Dispelling Rumors

There may be new apt.dat features in a future version of X-Plane. I really can’t comment on what will be in future releases. But I can ssure you this much:

You will not have to change existing scenery to work with the new version of X-Plane! No upgrade will be required.

When we introduced DSFs we continued to support ENVs; when we introduced OBJ8 we continued to support OBj7 (and OBJ2!)…whatever we do to the apt.dat file, whether it be a new version or just new features, X-Plane will continue to work with existing apt.dat files as it always has.

Austin announced that X-Plane runs on Intel-based Macintoshes, and Steve Jobs announced that the Intel-based iMacs are 2-to-3 times as iMac G5s. But here’s the catch:

X-Plane 832 is not a universal binary – it has no native Intel code in it. So it runs under emulation on the Intel-based Macintoshes, which really hurts performance. So do not expect god framerates on Intel Macintoshes in 832 – you’ll have to wait for a universal binary version of X-plane for that.

Hopefully I’ve dispelled some rumors. This won’t dispell anything, but..I am writing this post from Cupertino.

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The Crayon Rule

Quick note: the region system will go in a later version of X-Plane, not 8.30. It’s coming soon though! I will try to post a spec soon…I am way behind on my documentation.

Anyway, the “Crayon Rule”: the worst thing you can do for frame-rate in X-Plane is change textures.

The name comes from a conversation almost three years ago when Austin explained to Sergio the cost of changing textures like this: drawing in OpenGL is like drawing with a crayon. Changing textures is like going back and getting a different crayon. It’s a lot faster to do all the red in your picture and not have to change between red and blue crayons all the time.

The crayon rule continues to hold true and it’s understandable why:

  • The texture you are using to draw is usually in VRAM. But the texture you are changing to could be in physical RAM (and must be transferred to the card), or worse it could have been paged out and must come from disk.
  • The drawing pipeline must be stopped and reconfigured with the texture change.
  • These things involve the host machine and CPU. Graphics cards are getting faster but CPUs are not – at least not much faster. So while the new graphics cards will be able to draw twice as many triangles, they will not be able to change textures twice as fast. In other words, we’re stuck with the crayon rule for a long time.

The crayon rule is why Austin and I insist on using one texture per object, and always recommend that you use a few large textures rather than many smaller ones. Fewer textures means fewer texture changes.

So the big things you can do as an author to obey the crayon rule are:

  • Use fewer large textures instead of many small ones.
  • Group your polygons that use the panel texture together – switching to the panel texture is a crayon change.

The crayon rule makes it more difficult to make scenery, but that extra effort has a huge payoff in framerate.

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Irresponsible Programming

I like to pretend I am a responsible software engineer, but…I am not. X-Plane is RC2 right now but I have just hacked in the “region system”.

Put simply, the region system allows you to make collections libaries of objects (in scenery packages) to customize the look of the scenery, that only apply to a region of the world, rather than globally. This lets you make sure your custom Igloos are not used in the Bahamas.

The reason for this impulse is that we are setting the stage for non-US global scenery objects. I will try to post some docs soon and a tutorial showing how to customize X-Plane’s objects.

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What’s an Async Task? (Is it good with jam?)

You may have seen a crpytic message like “async task jammed” or “thread timed out” with X-plane 8.20 or 8.30, after which the sim quits. Here’s what’s going on and what we’re trying to do to fix this.

X-Plane offloads part of the scenery load process to a second processor. (If you don’t have a second CPU, the first one does the work, but the operating system switches between the two tasks automatically.) For example, X-Plane “outsources” the work of turning a road (which is just a line segment in a scenery file) into real 3-d roads using the second CPU.

X-plane puts a request for the roads out to the second CPU, and then every frame it checks to see if it’s done. If the roads are processed, it can then send them to the video card and go on to the next thing to be done. If the roads are not finished – no biggie – it just checks again on the next frame.

Now here’s where the trouble begins: sometimes X-Plane needs the outsourced work to be done immediately. For example, when we load new scenery we have to recycle the memory from the old scenery. This requires the work of building the roads on the old scenery to be done! When this happens, X-Plane waits (up to 2 minutes) for the background work to finish.

If after 2 minutes the work is not done, X-Plane gives up and puts up a cryptic message like “async task jammed” or “thread timed out”. (In 830 beta 11 the message should be less cryptic.) I must admit: I do not understand how this condition happens; it has never happened on my machine or Austin’s.

Here’s what we’re doing to alleviate the problem: in X-Plane 830 you can turn off the “load scenery in the background” option. When this happens, rather than outsource jobs to the second CPU, X-Plane will do the work immediately. This will have two effects:

  1. There will be slight pauses as you fly while x-plane stops and builds roads, etc.
  2. There is no way that we can get stuck waiting for the second CPU, because it won’t be used.

Besides turning off this option, you can reduce the amount of stuttering (either with background loading on or off) by decreasing the amount of roads and objects. For example, on my G5, the sim flies pretty smoothly on “default” settings with background loading on or off. But on “insane” settings the sim is still smooth with background loading on and stutters visciously with background loading off.

I hope to get the background code to be more reliable in 840, but in the meantime you should be able to fly (etiher with or without background loading) in 830 with reasonable results if you set your rendering settings appropriately.

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Fun With Numbers

“Scenery load is now twice as fast!” Where do Austin and I come up with these claims? Here’s a little bit of info on on our performance tuning process.

When we work on performance, we always measure a baseline metric and then look at this metric while we work on the code. The test situation must be the same every time, so usually we use a standard plane position (like on a known runway) for framerate, or a known scenery box (for scenery loading).

Here’s the statistics for X-plane scenery load before and after tuning the DSF loader:

Before optimize
DSF load time: 2297036 for file +33-119.dsf
DSF load time: 4614647 for file +33-118.dsf
DSF load time: 4251205 for file +33-117.dsf
DSF load time: 5137940 for file 34-119.dsf
DSF load time: 5038098 for file +34-118.dsf
DSF load time: 3893140 for file +34-117.dsf
After Optimize
DSF load time: 1634270 for file +33-119.dsf
DSF load time: 2156187 for file +33-118.dsf
DSF load time: 1882647 for file +33-117.dsf
DSF load time: 2418666 for file +34-119.dsf
DSF load time: 2318480 for file +34-118.dsf
DSF load time: 1819997 for file +34-117.dsf

These numbers are not normaly visible; a special sim option available to us causes it to print the load time per DSF. The nubmers are in microseconds and shown per DSF.

So on immediate inspection you can see improvements of between 140 and 205%. But to really understand what this means you have to consider all of the fine print:

These stats are with objects and roads turned off – this is with scenery on minimal settings in both cases. With more 3-d stuff turned on, the overall loadtime might be better or worse; these statistics don’t tell us anything about that.

These tests were done on the same 6 DSFs in both cases, but other DSFs might have different improvements. You can see that even in the six DSFs here the amount of improvement varies a lot!

These tests were done on my dual 1.8 ghz G5. This is a fast machine for a Mac (but probably only average compared to modern desktop PCs). The speed-up could vary on other machines that don’t have similar performance characteristics, since scenery loading depends on the speed of the CPU, memory bandwidth, I/O performance, etc.

This is all a long-winded way of saying “your mileage may vary”. When Austin and I post performance numbers, they are calculated in strict test conditions with all variables controlled. But since these statistics represent one datapoint, they are not universal changes, just indicators that our code changes are going in the right direction.

(Could it be that an optimization makes a difference on our machines but not yours? Yes – this is always a risk! We try to run on different machines and we listen to user feedback on performance to catch this case.)

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Crash bang crunch

Thank you to everyone who has reported the crash bug in beta-8. I found the crash this morning thanks to some helpful bug reports (technical explanation: boneheaded code on my part) and the fix should be in the next beta.

Also it looks like QuickTime 7.0.4 isn’t compatible with the AC3D x-plane export plugin. This makes no sense to me…I spent over an hour remotely debugging on a user’s machine (thanks Peter!) and came to the conclusion that something is borked deep down inside a library. Somewhere. Maebe. I’ll try to get this resolved soon but since I do not have an OS X 10.4 machine, it may take a little bit. In the meantime it is possible to go back to QuickTime 7.0.1.

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Fun with Animation

Here are a few notes on animation:

The animation commands for OBJ8 take two changes (a start and end rotation or translation) and two dataref values and map them. For example,

ANIM_rotate 0 1 0 -40 70 0 1 sim/flightmodel/whatever

would rotate the following geometry around the Y axis from -40 to 70 degrees as the dataref goes from 0 to 1. Intermediate values between 0 and 1 are interpolated. (For example, 0.5 maps to 15 degrees.) But what happens if the dataref exceeds the bounds specified?

The answer is: the result is extrapolated. If this dataref goe sto 2.0, the rotation will go to 180 degrees. (How I came up with that is left as an exercise for the reader.)

You can take advantage of this to animate rotating things like radar dishes and beacons using the sim/time/total_running_time_sec dataref. This dataref is the number of seconds x-plane has been running. For examlpe:

ANIM_rotate 0 1 0 0 360 0 5 sim/time/total_running_time_sec

This will cause the following geometry to continuously rotate, making a full rotation every 5 seconds. But why does the animation loop? Well, let’s take a look in detail:

When the time is 0 the rotation is 0. 5 seconds later the rotation has made it up to 360, one full turn. Then what? At 6 seconds the rotation will now be 420 degrees. But 420 degrees is the same as 60 degrees; you can’t tell whether the object has made one, two or a hundred full revolutions before turning the extra sixty degrees. So the rotation is actually becoming a huge number but the wrap-around cannot be seen, making the appearance of continuous rotation.

Rotations always occur around the origin. But what if we want to rotate around another point in our object? We must use the ANIM_translate command to move the rotation to where we want. For example:

ANIM_trans  3  4  5    3  4  5   0 0   no_ref
ANIM_rotate 0 -1 0 -90 90 -1 1 jetway/circle1
ANIM_trans -3 -4 -5 -3 -4 -5 0 0 no_ref

This will rotate the geometry around the Y axis from -90 to 90 degrees as the circle goes from -1 to 1. But the rotation happens around the point 3,4,5.

Note that the animation-translate commands have the same coordinates for both translations and no dataref. This is a static translation – it is alawys the same and its sole purpose is to move the rotation to be around a useful point. It is always followed by an opposite translation.

(It is possible to omit the second translation and simply build your animated sub-part with 0,0,0 = rotation point instead of 0,0,0 = the object’s location in x-plane. But this is an advanced optimization.)

An example:

A rotating radar dish

And from the end of the OBJ:

TRIS 0 1536
ANIM_trans 20.226 22.468 -0.039 20.226 22.468 -0.039 0.0 1.0 no_ref
ANIM_rotate 0 -1 0 0 360 0 6 sim/time/total_running_time_sec
TRIS 1536 1116

My Supervisor (no it’s not Austin)

One of the secrets of X-Plane development revealed…Nala supervises my work very carefully to be sure I not introducing bugs into the rendering engine. In the event you get threading errors in the next 830 beta, you now know who to blame!


Sceney Loading in X-Plane 830

Scenery loading has changed a lot in 8.30. Before 8.30 we loaded 6 scenery tiles (DSFs or ENVs) and completely built them up into memory before we let you fly. We then loaded any new texures while flying. The result is a possible hit on framerate right after a scenery load for textures, and a very long pause while the scenery area gets built.

In 8.30 we now read the full DSF and build the terrain, then start the sim running. We then build the local 3-d “clutter” (objects and roads) while you fly, but we only build up the ones near your plane. As you fly, the ones behind you are destroyed and new ones in front of you are built.

This means a few things:
– Shorter scenery pauses, because we have to do less before we can start the sim.
– Lower framerates right after a load because the sim is doing extra work while you fly to finish the load.
– Lower framerates occaisionally while you fly as a few more roads are built and objects destroyed. This is pretty minimal though because these items come into range gradually.
– Lower memory usage, since only the roads nearby are fully built, not all of them.

This strategy works out particularly well for people with dual-processor or dual-core machines; some of the work to do this “clutter-building” is done on the second CPU while you fly, so the framerate hit is a lot less noticable. Basically we are taking work that was done by the main CPU while you waited and doing it on the second CPU while you fly.

One problem still in 830: when you place the plane, we do not pre-build the roads (but we DO pre-load the textures). If we did not preload the textures, you would be placed in a gray area and see the textures come in. So hopefully in a later beta I will preload the roads if you place the plane using the airport-picking dialog box. This should help provide good framerates as soon as your plane is placed.

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The emperor is not as forgiving as I am….

I have a new program called “Env2Overlay” that converts the custom obojects in an ENV file to overlay DSFs. This makes it very easy to convert custom airports to version 8.

However one thing I’ve found is that a lot of ENV scenery contains references to missing objects. The ENV loader historically has worked even with objects missing. The DSF loader will refuse to load scenery if objects are missing.

For now I am going to leave the DSF loader in “unforgiving” mode so that missing objects are an error. I think this is more useful to authors: better to clean up the scenery package while converting it rather than convert it to DSF and continue to have missing objects; I think this indicates a packaging problem or other lost content!

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