Disappearing Objects Take 3

In version 840 I tried to fix a persistent bug in X-Plane where objects sometimes randomly disappear…turns out I got the math wrong.

The next 840 beta (which I think will be called “RC1”, but I am a poor predictor of these things) has a bug fix to the bug fix. I don’t know how many of these bugs this will fix (some appear to be due to driver issues) but I think we’ll be closer.

Comments Off on Disappearing Objects Take 3

The data wants to be free!

I try to avoid politics in this blog, but this article that Jonathan Harris posted to the scenery list is pretty topical to X-Plane.


Of course I am biased – low-cost high quality data (SRTM, Tiger, etc.) make it possible for a small company like X-Plane to create high quality scenery. But I would also add two more thoughts:

1. Knowledge is crucial to the healthy functioning of a democracy. Without good digital maps, you can’t apply modern tools to understanding regional trends, environmental problems, electoral districts, etc. Knowledge shouldn’t just be for the rich (or big companies).

2. Geospatial data is infrastructure – free or cheap GIS data make a country a better place to do business. I think in this global economy every national government needs to be asking “what are we doing to make sure our citizens will be competitive on a global level.” There’s no question that having good roads and transportation infrastructure are important for a national economy, but I would submit that having good digital infrastructure will become important too; in that context it’s appropriate for a government to make the data free (and take the local loss in the providing agency) to foster growth, employment, and the well-being of its citizens.

Just my 0.02…

Comments Off on The data wants to be free!

apt.dat – a fundamental shift

Long time no post – it’s been a busy few weeks, helping friends with projects and traveling. There have been some lively discussions on the scenery list though!

I’ve been tossing around the idea for a fundamental change in the way the apt.dat format handles metadata. It’s subtle but I think it has some far-reaching implications.

Right now the apt.dat format is pretty high level. Let’s use taxiways as an example: a taxiway has a light code that indicates whether the taxiway has blue edge lights. This has two effects:
1. The information about whether the taxiway has lights is preserved.
2. You can’t put blue edge lights down without a taxiway.

I am exploring the idea of entirely separating edge-lights/lines* from taxiways. This would mean that:
1. We’d have no idea about whether the taxiway has lights. There may happen to be lights nearby, but we can’t be sure of where they come from and
2. We can put lights anywhere.

The shift is fundamental because it is reducing the amount of high level data and increasing file size in return for gaining flexibility with a simple system.

Anyway, email me if you have thoughts on this…food for thought.

* The exception will be runways. We will be supporting all existing data in the apt.dat format – nothing’s geting dropped, and there isn’t a real replacement for runways. Generally runways will still be built by saying “this is a runway and these are it’s many properties” and letting the sim do the rest.


Objects and Drag

X-Plane 8.40b2 introduces the ability to attach multiple objects to the aircraft. This is a very powerful new feature.

Currently these objects are not part of th physics model. This may change during the beta, or not. Please do not attempt to do real work based on the beta – an X-Plane beta is a test version of the sim and may change radically due to bugs we discover. Until X-Plane is final please try features and report bugs, but do not assume the features will continue to work the same way.

Comments Off on Objects and Drag

Cockpit Lighting Weirdness

A few authors have asked me why the lighting looks different for the parts of a 3-d cockpit that use the panel. Here’s what’s going on:

The 2-d panel has 3-part lighting: the base background is lit by the ambient light levels with no regard to lighting angles. (Since the panel is 2-d we’re not really in a position to light different parts of the panel differentially.) The panel is then additionally lit by the cockpit flood lighting and any instrument gauges are further lit by the instrument brightness, for example an EFIS.

The 3-d panel is then built by taking an image of the entire 2-d panel. Here’s where things get tricky. The image of the 2-d panel has already been darkened by the ambient lighting levels and brightened by the flood lights.

If we were then to apply directional 3-d lighting (the way we do for normal lighting) we would further dim the panel (which would make it too dark) and we would dim the EFIS and other instruments.

But since we don’t apply directional lighting, the panel texture does not match teh surrounding non-panel geometry in a 3-d cockpit.

There are a few possible things we can do about this:
1. We can leave things the way they are and require 3-d panels to carefully use the clickable parts only for small areas to minimize the visual impact of the lighting effects.

2. We can build the 3-d panel separately out of multiple textures. This would allow us to do truly correct 3-d lighting, but would have some pretty big performance implications. The 2-d panel would have to be rendered multiple times, and the total VRAM requirements would increase by several megabytes. Since the panel is never compressed or resolution-reduced, VRAM is a serious consideration.

3. We could disable all 3-d lighting for the 3-d cockpit. This would cause all geometry to match, but give the 3-d cockpit a flat look. Authors would have to build their 3-d lighting into their texture.

None of these options is ideal.

Comments Off on Cockpit Lighting Weirdness

But you said that was impossible!

There have been a few cases where we’ve dismissed a feature request as “impossible” – only to then implement it later. There are a few reasons why this happens:

  1. Sometimes we’re just wrong…we didn’t understand an algorithm or know of a technology. Later research gives us a clue.
  2. Sometimes the underlying technology we use changes, either OpenGL and the operating system. (The joystick bundle was totally necessary for X-Plane through 832 since it was a CFM application – 840 is Mach-O, allowing us to run bundleless for the first time.)
  3. Sometimes we have to make changes in our own code. A feature that is near impossible in one version could be one line of code in the next. This is because so much of our features depend on underlying systems. We spend a lot of our time working on the underlying systems to make multiple features possible at once.

So please do not be surprised when the impossible becomes the possible; one of the things that makes flight simulation and enjoyable problem to work on is that new technology constantly unlocks new possibilities and lets us make each version of the sim better than the last in ways that would have been impossible.

Comments Off on But you said that was impossible!

Emailus Overloadus

It’s winter and I seem to have caught another case of Emailus Overloadus, also known as a clogged in-box. If you emailed me in 2005 and haven’t heard back, please ping me again. If you’ve emailed me in the last six weeks, I apologize for the extended delays; I am trying to get to everyone in the next week or so. Sadly email seems to come in faster than I can answer it.

Comments Off on Emailus Overloadus

Fast Framerate with Old Scenery

I was investigating Anacortes, WA with version 7, 8 US and 8 global scenery this morning and had a (probably obvious thought): if your framerate is inadequate with the new global scenery, use the old scenery!

What’s important to understand is: the amount of work to be done is defined by the scenery, but the capacity of the sim to do that work is defined by your hardware and the version of the sim! In particular, 835 is definitely faster than 820, which was faster than 800, which is faster than X-Plane 7. So you can use X-Plane 835 with old scenery and get a much higher framerate.

In the Anacortes area I’m seeing framerates in the 60s with global scenery, 90s with the US DSFs, and 120s with V7 scenery. In a direct comparison with all settings controlled, X-Plane 7.63 runs this airport at 52 fps, while X-Plane 8.35 runs the same scenery at 110 fps.

There are two things going on here: in newer versions of the sim we can use newer, more efficient techniques to talk to the card. These techniques are invented by the graphics card vendors to allow applications to take advantage of the ever-growing power of graphics cards. So a new X-Plane has an advantage over an old one in the way it utilizes modern hardware – hence the framerate improvements.

We are also “spending” that performance dividend to provide more realism. Here you can compare the look and framerate of the v7 ENV (top) vs the v8 DSF (bottom). We are utilizing higher performance to make scenery more realistic, but if you’d rather have the framerate just load up the old scenery. It still works, and it’s very fast.


It’s a miracle anything works…

Two bugs we resolved today …

One user couldn’t use our installer – it always failed on the file After reporting this to us, he figured out that parental-access controls in his router were banning the file due to a certain 4 letters in its name. I feel lucky that the user diagnosed the problem because I never would have figured that one out!

Also we’ve had reports that X-Plane 832 is slower than 816 when used in a two-monitor instructor/visual configuration. Finally we figured out what this was – a bug in my code: I programmed X-Plane to reduce CPU usage when a “dialog box” is up (the instructor station is one such dialog box) to play nice with other programs. But of course in two monitor mode the dialog box is up and you’re flying, so reducing CPU usage kills framerate. This will be fixed in 835.

What’s funky about this second bug is the reason why Austin and I could not reproduce it: the bug only happens if you do not touch the mouse! Austin and I, while trying to debug, would mouse around and click on all sorts of things, and this would hide the bug. Real customers, in real applications, don’t touch the mouse while doing commercial training. (UI programmers who understand how mouse cursor updates are done in Win32 and Mac OS will understand this behavior.)

If there’s a moral to this post (and there probably isn’t), it’s that bugs are hard to catch and reproduce because there can be system differences between users and developers that are very subtle – assumptions so basic that they don’t get questioned.

Comments Off on It’s a miracle anything works…

Boston Drivers

Last night I was looking at what it will take to put animated cars back on the roads in X-Plane. (I can’t say if or when they will come back…that’s a subject for another blog entry.) I found something that’s going to be a little tricky.

Simply put, the road data we use in the US (Census TIGER data) does not contain directional information. It might tell us a street is one-way, but it doesn’t tell us which way it goes!

In fact, the data doesn’t even say which highways connect to each other! The road data is “flat”. If two highways cross each other we see this as a four-way intersection. In the original US DSFs the roads were built with this flat pattern – sometimes you’d see a 10-way intersection of limited access highways.

In an attempt to clean up the highways for the global render I wrote some code that attempts to evaluate junctions for traffic flow. Basically it knows that two limited access highways at right angles cannot intersect and it moves one up in the air as a bridge. This process iterates a bit in an attempt to make a junction that is fully real-world navigable.

One advantage of processing the roads in this manner is that often it will remove bogus junctions, allowing us to continue straight roads. This both reduces file size and improves the look of the roads. (Our road junctions still look terrible – a throwback to a time when road-creation paused flying and therefore speed was at a premium over visual quality.) Unfortunately this “layering” code sometimes goes haywire and does some pretty strange things:

  • A highway may be put in a high layer in one junction and a low layer in an adjacent one; the result is a highway ramp that “dives” down at a steep angle to try to meet all of its crossing restrictions. The layering code isn’t smart enough to find optimal junction ordering when there are many junctions nearby.
  • Sometimes the number of layers just gets huge, creating a massively tall junction. (But then, in Los Angeles perhaps this is realistic!)

How does this relate to cars? Well, what is significant here is that the algorithm does not consider directionality of one-way roads since we do not know it. Furthermore, the DSF optimizer, because it was written under the assumption that we really have no way to know which way a one-way street is going, will flip the direction of one-way roads to reduce file size. (Sometimes changing the direction of a road segment allows for tighter file compression.)

The good news is: DSF files do have a notion of direction, so if we ever get better road data, the sim and file format will be ready. The bad news is: the current DSFs have a road grid that may be similar to driving in Boston.

(As a side note, I grew up driving in Boston, so none of those rules seems even slightly unusual to me. What did surprise me was: when I moved to Washington, DC I found the rule that the cheaper car has the right of way turned on its head. I’ve never been cut off by more reckless BMWs and Mercedes in my life. You can speculate about what this says about the culture of our capital.)

Clearly X-Plane’s car engine needs to address this in some manner. There are two cases that are tricky: one way roads that end at the destination of other one-way roads (leaving the car with no legal route) and one-way roads whose origin is not connected to anything but the origins of other one-way roads (giving cars no way to reach the road). The later case is not really a problem – it just means the road will have no traffic; the later is more of a problem as it requires the car to do something illegal to get out of the situation. I am still looking at what we can do about this but my guess is that cars are going to have to employ some Boston driving to get around the X-World.

1 Comment