Tag: inside x-plane

Draping Part 2: Draping a Polygon

In my previous post I described Z thrash and how polygon offset is used to fix it.  Now we can look at draping and runways.

The fundamental problem with sloped runways is that a runway is made up of tiles, but the ground underneath it may change slopes in unrelated locations.  The following diagram illustrates the problem.

This is a side view of a sloped runwy: the green line is the underlying bumpy terrain and the black line is the runway, divided into multiple tiles.  Recall from last time that we can only avoid Z-thrash using polygon offset if the two polygons are truly coplanar.  Therefore, we can look at four ways to handle the situation and evaluate them for quality:

  1. In the top picture, the runway simply runs right through the terrain.  This case won’t work, with or without polygon offset.  In some cases the terrain is on top, and in some cases the runway is on top.  The gap between the runway and terrain might be quite large for a significantly sloped runway.
  2. In the second picture, we move the runway up to clear all but the tallest part of the terrain.  This won’t Z-thrash, but it will look pretty bad.  There can be a large gap between the runway and the terrain; if you run your airplane off the side of the runway, you’re going to fall.  The gap between the runway and terrain will be quite noticeable.
  3. The third picture is close – each runway tile is clamped to the terrain.  But this still isn’t good enough; while most runway tiles now sit on the ground perfectly, a few will take a ‘short cut’ over or under the ground, like the case I have circled in red.  So we’ll have less Z thrash, but some parts of the runway will still be broken.
  4. The last picture is what we really need: we break the runway tiles up into even smaller pieces where the terrain makes a change.  The last tile on the right is now two tiles, each of which can sit perfectly on part of the terrain.  Only in this last case can we apply polygon offset and get a nice clean runway.

This last case is exactly what X-Plane does.  This technique is called draping – the sim ‘drapes’ the overlaid runway tiles on top of the runway like a rug.  The sim breaks the tiles into pieces so that each tile can sit perfectly on the underlying terrain.

The real work of draping is not just in figuring out where the terrain below the tile is, it is in cutting the tile into the appropriate pieces that can ‘sit right’.  The following pictures show this process in 3-d.

This is a screen-shot of a small piece of taxiway at KSAN.  I have drawn in the outline of the draped polygon in yellow and the terrain triangles in blue.  (The terrain triangles are quite large, so we can only see part of them running through the scene.

This is a wire frame of the problem at hand: we need to insert the yellow polygon into X-Plane, but we need to make sure that no triangles that we generate cross the blue line.  The blue lines essentially represent “ridge lines” in the underlying mesh.

If we just had to draw the yellow polygon, we could triangulate it using the thick gray lines shown in this picture.  However, this would not be good; the big gray triangle in the middle of this picture runways right across a blue line – there’s no way that this big gray triangle can “drape” properly across the ridge line.

This is what actually ends up happening.  The gray lines are cuts we need to triangulate the polygon; the red line are cuts we need to not cross the ridge line, and the purple lines are cuts we need to ensure that everything is broken down into triangles (since that’s what the video card really wants).  Once we have this triangulation, we can place every vertex on the ground and it will “drape” the way it should.

In my next post I’ll describe how you can use draping in your scenery to create images on the ground without Z thrash.

Posted in Scenery by | 13 Comments

Draping Part 1: Z Thrash

I think I have mentioned draping repeatedly, as well as Z thrash, without ever clearly explaining what either of these things are.  I happen to have some pictures from recent work on draping, so here’s the basic situation.

What Is Thrash?

Warning to OpenGL purists: I’m going to play fast and loose with a number of OpenGL rules.  Basically what I say here is the worst case.  Sometimes things work out better than I describe, but if you don’t handle the worst case, you’re going to have problems some times.  Nitpickers, consider yourself “on notice”.

When drawing 3-d polygons in OpenGL, if two polygons are coplanar, most of the time they will experience “Z thrash” – that is, the hidden surface removal hardware on the graphics card removes some parts of one polygon and some parts of the other, often removing different parts on each frame in a flickering pattern.  The results look like this:

In this picture on the left, we see the raw result of coplanar airport polygons on top of the sandy desert terrain that is the airport surface area.  As you can see, the graphics card has been a bit random in deciding whether the runways are “hidden” by the ground or not.  If you could see the camera moving, you would see that the pattern of missing taxiways ‘flickers’ as the camera moves.

If there is an overall take-away point so far, it’s this: you can’t just layer coplanar triangles without getting a mess.

(What about moving the runway slightly higher than the pavement?  This would have several problems.  First, the amount we’d have to move them is horribly dependent on camera angle and video card Z-buffer performance.  Second, we’d see the gap between layers; for markings that really do need to be coplanar, like a rubber mark on a taxiway, this would appear as a bug.)

The Cure is Worse Than the Disease

Fortunately if we have truly coplanar geometry, OpenGL gives us a weapon to fight this: polygon offset.  Polygon offset is a cheat that tells the video card: “listen, if you ever have doubts about the runway and the ground, favor the runways by a little bit.”  It’s a little bit like putting a spread on a football game: the ground would have to be closer than the runway by a margin before we stop seeing.

When we have coplanar geometry, we can use polygon offset to handicap the geometry we want on top.  Problem solved, right?  So why do I keep writing blog posts with titles like: The Sordid History of ATTR_poly_os or The road to hell is paved with ATTR_poly_os?

Polygon offset has three problems that can bite you:

  1. The amount of ‘margin’ you get depends on the camera angle.  This can make for some truly bizarre effects.  The only way to safely manage this is to carefully minimize the margin.  (This is a problem for the programmer, not the artist.)
  2. Because the margin must be minimized, the geometry really has to be coplanar – small cheats can exceed the margin.
  3. The margin you give your top polygon applies to anything else that is nearby, which can cause even more weird artifacts.  For example, if you polygon offset a taxiway, it can start appearing through the nose wheels of airplane.  That’s another aspect that must be carefully controlled.

If there’s a take-away to polygon offset, it’s this: polygon offset can help fight Z-thrash, but it must be used in a very narrow set of cases, and it must be used very carefully.

In my next post I’ll describe how X-Plane uses draping to fight Z-thrash for sloped runways.

Posted in Scenery by | 7 Comments

ATC Part II – A Dilemma

Here’s a thought experiment for you: you have a new version of X-Plane with new global scenery, new rendering engine options, new weather, a whole fleet of new planes, and new ATC. That new ATC is rewritten to support realistic IFR flight, the AI planes participate, the system includes ground ops and it uses audio files to “talk”. But the AI planes don’t have real-world liveries, and they don’t follow real-world schedules for major airports. What do you do?

  1. Release this new version of X-Plane with the ATC as is, and continue to improve it in a patch.
  2. Release this new version of X-Plane, but ship the old ATC from version 9, because while the new ATC is better, it’s missing some features that some users have asked for.
  3. Delay the release of the entire sim because ATC is missing some features that some users have asked for.

Clearly options 2 and 3 aren’t options at all; you wouldn’t ship the old ATC (which is clearly inferior to the new code) just because there are more features you could ship, and you wouldn’t withhold the rest of the new version’s features for the same reason.

No feature is ever done unless a program is dead. Austin hired me about five years ago to work on the scenery system, and it is no more done now than it was five years ago. It is a lot better than it used to be; more efficient, better looking, more realistic, etc. But the bar for what is possible keeps moving. That’s part of what makes working on flight simulation software so interesting.

In yesterday’s post I tried to clarify why we are using real physics in the AI ATC planes; a lot of the discussion surrounding Austin’s original announcement made some assumptions about where CPU time is spent that aren’t correct. This post describes how features are incrementally added to a release – without understanding release planning, Austin’s description of 20 AI planes makes no sense.

Stretching the Playdough
Programs aren’t just born whole, complete, never to be modified again. If there is one consistent misunderstanding of software, it’s this notion that you “build” software the way you might build a bridge or a house. The truth is, software is made out of a building material that is nothing like bricks or steel, and thus the engineering practices have to be quite different.

If you build a house and decide you want a different exterior floor plan, you’re pretty much hosed. No one is going to change the outer shape of a house because the materials used to build the house are not particularly pliable to modification. The cost of moving a foundation is about the same as building a new foundation, so modifications are made as minimal as possible.*

By comparison, code is a lot more like playdough than bricks or concrete. You want the master bedroom a little bigger? Great, we’ll just streeeeeetch it out. Code is always modifiable after it is “finished” – if treated right, it never hardens and sets, and this is why successful programs are often on version 10. The problems a computer program tries to solve change, but code can change with them.

So my first take-away point is this: if we add a feature to a program later, it is not because the program was “incomplete” or “unfinished” before – perpetual improvement is the expected norm in a healthy computer program. In fact, perpetual improvement is desirable because it lets us match changing technology and because it lets us incorporate feedback we get from our user base.

(There’s an old saw in computer software that the reason to put out version 1 is to learn what you did wrong so you can make version 2. That’s a bit of an exaggeration, but the truth is that the only way to get really solid user feedback is to put something out there and then listen.)


Given that we expect our software to change perpetually, how do we plan? The answer is that we need a road map for the next several steps that the software will take as it evolves. By knowing where we are going we can be reasonably well-informed on the features we code now.

A road map of features needs to be prioritized for two considerations:

  • What do users want first? The things we code first, we can ship first, so high priority features should go first.
  • What features have to be implemented first? You can’t build the roof of a house first, and we couldn’t have programmed orthophoto paging of DSFs until we programmed loading DSFs themselves. Sometimes the features “have” to go ina certain order for programming reasons.

ATC AI Airplanes

At this point we finally have enough common vocabulary to sanely discuss the AI airplanes for X-Plane’s ATC. So far I have tried to establish that:

  • The AI airplanes are going to get better over time.
  • We will almost certainly ship ATC AI before the AI airplanes are perfect.
  • The order that we implement ATC features will be a mix of what we think is most important and what has to go in first for engineering reasons.

The first stage of the roadmap was to use X-Plane’s built-in AI planes as AI traffic. There were a lot of reasons to do this as the first step:

  1. The code’s already there, so it let us get some airplanes moving around quickly. Inventing a fully separate system from scratch would have taken time away from the rest of ATC.
  2. Virtually every optimization that has been suggested (and several that haven’t been) can be applied to the AI ATC airplanes, so there’s plenty of room to get faster. (If we thought we couldn’t optimize, we might not have put the flightmodel in for AI at all.)
  3. By sharing the flight model code with AI, the optimizations we do pay twice – to make ATC faster and to make the framerate faster for all users.
  4. Inventing a new way to make airplanes (only for ATC) would mean two sets of editing tools, which would invariably mean worse tools in all cases.

With the AI planes running, we then have a lot of ways we can improve things. We can optimize performance. We can optimize memory use. We can increase the airplane limit. We can virtualize the flights (so far away flights exist only in the ATC DB and don’t use a “real” AI plane). We can sever the 1:1 mapping of airplane model to flight. We can recycle airplanes when they get too far away to be adding anything to the user experience.

Which techniques will we do, and what will the airplane limit be? I don’t know. Optimization really needs to be driven by testing – that is, we need to improve the system by measuring it to see what the next best step is, not by speculating about what problems we might hit in the future. Over time, we will repeatedly optimize and the capabilities of the system will steadily grow.

If you want to know the final specs and capabilities for the ATC system, I fear you’ll have to wait until X-Plane 10 ships. When we are in development, any planning for future features is just that: a plan. As we hit real bugs and learn more about the features, the plan can, and often does change. Thus estimates of what X-Plane 10 will do won’t be truly reliable until we ship.

* I should be careful here; I used to have these conversations with Sergio, who is a professional architect among other things, and he would have to correct me on a million different aspects of construction technology.

Posted in Air Traffic Control, Development, News by | 26 Comments

ATC Part I – Real Physics

This is a repost of a reply I wrote on avsim that sort of grew out of control. In part II I will try to explain what we’re doing with the ATC system and how far we will get in X-Plane 10.0. Anyway, post follows.

I will have to write a blog post to go into this in more detail later, but I think a lot of what has been written here is wrong. Y’all started under the assumptions that:

  1. The flight model is going to be too expensive to run on AI planes and
  2. A table based flight model would be faster.

Those are both questionable assumptions at best…pretty much everything you can conclude from that is, IMO, dubious.

(I should mention at this point that I am unaware of any “dumbing down” features in the FM right now. My understanding is that we will pre-process control inputs in a number of ways, and the frequency of the FM can be set to multiples of the framerate, but even at the lowest setting, we do a full physics integration and the plane is the sum of the physics that are applied to it. I mention this now because I am about to speculate on some optimizations that _could_ dumb down the physics model, and I want to make clear that shipping X-Plane 9 does not do this!!)

Here’s the short version:

The most expensive part of the flight model is ground interaction – that is, the flight model doing collision checking with the ground and various parts of the airplane. When an airplane is “clearly in the air” (e.g. an initial test shows it has high altitude) the FM isn’t very expensive; when it is near the ground, CPU time cranks up as we make sure to get the touch-down characteristics just right; same with taxiing.

So if we want to make the FM faster, there’s really only one place to attack: ground interactions – it’s the lions share of CPU time. There are a number of simple things we could do to improve ground interaction. For example, we could stop checking for body scrapes – since X-Plane has to handle physics correctly even if the user lands gear up and scrapes an engine, the sim normally tests the full geometry of the plane against the ground (which is not flat, even at an airport) – that adds up. If we are willing to trust that the AI planes don’t screw up a landing* we could cut down ground check to only real landing gear, which would improve performance.

Now what if we did some kind of ‘lo-fi’ AI, whether it’s table based or it simply says “move the plane forward by this much” (E.g. a sort of track-based system)? If we want the airplanes to ‘sit’ properly on the non-flat airport surfaces, we _still_ need to do the most expensive part of the FM – the wheel-ground collision checks. So the total savings of a ‘lo-fi’ AI flight model would be very small, because at best we might partly improve the performance of code that doesn’t have much impact on the sim.

(To understand why you can only boost performance by attacking the biggest pigs, see here.)

However, there would be a pretty huge cost to a lo-fi flight model: we would have to code a SECOND implementation of pretty much everything we already do in the real flight model! We would have to have new flight model files to support this new alternate flight model. The opportunity cost here is in developer time…the time spent building a separate flight model could have been spent performance-tuning the real flight model…even if we had a second flight model, performance tuning time would now be divided between the two flight models, and neither would reach its optimal performance.

Besides my explanation above of why a lo-fi flight model wouldn’t really be a win, two more comments:

In software development, it often pays to try the simplest thing first, see how it works, and go from there, rather than speculate how a system may perform and write a ton of code up front before you have real data. This is what we are doing…the simplest thing we can do is to run the real FM on the AI planes, and so far it looks like it’s going to work reasonably. IF we hit data that says “no we have to do something radical”, then we will…when the data says so, and no sooner. So far indications are that the real FM is going to be fine, and this makes sense from what we know about its performance characteristics. We also know that we have a lot of tricks we could pull to make the real FM faster for AI planes (e.g. removing engine scrape-checks, per above) before we have to go and write a whole new FM.

And finally, dude, the real FM looks good. With the real FM, the AI planes move the way big heavy airplanes should move. They track the ground perfectly. If the ground has a bump and the airplane’s suspension is loose, it sways like it should. The control surfaces deploy with their real time. When you’re at an airport performing ground ops, you can get really close to the AI planes, and at that point these things matter! I speculate: once you take follow an AI plane running the real FM on the ground, it’ll be hard to go back to a ‘synthetic’ FM.

* This may not be a safe assumption…what if a microburst hits an AI plane?

Posted in Air Traffic Control, Development, News by | 13 Comments

Light Levels: Don’t Panic

One of the comments on the suburban preview screenshots I’ve heard in a bunch of places is “I hope it’s not going to be that dark” or “could you guys add more light”?

At this point, light levels in our previews are not reflective (no pun intended) of how the sim will really look. The reason is simple: the light model is not fully debugged (who am I kidding — it’s not even remotely debugged) and it only takes one light model bug to completely throw off the light levels. So I think light levels are going to be a case where the sim’s lighting looks a bit funny right up until the last bug is swatted – it’s just the nature of those kinds of bugs.

To give you an idea of how much change there is in lighting from version 9 to 10, here’s a short laundry list of sim changes that affect lighting:

  • Dynamic Exposure. X-Plane 10 reduces the effect of emissive (_LIT) textures base on the brightness of the sun. In X-Plane 9, emissive textures have the same impact regardless of time of day; thus a lighting effect that looks good at night will look too strong during the day. (In real life, you eyes would adjust for the sun and the artificial light would seem less bright.)

  • Linear Light Mode. This gets confusing fast, but basically, our eyes perceive light in a non-linear way; we are more sensitive to low light levels than bright light levels. Computer graphics mimick this behavior; the result is that most computer lighting models are physically incorrect in some circumstances. Using the non-linear eye-based behavior has been the norm for while because it was cheaper hardware-wise, but these days it is possible to do physically correct linear lighting. We are adding this wherever we can; the correctness varies with rendering settings since physically correct lighting is more expensive GPU-wise and we don’t want to hurt fps for low-end users.

  • Deferred Rendering. X-Plane 10 has two rendering modes: a forward renderer (which is a lot like X-Plane 9) and a deferred renderer that supports global illumination and an HDR rendering space.* This creates a certain amount of chaos because the code for forward and deferred rendering are separate, and they seem to develop separate, unrelated lighting bugs.

  • Global Illumination. X-Plane 10 supports global illumination in deferred rendering mode, which means that thousands of lights can light up any part of the scenery system. Thsi means that (for the first time) an object may be lit by dozens of light sources at once. It turns out that the linear light model is a lot more important when we have more than one light source. (In fact, Alex and I realized that we needed a linear light model when looking at highways lit by streetlamps.)

  • New Light Billboards. X-Plane, like most flight simulators, uses billboards (textured squares that face the camera) to draw the light effects near a light source, like glare and bloom. The shape, textures, and equations for the light billboards are heavily revised in version 10.

  • Clouds. The weather system is being rebuilt, including new shaders for cloud puffs. Since cloud puffs aren’t like solid buildings or airplanes, they have their own shaders with their own light characteristics. We are also experimenting with increased ground visibility, which affects fog.

If there’s a take-away point, it’s this: lighting isn’t just one piece of code in X-Plane – it’s the sum and interaction of a large number of features, all of which are being heavily worked over. Only when all of the features work correctly and work together in harmony will we have what appears to be sane lighting.

* Some users may confuse HDR, which just means an image with increased dynamic range for light levels, with the more common effects that games ship once they have an HDR render: bloom and tone mapping. Bloom is when bright light sources “blow out” and splat light around nearby areas; tone mapping is a technique to visualize that high dynamic range on a normal monitor – often it is used to simulate your eyes adjusting to variable light levels.

I do not think we will ship bloom in version 10.0; I experimented with it and found it had almost no value. First, there are very few scenes in a flight simulator where bloom is that useful; it seems to be a lot more useful for interior rendering, like you’d see in a first person shooter. Second, X-Plane already has a number of bloom-like effects, including halos around lights via billboards, sun glare, etc. With most of the important cases already covered by ad-hoc effects, my early experiments with bloom weren’t very promising. We may revisit bloom later, but I don’t think it’s as important as other effects for now.

Similarly, I don’t think we will have dynamic tone mapping because we will have an overall dynamic exposure control running all of the time. Again, the value of tone mapping is more obvious with first person shooters, where you can go from interior to exterior and you want the world to be ‘overpoweringly bright’ for a while. By comparison, pilots do their best to preserve their night vision, and the interior of an airplane is designed to match that; instruments auto-calibrate their brightness to the overall light levels, making tone mapping less important.

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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.

Posted in Development, File Formats, Scenery by | 9 Comments

100 Mile Visibility

First, Happy New Year! As is typical, I’ve been quiet on the blog because things have been insanely busy here at work. Just to give you an idea of the insanity:

  • There will be a 9.63 relatively soon – the bug driving this is some Linux distros not finding the DVD. But we’ll get a few new datarefs in there too.
  • We have new updaters and installers to get tested, again addressing Linux DVD issues, but also with updated web download code that should give a nice speed boost.
  • Chris has been working hard on Android. X-Plane for Android is pretty much the biggest APK anyone has tried to ship, and as a result we’ve hit a number of problems with the market that we are going to work around.
  • All that’s just the side show; X-Plane 10 development is of course the meat and potatoes.

Now, about visibility. X-Plane 9 restricts ground visibility to 25 nm (about 46 km) in an attempt to prevent you from seeing off the edge of scenery tiles. Many users have expressed (some more persistently than I would have liked) an interest in longer range visibility. Austin recently posted a note to X-Plane.org discussing level of detail and distance management in the new weather system, and users immediately picked up on his mention of 100 nm visibility. Here’s what we’re thinking; all of this is subject to change as we keep working on the product.

First, visibility: you can come up with a formula for the distance to the horizon based on height above a sphere: d = sqrt((r+h)^2 – r^2) where r is the radius of the planet and h is the height above the planet. Since the Earth is roughly 6 million meters in radius, we get a visibility to the horizon of:

100 meters: 34.6 km
500 meters: 77.4 km
1000 meters: 109.5 km
10000 meters: 346.5 km

Clearly a little bit of altitude lets you see a long way.

But there’s more to it than that: X-Plane has always changed the visible distance with altitude. The 25 nm limit applies to surface observations (which is what you get from a METAR). As you move up into orbit, that distance is scaled out to the horizon distance, so that you can see the whole planet from orbit. That scaling can reveal the edge of DSFs, which are blended into the planet when volumetric fog is enabled.

So here is what I think we really need to do:

  • We do need a larger ‘surface level’ maximum visibility, so that distant features are visible from the ground.

  • We need a scaling from ground to upper atmospheric visibility that gives us more visibility sooner; one of the problems with version 9 is that the increase of visibility is slow, which gives mid-elevations a hazy look.

  • In the long term, we need to load more DSFs, probably twelve instead of six. X-Plane 10 already has some improvements in how scenery shift is done, but my guess is that we can’t productize this until we have a 64-bit build (since more DSFs chew more memory), so I expect this to happen in a patch.

  • We need to add elevation displacement to the whole-Earth planet render, so that the blend between DSFs and the planet don’t have huge height gaps at high-elevation locations. I am hoping we’ll have this in 10.0, but it is not coded yet. (Usually we recut the planet textures last, since they are cut off of the DSFs.)

  • We need to improve the quality of haze, fog and atmospherics. In real life, atmospheric scattering reduces the contrast of far away terrain. I believe that correct scattering could make a huge difference in the quality of the transition from DSF to planet, the required tex res (we need less if we scatter more), and generally it would be a big contribution to the realism of the image.

    I’m not sure how much of this we’ll get into 10.0; I have a prototype of Sean O’Neil’s atmospheric scattering shader from GPU Gems 2 running in the sim, but I don’t think it’s shippable. I do hope we’ll get at least some scattering in place, with improvements in patches.

That’s a road map, at least. If there’s a take-away point, it’s this: increasing visibility is complex and involves a lot of parts of the sim and there are still significant parts that need work. So I really don’t know if we’ll hit some kind of hitch or problem that requires us to back off visibility.

Austin’s comments about 100 nm visibility reflect what the slider in the sim happens to be set to now. It’s also a design goal of the new weather system – that is, we want the new weather system to handle significantly larger distances (and have better scalability) than the old one did.

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A Cliff Shader

I have been stingy with pictures of next-gen global scenery for one reason: it’s really hard to get a nice shot of the global scenery that doesn’t show unfinished features. With something like global lighting I can zoom in and show just the new trick, but with global scenery, I can’t take a picture of a new house without showing a city block that looks funky due to a bug and a road that isn’t finished. Posting a working shot of the global scenery where some sub-systems have bugs and artifacts would just freak everyone out.

I figure if it’s obvious that the shot isn’t a production shot, I can get away with posting it though.

A lot of the times when I work on the rendering engine, it is with test textures like this. Our art team does their best to hide the seams between different art assets, so that the scenery looks like one continuous world. The problem for me is that the better they do, the harder it is for me to tell if the underlying shaders are doing what they should do.

So alpilotx sent this test: it’s all of the Innsbruck area painted with a test texture. What’s new and interesting here is that the flat, hill, and cliff areas are all shaded by a single shader that selects between multiple textures (and rotates the textures) based on the underlying mesh.

We are adding the cliff shader to version 10 for a few reasons:

  • Often we can get better cliff and hill definition by processing in the shader than by painting different triangles with different textures; our ability to control the transitions using different .ter files is limited.
  • Using one slope-sensitive shader saves over-draw and triangle count, which makes the DSFs faster and smaller.
  • Some day we may have the GPU distorting mountains on the fly to make them more mountainous. If we do, we need the GPU to also apply the correct textures; if the cliff areas are precomputed then they won’t respond to GPU distortion.
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Will X-Plane 10 Have X?

If I could have a dime for every email I have received that asks some form of “will X-Plane 10 have X” (where X is a feature or enhancement), I wouldn’t need to actually work on X-Plane anymore. (If you think your email triggered this post, well, there are approximately 100 other users who have asked the same thing.)

Simply put: I have no idea and I’m not going to try to answer these questions any more. Here’s why:

For as long as I have been involved with X-Plane, Laminar Research has provided free patches to the simulator throughout a major version run, and those patches have included not only performance enhancements and bug fixes, but also major new features.

So the question “will X-Plane 10 have X” can really mean one of two things:

  1. Will X-Plane 10.0 have feature X immediately ‘on the DVD’?
  2. Will X-Plane 10.x ever have feature X in a free patch before the major version run is over.

I can answer the first question, because we are relatively locked down on what features are still on the table for 10.0 vs. what must wait, but I think it’s at best confusing to do so. If a feature isn’t on the DVD, it might be in a free patch within weeks; it might be available by the time you get your DVD. Whether a feature is on the DVD is of interest to us as we plan our release, but I don’t think it actually makes a huge difference to users with internet connections.

Consider 64-bit – it’s something we want to look at during the version 10 run but we’re not going dig into it until after we get 10.0 out. So will 10.0 be 64-bit? No. But there will probably be a 64-bit patch available for free. I think you can see why I don’t want to post “X-Plane 10 will not be 64 bit.”

I cannot possibly answer the second question, because versions run over several years, and what we code for the end of the version run will depend on market conditions and technology that don’t exist now. One of the nice things about patching X-Plane frequently is that we can revise our plans as conditions change.

Consider the question “how many cores will X-Plane 8 utilize” had you asked the question during X-Plane 8.0. When X-Plane 8.0 came out, the answer was “only one” and we had no road-map to change that. For that matter, multi-core machines were rare and exotic beasts at the time, so multi-core wasn’t a priority.

Within the three years of X-Plane 8’s major version run, we ended up supporting multi-core for scenery mesh loading, something that couldn’t have been easily predicted at the beginning of the version run.

Finally, a note on release planning: now is absolutely not a good time to ask for features. The features that will ship in X-Plane 10.0 have already been determined, and since we’d like to ship X-Plane 10 sooner rather than later, I don’t think there’s anything you can say that would make us add a feature to 10.0.

All future features are going into a 10.x “bucket” for planning purposes. Since Austin, Chris and I are up to our eyeballs in code and the art team is red-lined too, we’re not spending any time sifting through 10.x buckets right now. If you send us a feature request, the very best case is that we dump it in a holding list for later; the worse case is that we lose track of the request in the chaos.

That doesn’t mean that we don’t care about 10.x. It’s just that we are very much heads down in the 10.0 release now and won’t look up until it’s done.

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Orthophotos Are Not Going Away

I mean to blog this a while ago, and Austin has moved on to new missives about the future of X-Plane, but:

A while ago Austin posted to the news list describing our approach to global scenery (that is, the scenery that ships with the sim), and he said some, well, rather disrespectful things toward orthophotos:

Orthophotos are garbage. I see this all the time. I am zooming along in an airplane looking that rooftops of WalMarts painted flat onto the ground. And the rooftops are blurry. And pixelated. And with a magenta or purple tint. And with big blurry shears right through the middle of them when they fall between offset satellite passes. It looks just terrible.

So first let me point out a few obvious things:

  1. There was never any chance that the global scenery would be based on orthophotos – not in v8, not in v9, not in v10. Simply put, we can’t ship you 900 DVDs in a dump-truck. Orthophotos of any reasonable quality are too large for covering the entire world in the base X-Plane product. This is not a change or new to v10.

  2. X-Plane is very capable of handling third party orthophoto scenery. We invested a bunch of engineering in this in the v9 run, and that code is not going away in v10. X-Plane will page orthophotos on multiple cores so that you get smooth flight and crisp images. If you want to see some orthophoto that don’t look blurry or pixelated, look here.

  3. DSF-based scenery that works in X-Plane 9 will work in X-Plane 10, unmodified. We are not getting rid of any modern scenery file formats.

Beating Ourselves Up

Austin continues in his rant^H^H^H^Hdiscussion, with this:

Then, to make the 2-dimensional, blurry, pixellated, mis-colored, distorted roof of a WalMart painted on the ground look even worse, if you throw in some REAL roads or auto-generated buildings, they invariably fall ACROSS the roof of the WalMart painted on the ground, compounding the wretched orthophoto with an Escher-like rendering-error. This looks terrible, and is not even plausible.

This is a critique of the version 8 and 9 global scenery. In fact, it is an observation of the fundamental problem with the urban global scenery: we never found a way to synchronize the real-world-driven and real-world derived 3-d scenery (real roads with plausible buildings and forests in between) with the photo-based land-class textures running underneath.

Ironically, this is not a problem with orthophotos (that is, specific photos placed in the world where they belong) per se. It’s really a problem with how to combine 3-d with land class textures. I don’t believe anyone has solved this problem yet for global scenery; if you look at FSX, there isn’t a lot of real world vector data to interfere with the land classes and their autogen.

In fact, orthophotos can look very good when they are combined with 3-d in a correlated way. For example, take a look at this screenshot of FlyTampa’s KBUF . They are using an orthophoto but they are putting matching 3-d on top of it, which makes things look good close up.

The Global Scenery Problem

I’ll leave you with this thought: the problem for the version 10 global scenery is to combine:

  1. the plausibility that you get from having synchronized 3-d and ground textures.
  2. the detail we’ve come to expect in photo-based scenery textures.
  3. the realism you get from using real vector data for the real world.

The current global scenery manages points 2 and 3 but fails pretty badly on point 1. That is what we are trying to address in X-Plane 10.

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