlying terrain from urban and forest to grass. This is a design trade-off; full pre-processing has both good and bad aspects.

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Warning: Orthophoto Polygons Are Not a Magic Bullet

With the new version of DSF2Text and X-Plane 8.60 it is possible to overlay orthophotos on top of DSFs. I’ll get into this in more detail in a future blog post…the purpose of this post is to be a wet blanket.

Using polygons to overlay orthophotos on the base mesh might seem perfect. It’s easy to do and you can clip polygons to any shape. Perfect, right?


There are a few major problems with overlay polygons that make them well suited to some cases but not a general purpose cure-all for orthophoto-based scenery. Here’s the fine print:

  1. Orthophoto polygons draw over the base mesh. In particular the base mesh is still drawn underneath the orthophoto. This means that wherever you use them, X-Plane is drawing twice! This is bad for performance…it will cut the speed of mesh drawing in half. Ouch! (This is not an issue when working on very small areas like airports, but it is an issue when working on very large areas.)
  2. Polygons are not built up until you fly near them. Polygons are part of the 3-d “clutter” part of the scenery not the base mesh, so we defer building them until we get near them to save memory. (This is the same as airports – look at an airport in textured map mode when far away from it — no taxiways!) But this means that if the orthophoto is huge (and thus visible far away) its absense will be quite obvious.
  3. Polygons burn CPU time when they are built up. For very small polygons this isn’t that noticable, but the larger the polygon, the more work X-Plane does “draping” it over the base terrain. So on single-CPU systems with large polygon orthophotos, the framerate may be adversely impacted.

Basically overlay polygons are great for texturing small areas like airports or special landmarks within a city, but are not appropriate for making wide-scale orthophoto base meshes. For that you really need to replace the base mesh DSF!

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The Future of Facades

Sergio and I were discussing the future of facades today. Facades are DSF polygons that are extruded into buildings by pushing up the polygon into a roof and texturing the roof and walls from a single texture using some simple formulas. The rules for facades are very simple – we originally thought them up for the purpose of creating buildings that precisely fit a city grid no matter what the block spacing.

The problem with facades is that the rules make very simple buildings – especially the roofs. So we want more powerful facades and the question becomes: how best to do this? There are two possibilities and I mention them here because they underscore what I think is the largest overarching design decision when creating a flight simulator scenery system.

1. We could make the facade builder in X-Plane smarter (write new rules).
2. We could write a stand-alone tool that converts the DSF polygons (plus a rule file) into OBJs that are then placed directly into the overlay.

This second strategy would turn the facade from a type of scenery element into a tool for making OBJs. We would be “pre-compiling” our facades.

Here are some of the considerations:


Precompiling primitives is a space-vs-speed trade-off. For any given algorithm, it’s usually faster to pre-build the entities and load them from disk than to build them in the sim. But pre-building means more files on disk, meaning bigger downloads, more DVDs, etc. Building up buildings from facades inside X-Plane is actually a form of compression.


This is one criteria where I think pre-compiling scenery is a clear win: it’s easier to build a stand-alone facade-to-OBJ converter than to implement it inside X-Plane. X-Plane is a relatively unfriendly place to build scenery algorithms because it’s busy being a flight sim.


If we make OBJs out of facades using a tool, the objects will look the same even if we create a new version of the tool. This is good in that it means that custom scenery won’t change how it looks, but it is also a limitation because improvements to facade technology require rebuilding a lot of objects to take effect.

(Another aspect of this: if the objects made from the tool use a texture, that texture can’t have its shape changed even if we make new rules, because some objects will be on users machines that were built with the old rules.)

X-Plane mostly errs on the side of pre-building scenery, and this is the one area where we really take it on the chin for that decision: the inability to easily create a new composite look from changes to some parts of the scenery. (Airports that don’t cause grass to appear under them is an example of this.)

Perhaps some good questions for weighing these options are:

1. Would an author care whether the facade looks exactly the same in the future?
2. Are there enhancements coming along in this technological area that we would not want to miss out on for older scenery?


Information loss: when we turn a facade into an object, the original polygon information is lost. Is it important that we ship the actual base polygons that make up buildings, or are the OBJs that represent them good enough?

Contextual Information: often we know more about a given situation when making scenery than when displaying it. Could we do better at building up a facade inside the sim (where we have context from other scenery packages), or when creating the sim (where we have a much larger dataset)?

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Polygons Part 2 – Real World vs Custom

In a previous post I discussed polygons, how they can be used, and a little bit about how they relate to X-Plane 850 and the new apt.dat system. I have been working on some demo scenery that will make this all clear, but the great is the enemy of the good, so rather than wait I’ll post more on this now and get the demos done as soon as I can.

There are essentially two ways to get at the new polygon code: via the apt.dat system or via an overlay DSF. When should you use apt.dat and when should you use an overlay DSF?

  • If you are trying to model something that is directly in the apt.dat spec, use an apt.dat file. For example, use apt.dat if you are making blue taxiway lights.
  • Use a custom overlay DSF if you are modeling outside an airport. (Do not make “fake” airports to use apt.dat features.)
  • If you need a custom look not supported by apt.dat, use an overlay DSF – it’s the only way.
  • Use a custom overlay DSF if you are modeling something that isn’t found in an airport, even if it looks similar. (For example, if you want blue lights to model some unique architecture in an airport, do not use apt.dat taxiway lights – your lights may look the same, but they are not the same!)

This last point is important: do not “abuse” the definitions of apt.dat files just because they look similar.

There are essentially two kinds of features in the scenery system:

  1. Features that do not change how they look, ever. For example, we do not change the way a textured triangle looks in an OBJ file.
  2. Features that are designed to model the real world. Over time, we change them to look more like the real world. For example, approach lights have changed a lot in 850 to look more like the real world ones.

This second type of feature is the one where I issue caution: if a feature in apt.dat or the sim is meant to emulate how the real world looks, we will change how it looks to improve rendering! This is why it is important not to use apt.dat feature for purposes other than they were intended for. It might be that in X-Plane 8.50 the blue taxi lights look just like some other feature you want to code. But in a future version, we might make them more realistic for an airport and they will look worse for your other application. By using apt.dat features only for their true purpose, you help us ensure that our changes to the base artwork make scenery better, not worse.

(This division of all scenery features between ones that are “stable” and ones that are “based on the real world” can also be seen in most parts of the sim. In particular, the flight model is designed with a “based on the real world” philosophy, a very controversial decision I’ll have to blog about some other time.)

Next: fixing airport terrain with polygons.

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I’m not dead! (Just coding tools)

I realized when I saw the “view stats” of 0 that I haven’t posted in maybe two weeks! Nothing’s wrong, I’ve just been busy coding scenery tools since 860 went into RC. I’ll post something useful here soon!

Just a quick random note: the maximum number of vertices in a DSF polygon is 65,535 vertices. If you have a polygon with more sides, you’ll need to simplify or subdivide it.

I would also add that the speed with which forests are built up is related ot the polygon complexity (as well as the total trees made). The algorithm is pretty fast, but you may want to consider a simplification algorithm that removes sides, making the polygon smaller, if the total error is less than a few meters (or whatever your tree spacing is), nuke the side!

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Airport Flattening, the Untold Story

I’ve been meaning to write this blog entry for about a year now. X-Plane 8 allows the airport surface area to be sloped. Here’s some of the back story and details.

Back when Austin and I were doing the design work for teh X-Plane 8 scenery system, we made a decision to allow sloped runways. The issue is that flattening the airport area requires the sim to edit the mesh on the fly, something we wanted to avoid.

(X-Plane’s scenery system is based on removing the editing of scenery from the sim itself…”X-Plane is not a GIS”. I did some slides on this once, I’ll try to post them soon.)

Instead we decided to simply drape the airports over the terrain, no matter how it was built. We figured that the sim’s engine could handle this (as it turned out, there were bugs that were fixed in the 8.20 patch) and in real life runways are often quite sloped!

Unfortunately when theory meets practice, things can get ugly…the biggest problem we saw was that in the original set of DSFs, the underlying terrain was very bumpy, and the smoothness requirements for a plane to take off are very high. (Flattening is also necessary to match the ground height with other sims for online flight.)

So we retrofitted the X-Plane engine with terrain flattening for airports. The flattening engine is meant as a last resort, to get absolute flatness and repair an already-flattened DSF. Its goal is not visual quality, but rather speed — that is, we can’t take 10 minutes analyzing the DSF each time we load one, or the sim will freeze pretty badly. (If this requirement that the flattener be fast ever goes away, we could do a much nicer job of flattening.)

The current flattening engine has two unfortunate properties designed to keep it fast:
– It flattens an area that is larger than the airport surface area (it rounds up) and
– It flattens vertices that are in the flattening area, not whole triangles.

The first limitation means that it may crush mountains around the airport, and is not appropriate for airports that are embedded in complex, hilly terrain.

The second limitation needs more examination. If a mesh triangle is partly inside the flattening area and partly outside, then the triangle is not flattened – one vertex is moved, and the others aren’t, which cause it to be sloped.

In these pictures, the blue lights represent the airport area perimeter, but the red lights show the full area that is flattened. I have artificially set the airport elevation much higher than surrounding terrain, to make the flattening obvious. Notice how some triangles become highly sloped!

When we make the global base scenery, we use the default airports from Robin’s database. So even if you do not want to submit your custom airport layout to Robin’s database, consider submitting some kind of layout to Robin. If an airport is present in the default scenery, then the area will be pre-flattened, which makes the sim’s flattening both work better and maybe even unnecessary.

Also, you can use the 130 code in a custom airport area to increase the airport boundaries, increasing the amount of flattening. But this is a mixed blessing – as you can see the mechanism is very imprecise. If you do use a 130 code in a layout and you submit the layout to Robin, please remove any 130 boundaries that have been set to a large area to flatten an airport.

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So Why is WorldMaker Still Around?

I realize this blog post will probably just inflame a bunch of email about how the scenery tools aren’t available yet, but I’ll answer the question and take the flames, because it’s a fair criticism and scenery tools are a fair feature request.

The long term direction of scenery tools is this:

  • Scenery tools will be separate from the X-Plane distribution, free, and open source. (This separation allows us to post scenery tool source without posting X-Plane source, and to use GPL code in the scenery tools.)
  • A few very basic editing functions (like adding nav-aids) are integrated into the sim to allow instructors to correct nav data during a training session.
  • WorldMaker therefore is no longer a scenery editor at all.

So why haven’t we killed it? We’ve been tempted to. But it will serve a long term purpose in the scenery tools ecosystem: it will be a small-footprint 3-d scenery previewer.

Because the scenery tools don’t use X-Plane code, the scenery tools will have two limits to their previewing capabilities:

  1. There is always the risk that with different code, the tools will preview scenery differently from X-Plane’s final render.
  2. Because the scenery tools don’t use X-Plane’s renderer, we basically have to rewrite viewing code in the scenery tools from scratch. That’s a lot of code, so for a while the preview in the tools will be limited.

Running X-Plane and the scenery tools at the same time isn’t a great option – since X-Plane loads a lot of scenery, and a weather model, and a plane, and then tries to run at max fps, it tends to be a bit of a pig in terms of system resources. WorldMaker will be a viewer that can reload your scenery quickly so you can have a 3-d view of what your end result will look like that will match X-Plane.

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It Ain’t Over ‘Til It’s Over

Just a gentle reminder: please do not consider anything in a beta to be final!

By this I mean: there is a risk that file formats that are new to the current sim (860) could further change during the current (860) beta run.

So…if you are working on scenery that depends on new 860 features, please do not ship your final scenery until 860 is finished! This way you can be sure you’re compatible with the final formats.

If your scenery uses 850 features and 860 breaks them, please report a bug, as 860 should handle any legal scenery that 850 handles.

(This goes for plugin datarefs too!)

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Scenery Polygons – Part 1

X-Plane 850 introduced some new polygon features that are worth looking at. Polygons are always legal in overlays, which makes them particularly flexible.

A polygon is any DSF entity described by one or more rings of points. A polygon’s look is defined by its artwork file, which also constrains its properties. Polygons may:

– Be draped, meaning the polygon in the DSF does not have altitude information, but instead takes its height from the underlying mesh. A non-draped polygon has altitude information. (Only draped polygons should be used in overlays because a non-draped polygon might not match the base mesh correctly. You can’t know whether a user has the 7-DVD DSF scenery, or old ENV scenery, or the older US DSFs.)

– Be curved (via bezier curves). This is only legal in some polygons.

– Have holes: some polygons can have additional interior polygons cutting “holes” in them.

X-Plane 8 shipped with one original type of polygon: facades. Facades are draped polygons with no holes or curves. X-Plane builds a building along the perimeter of the polygon. Facades were used heavily in the original US DSF scenery, and are useful for airport terminals, and even possibly fences.

X-Plane 820 added two new types of polygons: beaches and forests. Beaches are non-draped, non-curved, no-holes polygons that add a strip of beach texture between land and water. As mentioned above, I don’t recommend using them in overlays because they are not draped. (They are legally allowed in overlays but the chance for ugly results is large.)

Forests may have holes and are draped, but cannot be curved; they fill in their interior with 1-4 polygon trees. We do not have any forest polygons in the current global scenery, but some users have built their own overlays to add trees to X-Plane using these polygons.

X-Plane 850 introduced the new apt.dat format with a number of new features. Rather than make the apt.dat file customizable (which would make the file format complex and change its fundamental purpose), we added new polygon types that let authors use the same facilities as apt.dat files have, but with a lot more control and flexibility. Three new polygon types implement some of the new X-Plane 8.50 apt.dat features:

1. Object chain polygons. You can make a polygon and X-Plane will string a series of objects along the polygon. The polygon is draped and may be curved, but may not have holes. X-Plane 850 uses this for taxiway lights, but this could also be used to plant trees along the edges of fields. (You could also use this to put streetlights along roads, but the road file format can do that already without adding extra polygons to the DSF.)

2. Painted line polygons. You can make a polygon and X-plane will drape a painted line along the ground. These polygons are draped and may be curved, but may not have holes. X-Plane 850 uses this for taxiway lines.

3. Draped polygons. You can make a polygon and X-Plane will fill it in with some kind of texture along the ground. These polygons are draped, may be curved, and can have holes. X-Plane uses these to make curved taxiways, but they have a lot of other possibilities since they provide a way to change the terrain.

I will comment more on draped polygons in a future blog posting, but one immediate note: before X-Plane 850 if you added an airport it was impossible to convert the terrain underneath to grass (from whatever terrain might have been present). With X-Plane 850 you can now make a draped polygon using a grass texture and a DSF overlay with the perimeter of the airport and thus “patch” the texture of the mesh to look like grass.

This technique is not as good as the grass in the native airports for four reasons:
– Draped polygon performance is slower than the mesh itself – I’ll comment about that later.
– Our DSF creation program flattens airport areas – a draped polygon doesn’t so there can be SRTM DEM noise that makes the airport area too bumpy to use.
– The DSF terrain grass can have a soft border, but right now draped polygons always have a very sharp border.

Draped polygons still represent a better option than putting the runways over the existing terrain though.

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DSF Object Order is Not Draw Order

The order objects are listed in a DSF file (either the OBJ definitions or the actual OBJ instances) is not the same as the order X-Plane will draw them! X-Plane will change the order to maximize frame-rate, and the way it organizes them has changed in 850 (and will change again in futur versions).

Until X-Plane 850 there was no way to control the order of objects (although some packages do rely on X-Plane 840’s behavior and now look funny in 850). But with 850 there is a clean way to control draw order that will work well into the future.

Starting with X-Plane 850, there is a new command ATTR_layer_group…it let’s you override what part of the rendering process your object is drawn in, which is useful in a number of cases. Here’s how it works:

X-Plane draws the scenery in a series of steps or “layer groups”. Because of the way OpenGL transparency and polygon-offset affect Z buffering, X-Plane must draw the terrain first, all decals over the terrain second, 3-d buildings third, and anything translucent (clouds, smoke puffs, etc.) last. Layer groups organize all of the scenery into these catagories.

Objects are normally drawn in the “objects” layer group, which is intended for 3-d buildings, static airplanes, jetways, and anything else you might model with an OBJ. However ATTR_layer_group will let you specify that some objects be drawn earlier or later than others.

The command’s syntax is:

ATTR_layer_group objects 1

Where the name (“objects”, but others are allowed – see the OBJ spec) specifies a group, and the number makes the object be drawn after (positive numbers) or before (negative numbers). Objects with the same group and number are drawn together (in an unspecified order), but X-Plane goes by the numbers.

So for example if you want to ensure that a transparent hanger is drawn after a static plane, use ATTR_layer_group objects 1 on the hanger to draw it after all other objects. To draw some apron lines before the terminal, use ATTR_layer_group objects -1 to draw before buildings.

A few other tips:

  • Try to use as few offsets as possible – e.g. try to put all “draw last” objects into the group objects/1 – rather than using a different number for each.
  • Avoid using layer groups when they are not needed – they inhibit X-Plane from optimizing framerate.
  • If you do not use a layer group, but do use polygon offset, X-Plane sets the layer group to objects with a negative number. This is inhibited when you specify the layer group yourself, so be sure to use a negative offset layer group for polygon offset!
  • Try to organize your textures so that one texture is not used in multiple layer groups. For example, if you have translucent buildings that must be drawn later, group them in their own texture.
  • Related – try to keep polygon-offset geometry (pavement markings) in their own texture and objects. Share one texture for all markings, but don’t put the markings and object together.
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