Category: File Formats

Airport Layout Cutoff for X-Plane 10.0

If you want to submit an airport layout to Robin in time for the X-Plane 10.0 scenery render, please submit it to him by October 1st, 2011.  See // for more info on the apt.dat file.

(And yes…my previous estimate for when we’d be cutting the scenery was off by a lot. I apologize to everyone who crunched for that deadline.  For what it’s worth, I told Austin that if we didn’t get X-Plane 10 out this year I’d cut off one of my fingers.)

We will not be accepting ground traffic flow layouts for the new ATC system or user-submitted building placements before the sim is released.  Once the sim is out there we’ll start working on that process.  Our intention is to collect and redistribute airport building layouts and ATC data (using X-plane 10’s new airport element art assets) the same way we do for apt.dat files.

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Three New Things for WED

WED 1.1 beta 4 is now posted – see the Scenery Tools wiki for download links.  Thanks to Matthias for the patches to remember the hierarchy’s open/close state – I suspect a lot of users will appreciate this detailed touch.  Thanks also to the handful of users who tested some early versions of this beta – there are a bunch of big changes in beta 4 that needed a few tries to get right.  (If you find bugs in WED or any of the scenery tools, please report them here.)

This will hopefully be the last beta for WED 1.1, setting us up to move on to WED 1.2 with X-Plane 10 editing features; beta 4 contains three areas of improvement that had needed attention.

Correct Preview Order

WED used to preview scenery based on the order of your work in the hierarchy.  This isn’t so good because X-Plane has its own draw order based on layer groups.  The next beta will do a much better job of showing the final draw order that X-Plane will use, so you aren’t surprised when you view your exported scenery.

Saving Work When the Undo System Blows Up

There has been one data loss bug in WED that has persisted since version 1.0: if the undo system blows up, you get a nasty error (“WorldEditor has hit an error due to a bug.  Please report the following to Ben…”) and no chance to save.

The next beta will automatically save your work in a special file (earth.crash.xml) that you can use to get your work back in the event that the undo system blows up.

Why does WED crash like this at all?  The problem is that the undo system saves incremental deltas as you edit your project, to save memory (so we can have a gajillion levels of undo).  If the book keeping gets screwed up, we are hopelessly borked because with missing deltas we can’t ever get the project back to a state that makes sense.  Once we lose deltas, we’re lost.  The error dialog box pops up when it does so that we can know that the problem is with the undo system – otherwise we’d just have an inexplicable crash with no idea what went wrong.

When last I checked, there were only one or two users still seeing undo problems – I’ve added additional logging, so if you do see problems please file bugs!

Changing the File Format

WED 1.0 uses a database as its file format.  While this seemed like a good idea at the time, it has since proven to be a performance and maintenance problem.  The next beta of WED will use an XML file format.

WED 1.1 will still read the old database format (including the format used in previous betas), so you can open your old projects.  Projects saved in the new beta will not open in the older betas, but this is no surprise.

For the nerds: when I first started working on WED, I thought that it might be useful for WED files to be databases so that bulk operations could be performed on large quantities of data.  Since then I’ve realized that WED isn’t a GIS or spatial database app – rather it’s a modeling tool.  The new XML file format is basically a direct output of its internal editing-centric format.

Posted in File Formats, Scenery, Tools by | 6 Comments

The Future of Precomputation

My previous post announced that a previously pre-computed operation (setting up the elevation of a DSF) will move into X-Plane in version 10.  We have multicore and we can save some space and improve quality.

This led one commenter to speculate as to whether the entire meshing process can move into X-Plane.  Unfortunately it cannot; a lot of other scenery creation processes are tied to pre-building the mesh, and some of those are still very, very slow.  But it does beg the question of how more we might move into X-Plane.

The Original Precomputation Decision

The original DSF strategy (designed for X-Plane 8.0 a long time ago) was to pre-compute all of the difficult problems for scenery tiles so that X-Plane could simply load the data and draw it.  This got us four things:

  1. We could use expensive algorithms to build scenery without hurting sim performance.
  2. We could pre-optimize the scenery for fast frame-rate, even if those optimizations were slow to compute.  (No one cares if building the scenery takes longer, that’s done before your DVD is burned.)
  3. In some cases, we could reduce data size if the finished computed output was smaller than the input files.
  4. Scenery is deterministic – because the work is done ahead of time, changes in the sim don’t affect how scenery looks.  New scenery technology mostly goes into the off-line scenery generator, which means less backward compatibility work.

The down-side of this decision was that we didn’t have a good way to integrate or modify scenery data on the fly.  Changes in apt.dat files, for example, are not reflected in the scenery, since the scenery is already “fully baked” when the sim boots.

From Precomputation to Multicore and GPU

With X-Plane 10 we’re doing a few things to move rendering work from “ahead of time” to “while you fly”, using multiple cores and the graphics card for the work, since those are areas where computers have gotten a lot more powerful.

  • In X-Plane 9, terrain is selected by slope (among other things) – this is why you see steep cliffs and moderate scree above grass and forests on a mountain side.  In X-Plane 10, some of this happens on the GPU – we build a single shader with three terrains, and the GPU picks amongst the terrain on the fly.
  • In X-Plane 9, we pass the raster elevation mesh to the DSF loader, rather than bake it into the mesh.  Eventually we may be able to pass this data directly to the GPU.

These two techniques are meant to someday play together: if the GPU has the elevation and knows how terrain changes with slope, then we can have detailed terrain on the GPU without bogging down the rest of X-Plane.

Going Further

How might we further push this technology?  I can imagine that at some point, if the sim can arbitrarily increase the triangle density of a mesh using the GPU and the raw elevation data, then we could reduce the triangle density in the DSF to only the triangles needed to represent land class changes, saving DSF space.

Using a triangulation to represent polygonal vector data isn’t as crazy as it might sound; there are a number of computer algorithms that subdivide polygons into triangles to lower the computational cost of processing them.  (We even use one such algorithm to simplify complex polygonal shapes – see MapHelpers.h in the scenery tools code for the scary details.)

So in the long term the triangle mesh that drives X-Plane scenery can change from an approximation of elevation (with terrain along for the ride) to an optimized way to store terrain polygons (with elevation to be applied later).

Here’s a clearing house of past links on the topic of precomputation and global scenery:

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Coping With New Features

Two more pictures from the test package Tom sent me.  These illustrate both some cool things that happen in X-Plane 10 with global lighting and the process of adopting the new features.  Our strategy for the new rendering features in X-Plane 10 is 3-fold:

  • When the new features (shadows and global lighting) are off, scenery that works in version 9 should just work.  So users always have the option to turn off the new features, use existing scenery, and get some fps back.
  • We try to minimize the artifacts between new features and old scenery.
  • We try to minimize the amount of rework necessary to be fully compatible with new features.  For example, switching from ATTR_poly_os to ATTR_draped is a simple search-and-replace job.

The hangar during the day, with shadows with visible skylights, required some bug fixing in the engine, and a new attribute.  Normally an object is either blended or not blended.  The problem is that a blended object doesn’t let light through for the purpose of shadows.  (Even with the “bug”, the hangar with shadows still looked pretty good!)

X-Plane 10 introduces a new OBJ attribute, ATTR_shadow_blend, that will make an object translucent for rendering (note the grime and dust on the windows) but fully transparent for shadows (hence the skylights let light through).  The attribute works the same as ATTR_no_blend syntax wise, making the update quick.

(You don’t have to use this attribute, but without it, the sim may not be able to produce quite as nice shadows.  Note that this attribute is not necessary for objects marked “glass” on an airplane – they are already handled by a separate process.)

There’s a second bug, visible in both pictures, but more visible in this second picture; the static airplane, which is from version 9, contains a ‘fake’ shadow, consisting of a single quad on the ground via ATTR_poly_os.  During the day, we have a double-shadow (both the one generated by the sim and the fake one) and at night the fake shadow does not go away.

With X-Plane 10, this kind of problem (a technique that is useful in X-Plane 9 clashes with new rendering settings) can be addressed via the conditional OBJ commands.  The conditional commands let you specify that certain parts of the OBJ are only to be used if the user has shadowing off (or global lighting off), for example.  Thus the old shadow works for users who turn off shadows, but goes away when the sim takes care of it.  The same technique can be used to have two versions of LIT textures (or even remove LIT textures) when global lighting is turned on and off.

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Three Things You Need for Fast Orthophotos

One user asked me about slow performance with an overlay scenery he created using GMaps.  There are three things you need for fast orthophotos in X-Plane, and unfortunately his scenery is missing one.


I’ve written before about DDS.  They key points here for orthophoto performance are:

  • Since DDS is already compressed, the CPU has to do less work preparing the texture if it is going to be compressed.
  • Since DDS is already pre-minified (meaning the smaller versions of the texture are already computed), if you are not running at ‘extreme’ resolution, the sim can simply load a lower res version.  With png, X-Plane must load the full size version and scale it down on the CPU.
  • Since DDS is already pre-minified, the sim doesn’t have to compute those minified versions of the texture on the CPU.

All of these things lead to much faster texture load times with DDS.


LOAD_CENTER is a directive that can be put in a .pol or .ter file to tell X-Plane at what location the texture needs to be at maximum resolution; X-Plane will reduce the resolution of the texture as you fly away from that point.  LOAD_CENTER is important for orthophotos for a few reasons:

  • It saves VRAM, since textures that are far away won’t be loaded at full resolution.
  • When combined with DDS, it improves load time.  Since some textures are loaded far away, they can be loaded at lower resolution, which (per above) is quick for a DDS file – less data, less load time.

Note that LOAD_CENTER causes X-Plane to reload the textures while you fly, so it requires at least one extra core to work well.  It’s really important to use DDS with LOAD_CENTER; otherwise that reload time can get expensive.

Use a base mesh, not draped .pol overlays.

This is probably the most important thing: if you want to cover a lot of area with orthophotos, you need to rebuild the base mesh using .ter files, not cover it with .pol files.  There are a few problems with using .pol files:

  • Draped polygons are only ‘built’ for areas relatively near the airplane.  So even under ideal circumstances, they are going to disappear in the far view.  This way of using them (only when near the airplane) is a memory savings based on their intended use: for small surface areas like airports.
  • Similarly, since the draped polygons are being built and destroyed as you fly, the amount of extra CPU work while flying is quite a bit higher with .pol files than with a base mesh (which only has to page the actual terrain).  So a computer that might be fine paging .ter files can get behind in its work for .pol files.  (Authors often use .pol files because they are easy to work with – specify a rectangle and X-Plane does the cutting and slicing…well, that work is happening while you fly, burning up CPU power that could be used to page the orthophotos.)
  • Since .pol files cover the base mesh, you pay for your mesh twice – once when X-Plane draws the base mesh and once when it covers over it with polygons.  This means twice the VRAM used to draw a frame and twice the fill rate.

If you want high performance orthophotos over an area any larger than an airport or down-town, please use .ter files!

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These four buildings are facade-scrapers, a new feature in X-Plane 10.

X-Plane 8 and 9 support facades.  A facade is a building definition that is extruded to an arbitrary shape and height specified in a DSF.  Facades allow us to build custom-shaped buildings that fit a precise footprint.  The walls are created by cutting and repeating a grid texture of windows.

With a facade-scraper, two objects are placed in the center of the facade to form a tower.  The facade base itself can exactly fit a city block, while the OBJ pair gives us a tower with architectural detail.

This is a wire-frame view; the facade itself is wire frame, and the ground is wire-frame, revealing the two objects.  The base of the facade sits on the ground, but the tower can move up and down, with excess underground.  This allows us to customize the height of the building to precisely meet the DSF height.

(This facade is a test case that I built out of Alex’s Chrysler building; the UV mapping on the base is pretty rough since I was just creating test files.)

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Two Features for Scenery Conversions

We have not been adding features into X-Plane specifically to aid conversion of scenery from MSFS (and I don’t think we will), but sometimes features we want outright help solve conversion issues too. Here are two new features I expect us to ship in version 10 that should help the conversion of overlay scenery packs.

Draped Geometry: I’ve mentioned this feature before; basically in X-Plane 10, the horizontal geometry that sits “on the ground” can be marked for real 3-d draping, rather than just polygon offset.  Draping is the process of actually cutting and shaping that geometry to perfectly ‘hug’ the ground – it’s how we can put a runway or taxiway on top of non-flat ground without Z thrash.

In X-Plane 9 you can create draped geometry, but you have to use .pol files; this can be awkward if you want to put an element in many times, share an element with other authors, or have an element on the ground match a 3-d object.

In most of the cases where I have seen serious Z thrash in scenery conversions, simply changing ATTR_poly_os to the new proposed ATTR_draped will fix the problem.

OBJ Elevation Control: X-Plane 10 will allow you to position an object vertically as well as horizontally, probably with something like this RFC.

For scenery conversions, this means that a scenery pack authored to a specific height elevation can be duplicated precisely.

For authors creating new work, this makes certain kinds of scenery possible that would have been very difficult before.  For example, we get asked fairly often about LPMA. which features a runway on pillars.  You can model as this as an OBJ with a hard surface, but in X-Plane 9 it’s very hard to predict what height that OBJ will be placed at. (Hack/work-around: place the OBJ in the water and model the geometry offset from the runway location.  Most scenery packs should have the water at MSL 0.)  With elevation control, the runway can be built at the real runway surface height and will be placed their explicitly.

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Plausible, Realistic, Procedural and Algorithmic

Alpilotx pointed me toward a thread on the org discussing Austin’s work on the weather system. The thread turned into a bit of a he-said-she-said with regards to Outerra and whether it could some day be combined with X-Plane.

This blog post will be a discussion of various general approaches to scenery and the trade-offs we have to consider, e.g. plausibility and realism, procedural vs. algorithmic and data driven design. But first, a brief note on Outerra. As I have said before, we are already aware of Outerra, so there is no need to email us. The bottom line is that we have a set of mostly done features for X-Plane 10, our goal is to finish X-Plane 10, and we are not even spending one brain cell considering putting a new rendering engine into X-Plane while we are trying to get 10.0 done.

Defining Some Terms

One of the problems with comparing scenery system approaches is that a real productized approach to scenery rarely fits into a perfect bucket or matches a single theoretical techniques. So here are some approximate terms, designed to generally describe an approach. They’re not going to be perfect fits, and even the definitions will fluctuate in different contexts and forums.

  • We can say scenery is plausible when it looks like it might exist somewhere in the world. Plausible means that roads don’t go straight up over a cliff, trees don’t grow in the ocean, etc. In other words, plausible scenery is scenery where absurd things don’t happen. Plausible scenery is great when you don’t know what an area should look like. A lack of plausibility is often a bug.
  • We can say scenery is realistic when it correlates closely with what is really present at a given location on the Earth. So if there really is a lake behind my house, realistic scenery has that lake. Plausible scenery might have a lake, a forest, or something else believable for where I live (the Northeastern United States). A giant sandy desert would not be plausible for my location.
  • We can say scenery is procedural if the detail in the scenery comes from some kind of algorithm that produces results. For example, a fractal coastline is procedural.
  • We can say scenery is data driven when the detail comes from some source of external input data. Our mountains are currently data driven – that is, the mountain shape basically comes directly from the DEMs we use.
  • We can say scenery is artist driven if the look of the scenery comes from art assets created by an art team.
  • We can say scenery is algorithm driven if part of its look comes from the transformational process that converts data from one form to another.

(I’m sort of drawing a line in the sand here with procedural vs. algorithmic, but what I’m trying to contrast is a program that generates ‘information’ out of thin air vs. a program that creates information out of other information. For example, in X-Plane 9, European capillary roads were procedural. We had no real data, so I wrote an algorithm that made them up in a manner that was consistent with underlying terrain. In version 10, these roads will be algorithmic; we take OSM data and then do some processing to make it suitable for X-plane. This is definitely a line in the sand kind of definition.)

So Are We Plausible or Realistic?

So the first question is: is the goal of X-Plane global scenery plausibility or realism? The answer is: a bit of both. Austin’s posts on the subject virtually always bring up plausibility. The reason for this is simple: he is not too worried about the amount of realism we’ve put into the scenery, but he is not happy with the bugs. He wants the bugs gone. So every time he and I speak, he says “and make sure it’s plausible!”

But we’re not going to remove realism just to fix plausibility bugs. I expect that the next global scenery render will be at least as realistic as the last – that is, we’re going to use better data and we’re not going to make up data where we had real information before.

There are limits to realism. We don’t expect the global scenery to ever be as realistic as a custom scenery package for a small matter. But realism does matter. Part of the joy of flying in a flight simulator is seeing the real world. Where we can have more realistic global scenery, we consider it to be a win, and we are always looking to be more realistic than the last render.

Plausibility for the version 10 render is going to take two forms:

  1. Bug fixes. Any time something screwy happens, it’s not plausible. Sometimes these are code bugs that must be fixed, and sometimes they are data conflicts. For example, the water data sasys “water” but the elevation data says “hill”. Combine them and you get water going up a hill. We have to write code to resolve this, somehow.
  2. We are reworking the way cities are rendered, because even at their best, the old approach, procedural buildings with algorithmic roads over land class photos, did not look plausible, even at its very highest setting. So this is a feature request to fix a plausibility problem.

Algorithmic or Procedural

I’ve discussed this before (and forgotten about the post). But to expand the discussion, we need to consider not only algorithmic and procedural data processing, but whether we are driven by procedural generation, input data, assets created by artists, or some combination. (In practice, all systems require a mix of data, art assets, and procedures and algorithms, it’s a question of the blend.)

I’ve been working on global scenery for a few years now, and over time I’ve come to appreciate the importance of artist input (via art assets) into any scenery process. Simply put, if you want scenery to look good, you need to make it reasonably straight forward for people who are good at making pretty pictures to control the look of your visual results. A few years ago I viewed the scenery process as strictly a question of data conversion and visualization, but now I see it as finding a way to merge art assets and data into a cogent final product, with the art assets being used in a way that the artists can control. In practice, this often means making sure that the art assets come in a format that artists are comfortable with or can learn without too much pain.

As I said in the previous post, our approach is becoming more algorithmic and less procedural as higher quality source data becomes available. (For example, we don’t have to generate European roads when we can import and reprocess them.) But our approach over time has always been heavily artist driven. By this I mean: our input data is algorithmically processed into a final form that makes sense only in the context of art assets, and we have a pretty good idea of what those art assets will look like when we design the algorithms. To use roads as an example again, our task with OSM is to convert OSM road data into a road network that will visualize nicely with road art assets created by an artist.

Procedural Compression

One way to view procedural scenery is “creating lots of information from little or no information”. But another wa
y to think of it is as a compression technology. As was correctly pointed out on the org forums, you use less storage specifying the overall location of a forest than you do specifying every tree individually. The compressed form (store the forest location) can be equally plausible. It will be less realistic if the original tree locations were based on real world data, but it will be equally (unrealistic) if the original tree locations were procedurally generated. Put another way, pushing procedural processes out of the scenery generation process and into the flight simulator makes DSFs smaller.

When I first started working on X-Plane 8 DSF scenery, not only was DVD size a factor, but so was load time; we had one core and it wasn’t a very fast core. Anything we could do to make loading faster, we did. Thus we pushed a lot of work into the scenery generation process, including procedural processes, to keep load time down.

Times have changed; we now have dual core machines as a baseline, and often quite a few more cores. Thus over time we are starting to move procedural processes back into the simulator, trading load time (which runs on multiple cores) for generation time and file size. So perhaps a more accurate statement would be: our scenery generation process is becoming more algorithmic and less procedural, and X-Plane itself is becoming more procedural. This is driven both by more input data (which must be processed up front) and more compute power on the host (which lets us shrink file size, and thus use DVD space for other things).

X-Plane 10

Here’s how this plays out in practice in version 10:

  • Some (but not all) of the building placement work* has been moved into X-Plane; a bit of expensive precomputation is still done at DSF generation time.
  • Some (but not all) of road processing has been moved into X-Plane; a lot is still done at DSF generation time.
  • Where possible, we are moving from a multi-layered approach to terrain to a pixel-shader-based approach to terrain. This cuts down overdraw and uses the GPU more efficiently. (The simplest example: in X-Plane 8 and 9, cliffs have separate terrains from hills. In X-Plane 10, a single terrain sits on both the cliff and the hill and changes its appearance based on the actual slope; this texture change is computed by the GPU.)

In other words, X-Plane 10 is making the logical evolution to better balance the computing resources we have to improve plausibility and realism.

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Curved Roads

At this point I can say with 99% confidence that X-Plane 10 will feature bezier curved roads. In X-Plane 9, a road is a line segment; you can simulate curved roads by using a lot of line segments, but the global scenery roads are pretty chunky.

X-Plane 10 allows for a road to be a bezier curve, allowing the specification of smooth curves with a small amount of data. This sets us up to trade off visual quality and performance using a rendering setting.

A few notes for authors:

  • Like all of the new v10 road features (and pretty much all of the new v10 scenery features), you don’t have to use bezier curves in your roads. They are there as an option if you want them.
  • X-Plane 10 will not make curves for you; road data that is defined as line segments in the DSF will be rendered as line segments. (This follows the principle that DSFs contain pre-processed scenery data, and the sim shows DSFs exactly as they are written.)

Pay No Attention to the Documentation

The DSF specification alludes to bezier curved roads; this “old way” of encoding curves was never supported in the sim – all versions of X-Plane ignore this data. The “old way” was how we thought we might do curves some day.

The version 10 curve encoding is different; the “old way” will continue to be ignored in version 10. So: do not use the DSF spec to try to make curved roads now. I will post detailed documentation on curved roads once version 10 is available to authors.

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Draped Object Geometry in X-Plane 10

I have mentioned a few of the scenery engine features coming in X-Plane 10 that will be of interest to authors: global illumination, conditional parts of OBJs (to cope with variable rendering settings). There is another general feature coming that will make authoring scenery a lot easier, I hope.

X-Plane 9’s rendering engine has the ability to drape geometry. Draped geometry are meshes that are ‘dropped’ onto the terrain and hug the underlying base mesh perfectly. The most common example of this is the runways: because the runways ‘drape’ the ground, the runway shows any curvature and bumps from the underlying base mesh. This is who we create sloping and non-flat runways.

Authors can drape geometry as well, using a draped polygon (.pol) primitive in an overlay. Such draped geometry is useful any time you want to add more “paint” to the ground, e.g. to put down a taxiway, parking markings, dirt, grass, a driveway for a house, you name it.

There is one case in X-Plane 9 where you cannot drape geometry: in an object. In an object, all geometry is aligned to the object, and will only interact nicely with the ground if you get lucky. For example, if you model a house with a sidewalk, the sidewalk won’t “sit” on the ground if the ground turns out to be sloped. You can use ATTR_poly_os to hide the artifacts, but ATTR_poly_os really can’t cope with mismatches between the OBJ and the terrain under it.

X-Plane 10 will introduce a new object attribute: ATTR_draped. Draped geometry in an object is actually draped down onto the terrain when the object is placed in the scenery. This means that the draped part of the object will hug the ground perfectly with no interference or Z thrash. You get all of the quality of a draped polygon with the convenience of an OBJ.

There are a few possible uses for ATTR_draped:

  • Any time a 3-d model needs some ground details attached to it, e.g. the driveway near a house, draped geometry provides a good fit with the ground and good alignment with the object.
  • Any time you want to include a pre-made ground decal (E.g. a painted parking spot on a taxiway), the ground detail can be modeled as an object using draped geometry.

ATTR_draped will facilitate creating and sharing custom details for airports and streamline the authoring process.

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