Pie in the Sky: Future Rendering Tricks

I've had a few inquiries about environment maps, normal/specular maps in more places than just OBJs, etc. The short answer is: a lot of these future rendering engine enhancements are good ideas that we like, but we have other features that are already partially implemented that we need to productize first.

In particular, one of those half-finished features not only could use to be shipped, but also affects the way just about every other rendering effect works. So better to get these features finished first, and build new effects within the context of these "new rules".

Here are some of the ideas that I've heard kicked around:

• Environment maps on the plane - I like it, it's not that hard to do, and the framerate hit could be ramped up and down with detail. See above about finishing other features first.
• Next-gen texturing on runways (wet runways, environment maps, bump mapping) - I like all of it. The runways really need to be addressed comprehensively, not piece-wise, in order to find a rendering configuration that meets our scalability and efficiency needs. (In other words, we can't just burn tons of VRAM on the runways, and we need a way to render them that works on low and high end computers with one set of art assets.)
• Normal mapping on the ground. I like it, but I wonder if this isn't part of a bigger idea: procedural texturing on the ground. E.g. if we want to add detail on the ground, can we add it with multiple layers at different resolutions with a shader adding yet more detail.

Generally speaking, it's not that hard to push a feature out from one part of the rendering engine to the other. For example, normal maps on facades (if anyone cares :-) would make sense and be a fairly trivial change. I hesitate on polygons only because the ground might require something a little bit more sophisticated.

Going Crazy With Choices

Here's a straw man of why polygons might be different: draped polygons can't have specular shininess right now, and I don't think anyone is complaining. So it's a bit of a waste to use the alpha channel of a normal map for shininess. Perhaps it could be used for something else like an environment mapping parameter.

Hrm...but we would like to have environment mapping on airplane objects too someday. Well, we could go two ways with that. We could just use the alpha channel for shininess and environment mapping...not totally unreasonable but it wouldn't let us have a glossy non-reflective material, e.g. aluminum vs. shiny white paint.

Math nerds will realize that the blue channel in normal maps can be "reconstituted" from the red and green channels by the GPU (at the cost of a tiny bit of GPU power). That would give us two channels to have fun with - blue and alpha. Perhaps one could be shininess and one could be an environment mapping material.

Well, shininess is still no good on the ground. But...perhaps that would be a good place to store dynamic snow accumulation? Hrm...

My point of this stream-of-consciousness rant is that the design of any one rendering engine feature is heavily influenced by its neighbors. We'll get all of these effects someday. If there are features that are really easy, we can get them into the sim quickly, but the only obvious one I see now is using bump maps on other OBJ-like entities (which at this point would mean facades).

Removing Third Parties is a Diagnostic Test, Not a Cure!

In response to so many bug reports I say:

Please temporarily remove your third party scenery, airplanes and plugins and retry. Do you still see this bug?

And in response I get:

No - the bug was with XXX. Thanks!

But...but...but...we're not done yet! The point of removing the third party add-on is divide-and-conquer: to see if the bug is in the core of X-Plane or is caused by an X-Plane-add-on interaction.

If the add-on used to work but no longer does, I want to know where I can get the add-on, who created it, and when it used to work, so I can investigate further! If an add-on stops working because X-Plane was updated and the add-on is not doing anything naughty, that is still an X-Plane bug.

So please: don't give up on the bug just because removing the add-on makes it go away...there is still more to debug!

Marking the Ground? Use a .pol or .lin.

The rule is very simple: if you want to put a marking on the ground, your results will be much better if you use a .pol (draped polygon) or .lin (draped line) than if you use an OBJ. In fact, an OBJ is probably the worst way to put markings on the ground.

Requirements for Good Looking Markings

In order for markings to look good you need to have three things happen:

1. You need to make sure your markings are at the exact same level as the ground itself.
2. You need to use polygon offset to tell X-Plane to tell the video card that these are "coplanar" (at the same height) triangles that must be managed in a special way.
3. You must guarantee that the draw order is: the stuff under your markings, your markings, then the 3-d buildings and other 3-d stuff.

With these requirements we can compare .pol/.lin files to OBJs:

1. .pol and .lin files always "drape" to the ground, so they always meet rule 1 perfectly. With an OBJ you can set the height of your OBJ to 0 but on sloped terrain this won't be correct.
2. .lin and .pol files are always "polygon offset" automatically, so they always meet rule 2 perfectly. With an OBJ you need to use ATTR_poly_os.
3. ATTR_layer_group (or LAYER_GROUP) tell the OBJ or pol/lin in what order to draw, so you can set this correctly in all cases. The default values if you don't specify a layer group are more appropriate to the task of markings on the ground when you use a pol/lin - that is, by default they meet rule 3 perfectly. By comparison, an OBJ may not be in the right draw order.

So we can see from these 3 rules that you will always get the rules right just by using a pol or lin, but you have to be very careful and may still not get the rules right when using an OBJ.

Performance? Think .pol

When you have a large number of small markings, the performance of a .pol is going to be significantly superior to the performance of an OBJ. For example, imagine an airport where you want to draw on-pavement signs for all taxiways and runways. With an OBJ, to get the height right, you'd need to use a large number of small objects. (With one large object, it will be nearly impossible to make the OBJ markings be on the ground when far from the object center.)

When you use a large number of .pol markings that share one common texture and all have the same .pol file, X-Plane merges them behind the scene into one huge object-like pile of triangles. The cost of drawing all of those polygons will be similar to the cost of drawing just one object! That's a huge speed win.

Floating Objects Are Wrong

What I see most commonly in scenery packs that are sent to me and have thrashing problems are OBJs that have a height other than 0. This is simply the wrong way to create overlaid geometry in X-Plane, and it will produce artifacts in a wide variety of situations. At best, the "floating" objects will cause airplanes driving over the marking to look like they "sink in". At worst, the offset you pick will be too small for the video card's resolution and you'll get thrash anyway.

There is no one right vertical offset for all scenarios, and even if there was, it would still look ugly! See the above rules for what an OBJ really has to do.

How Do I Get a .pol into my scenery.

Well that is the $10,000 question. I must admit that I don't know what Overlay Editor's capabilities are in this regard. WED 1.1 will be able to add draped polygons, including texture coordinate editing of the polygons. I've been working on the texture coordinate editor in WED this weekend and am hoping to get some kind of WED 1.1 preview built this week.

With WED 1.1 the process is fairly simple:

• Create your texture.
• Create a single .pol text file that names the texture, so you can use it.
  
• In WED 1.1 you can select the .pol from the list of resources for your scenery pack, and then use the "polygon create tool" to simply draw the draped polygons into place.
• Once the draped polygons are created, you can select the polygon and open the "texture coordinate editor" tab to edit the way the texture is applied to the polygon.

My hope is that this process will be easier than creating markings using a 3-d editor - you can still edit the texture coordinates, but you can do so directly in WED.

Higher-Order Beziers? Probably Not

I was revisiting Bézier curves today. First I should say that the current scheme for handling Bézier curves in apt.dat and DSF is horribly convoluted - I am looking to replace the syntax with something less weird for the next set of features that use them. But I was also looking at the problem of generating curves from high-density line data.

Did you know that a Bézier curve can have any number of control points? This picture illustrates:

Each curve has one more control point than the last. Zero control points is of course a line segment, and two control points is a "cubic" Bézier, which is what we have now. (Note that having one control point in WED does not give you the one-point quadric Bézier you see here. WED gives you a cubic Bézier with one control point on top of an end point.)

But it also show: having tons of control points are not particularly useful for modeling a shape in detail - a string of connected cubic Bézier is probably just as good. And the high-order Bézier doesn't really trace out the control points, so it's not a good way to convert line segments into a curve.

Parameterized Lights, Generic Lights and Plugin Controlled Lights

There are a few changes to 940 regarding airplane lights...I will try to get some permanent documentation on the Wiki, but here's the basic ideas:

There is a new "type" of light in the OBJ8 format, called a parameterized light. A parameterized light is somewhere between a named light (totally as-is, can't be modified, simple to use) and a custom light (totally complex, can do anything, requires a lot of work). In a parameterized light, you control just one or two aspects of the light.

Parameterized lights are aimed at airplanes, not scenery, because typically parameterized lights are customizable and slow.* The goal is to give airplane authors some flexibility without having to invent a huge number of named lights.

Consider, for example, landing lights. A landing light could vary based on what switch controls it (we have 16 now), how big it is (many authors have pointed out that one size does not fit all) or how wide it's view angle should be. (Lights that are inset in a structure might not be easily viewable from the side.) With a parameterized light, we can provide one light definition with 3 parameters instead of a huge matrix of lights.

Generic lights are a new collection of 64 lights that can be used for any purpose, sort of like misc wings, misc bodies, and sliders. The main difference between a landing light and a generic light is that the landing light halo won't show up on the runway when a misc light is turned on. They are meant to be used for logo lights, inlet lights, etc. A series of new named lights will "listen" to the generic switches.

(Tip: combine ATTR_light_level with generic lights to have a light turn on and your lit texture appear at the same time.)

Finally, there is now a plugin override for the beacons and strobes (and in the systems model there can be up to 4 separate sets of beacons and strobes flashing at different times). With parameterized lights you can make two sets of strobes and use a plugin to control when they flash.

The combination of these three things let an author create an airliner that models all of the various lights and their behaviors.

* Slow needs some qualifications here. There are two code paths for lights, the fast and slow path. The slow path IS pretty fast, just not as fast as the fast path. The fast path is expected to be able to draw at least 10,000 lights in a single frame on low-end hardware, while the slow path is expected to be able to draw at least 500 lights per frame on low-end hardware.

500 lights is a lot for one airplane, especially if you have to place them by hand. And most modern computers will easily do thousands of slow lights.

Basically slow lights are not appropriate for scenery objects in the library that might be placed a huge number of times: OBJs attached to roads (e.g. street lights), OBJs used for buildings, taxiway lights. The are plenty fast for airplanes. In the X-Plane world, slow doesn't really means slow, just slower than something else.

Yank That Stick Back

A quote from the .org:

I actually did try the demo before I bought it, and I found it sorta arcadish as I took off with a King Air 200 and performed barrel rolls on t/o lol! That kind of tuned me out!

I don't mean to pick on that particular poster - we hear that a lot, particularly from people who use MSFS. Andy Goldstein points to Bob Hoover as a counterpoint.

More generally, the criticism is that since X-Plane will allow you to do things in commercial airliners that you've never seen in real life, X-Plane must not be realistic. And phrased like that, you can see the possible flaws in the reasoning:

1. How do you know your control inputs match real life?
2. How do you know the real plane is physically incapable of doing such a thing?

The first is the age-old problem of consumer joysticks - to really maneuver an airliner hard you have to put a lot of pressure on the controls - they put up a lot more resistance than a $20 SideWinder.

The second is an issue of falsifiability again - the absence of evidence is not evidence of absence.

Now in truth there are things that X-Plane models and X-Plane does not model. If you load up the airframe too much with fail-on-over-G on and the G limits are set correctly, X-Plane will start removing parts as you try to fly a 747 like an F-16. But we don't necessarily simulate some of the internal things that might go wrong. For example, accelerations on the fuel system might potentially cause flow to the engines to fail, stalling an engine. I don't know how much of this Austin simulates, but we certainly don't simulate the geometry of the fuel lines and the fuel as a fluid flow. So there may be cases where a maneuver is impossible for logistic but not aerodynamic reasons (e.g. you could do it but you'd lose your engines).

With that in mind, here are a few youtube links:

• Bob Hoover flies a Commander like an aerobatic glider - thanks Andy!
• Roll an airliner? It's safe, really!
• The 757 has a lot of power - thanks Andrew.
• I have no idea what happened here...I thought the Airbus computer prevents you from doing this kind of thing.

On that last video: the first time I flew with Austin in his Cirrus (this is before he traded it for a Columbia) he pulled the same maneuver: take-off, very slow rate of climb to pick up speed, and then: yank. Our climb rate was well over 2000 fpm in a single-engine prop for a while.

(Logistic note: please don't interpret this post as an invitation to contact me regarding any aspect of the flight model - when it comes to the physics engine I am just another user, with no special insight. Physics is Austin's domain, definitely not mine!)

Proximate And Ultimate Crashes

One last comment about driver crashes: I try to be fair about the distinction between causation and correlation when it comes to driver bugs; the piece of software that "fixes" a bug may not be the piece of software that contains a bug, and work-arounds happen in both directions between drivers and apps.

Similarly, seeing "ATI" or "NVOGL" in your crash report does not imply a driver bug. It does imply that the crash happened in the driver, but the driver crash may be a proximate cause, while the ultimate cause may be in X-Plane. (Sometimes the stack crawl is also just plain wrong, depending on the severity of the crash.)

OpenGL drivers need to be very, very fast; the whole point of buying that shiny new GPU was to get faster framerates. So driver writers have to make trade-offs about how much error checking they do when an application passes in junk data. If I were writing a driver and debating an error check that would slow performance and prevent a crash of an application that passed illegal data (but not the machine), I might very well skip the error check and say "if the application doesn't want to crash, it should pass better data."

Of course, it's not up to me, and it's not always up to the driver writers; the OpenGL specification actually dictates what outcome are acceptable in various error conditions. Quite often, "program termination" (read: the application crashes but the machine does not) is an allowed response to certain kinds of bad data from the application to the driver, particularly where checking the data would slow performance for all applications and the application should know better.

Pay No Attention To That Last Post

My last blog post may have expressed some negative views toward software upgrades:

I know I'll take some slack for this, but: why did you all go and grab an OS update so fast? What have you gained? The down-side is lost time and application compatibility problems...was it worth it?

That blog post was of course not my real opinion, but rather a recreation of the evil Ben. Always upgrade your software to the newest version whenever it comes out!

Caught With My Pants Down

My friends say I have become a technological curmudgeon...whenever a new gadget or device or operating system comes out, I just grumble about how that it just means more bugs and mutter about how a byte only had 7 bits when I was young.

How did this happen? Pretty simple. New driver comes out, new bugs and interactions with X-Plane, rinse, wash, repeat. This is not a criticism of the driver writers. Those guys have an impossible job: write code that is super high performance in an environment that is highly unforgiving (an X-Plane crashes X-Plane; a video driver bug crashes your entire machine), then do it again when the next hardware comes out 9 months later. As the apps guy integrating on top of this ever-running treadmill of new hardware and drivers (think I Love Lucy) I sometimes dream of a week when none of the software that X-Plane runs on top of changes.

This is not one of those weeks. Snow Leopard came out, and it looks like Apple has been quite successful in keeping the price low; I'm seeing plenty of bug reports against 10.6 and it's only been a few days. Apple ships video drivers with the OS, so Snow Leopard isn't just a new OS, it's new drivers too, as well as possible changes to the OpenGL "middle" layer that Apple provides.

Now about my pants...over the last few years I have been accumulating graphics card + operating system combinations, so I can see a higher percentage of hardware-related bugs "in the lab".* But my wife and I are in the process of relocating to Boston, and this has temporarily separated me from a significant subset of my hardware. The result: driver bugs are coming in and I can't easily reproduce them.

To paraphrase Airplane, I picked the wrong week to quit sniffing OS betas.

(I know I'll take some slack for this, but: why did you all go and grab an OS update so fast? What have you gained? The down-side is lost time and application compatibility problems...was it worth it?)

*Just because it's hardware related doesn't mean it's a driver bug - X-Plane can be the broken app too! Sometimes drivers work around X-Plane bugs and sometimes X-Plane works around driver bugs. I mention this because plenty of apps developers complain on forums about "driver bugs" - but how many of them repost when they discover a null ptr in their code? A driver and an app are two pieces of software that have to be integrated, and bugs crop up on both sides.

Scalability and apt.dat

Beta users, see the bottom of this post for how scalability turns into possible bugs.

In computer science, a program or architecture is scalable if it doesn't totally vomit up a lung as its constituent parts become bigger. For a cleaner definition, see Wikipedia, source of all internet knowledge.

An ant is not scalable - if you made an ant 100 times larger in every dimension, its tiny legs would break under its new weight. (An ant is not scalable because its weight grows faster than its structural strength. Thus elephants are not built like ants.) Geeks: scalability is to computer science as marginal cost is to economics.

Before X-Plane 940, the apt.dat file was distinctly not scalable. The entire file was loaded into memory; as users created more and more taxiway lines and signs and details, we simply used more and more memory. This approach isn't very scalable because authors have the potential to grow the apt.dat file faster than our system requirements can increase.

X-Plane 940 fixes this by not loading the entire apt.dat file into memory. Instead, only essential airport information is loaded into memory, along with a note as to where in the file the airport lives. Whenever an airport actually has to be built into a 3-d mesh while you fly, we go back to the apt. file and load the rest of the data for the one airport we are building, use it, and throw it out. Since 3-d airport meshes are built on a second core, the cost of loading one airport off disk is pretty harmless.

The problem with this fix is that it introduced a new scalability problem. Consider:

• Meshes in 940 are built on as many CPU cores as you have - some users have 8!
• Each CPU core could be working on a different airport, depending on how many are nearby.
• Each airport has to load up the apt.dat file to get the extra airport data.

This means that at times on an 8 core machine we could easily have allocated 50 * 8 = 400 MB of memory just to temporarily hold 8 copies of the apt.dat file.*

This is of course completely silly - there's no reason to load the whole apt.dat file to get one airport, and the fix that is going into beta 8 lets the airport loader surgically grab just one airport. Thus we will be scalable again, because adding more cores won't cause memory usage to go up.

Beta Users: Please keep an eye out for X-Plane running out of memory - if it starts to do so in the next beta it means that some part of this code change munged memory management. We're running stress tests on the sim now, but touching the low level memory nd file handling code late in beta isn't something I like to do.

* While loading 8 copies of the apt.dat file is wasteful of memory, it is not slow; X-Plane uses memory mapped file I/O, so reading a small part of a large file is very fast - just not very virtual-memory efficient.

Marketing And Fact

A while ago, Austin was on FSBreak, and I wrote this post as commentary on his interview. The main point I meant to make was this: from what I have heard from other engineers and seen in my own experience, most software companies prefer to develop new features on top of implementations that are known to have architectural problems. At LR, we fix the implementation architectural problem first, and that has been a net win for us.

Now that's basically a statement of my opinion on software engineering - in hindsight it probably belongs on my programming blog and not here. Unless you develop X-Plane plugins, you're not a programmer; I will try to constrain future scenery blog posts to things that non-programming X-Plane users will notice. If you are a plugin developer, you might want to look at the "Hacks of Life" posts tagged with OpenGL.

Anyway, back to the story...the responses to that blog post were all well thought out comments on X-Plane's quality control process. At the time my immediate reaction was: that's totally off topic - I'm commenting on architecture and they're talking about QA. I do think the authors made fair points.

But in hindsight, I think that there's a deeper issue: one of verifiability. Simply put, my statement that we (LR) rewrite stale implementations is nearly impossible to verify without source code access, something that you can't get for X-Plane. So from the perspective of anyone outside the company, my original statement is not falsifiable (it cannot be proven false) and thus rather useless as a statement of fact. Even though I claim that we make rapid progress on features by keeping implementations clean, you as a user don't care how we develop our features - clean architecture, more developers, or the use of time travel and voodoo dolls, it's a bit moot.

Thus the comments were off topic, but also they were moving away from an unverifiable topic and toward one that users can measure, namely the quality of X-Plane's betas.

There's a fair amount of marketing that gets put out in the tech and games industry. It's a slippery slope from giving a new, real technology a whiz-bang name (e.g. HyperZ is a real technology, and it is good for your frame-rates) to using techno-babble to make the bad seem good (e.g. HyperMemory just means that your video card lacks VRAM and is going to be slow). When new products come out, the feature list is parroted, but it's not always clear whether the new features turn into real benefits.

So what I'm going to try to do with the scenery blog is: try to keep the blog limited to verifiable, measurable aspects of X-Plane. If we ship X-Plane with "psychoactive rendering*", I'll try to explain what the heck that is and why you'd want it, and how you might notice that it's working.

* X-Plane does not have psycho-active rendering, except possibly when I make a mistake in the shaders and everything turns purple.

Test Your Add-Ons Now!

X-Plane 940 beta 7 is out. Now is the time to test your airplanes, scenery, and plugins. We've reached a point where we think we have the new systems code working. Please try the new beta, test your add-ons, and if something works in 930 but is broken in 940, file a bug immediately!

Two Warnings About Normal Maps

Two warnings about normal maps:

1. Make sure that the RGB color underneath transparent sections does not turn black or white! Some image editing programs (in particular Photoshop) will lose the color beneath a transparent area.

   With a normal map, this is very bad - black and white are not legitimate normal map colors, and the result will be bogus normal vectors under the non-shiny part. Normal maps affect more than just specular shiny hilights - the normal map affects all lighting, so having black or white under your transparent (non-shiny) parts is bad news.

   To check whether this has happened, I recommend Graphics Converter, which will show you your alpha and RGB channels separately, exactly as they are in the file.

2. Make sure your RGB value are normalized. The "length" of the normal (as encoded in RGB) must come out to a distance of 1. This is virtually impossible to do using PhotoShop or an image-oriented program...I suggest you use a real plugin to PhotoShop or Blender to create normal maps that are correctly "normalized".

It is also very possible that X-Plane's gamma correction is distorting normal maps, but that's one for me to fix.

Plugin Broken? It's Temporary

I just found out from a user that Chase View Delux doesn't work with X-Plane 940 beta 4. Here's what's going on:

• Plugins often end up running X-Plane code - when you make a call to the X-Plane SDK, the SDK sometimes passes the call right to X-Plane.
• X-Plane 940 beta 4 has heavy error checking on all of the X-Plane code.

Put these together and...X-Plane 940 beta 4 is putting heavy checking on plugins that wasn't there before.

Typically the kinds of problems caught by the error-checking code inside a plugin are not critical, but aren't bad things for authors to fix. Since we will turn off heavy error checking as soon as we get the systems code fixed, plugins will start working again..there's no need for plugin authors to recut anything.

A Slow Motion Car Crash

I'm driving at 55 mph on the highway. I drive over a nail, lose a tire, skid off the road, crash through the guard rail, plunge off a cliff, and die. That's not much fun. But when you get to the accident site, you'll probably be able to piece together what happened. The skid marks, the nail, the hole in the guard rail, the car wreckage below - you can connect the dots.

Now...let's say I'm driving 500 mph. Same nail, same out of control crash. But this time it's going to be a lot harder to tell what happened. Lord knows where the nail ends up, the distance from the nail strike to exiting the highway is going to be a lot bigger, and the car is going to be in smaller pieces scattered over a wider distance. It's going to take a lot longer to piece together what went wrong.

That, in a nutshell, describes the motivation for an X-Plane beta with all of the debug and safety checks on. X-Plane's normal operation is like the car doing 500 mph - when it crashes and dies, there's very little left that can be used to figure out what went wrong. When there is a bug in the code that destabilizes the sim, finding it via crashes in release builds takes a lot of developer time and slows the whole beta down.

With the safety checks on, X-Plane still crashes when something goes wrong - but the bodies and wreckage are all a lot closer to the scene of the crime, and the evidence left around is in much better shape.

One of the side effects of the safety builds is that they catch "harmless" coding mistakes - (harmless in quotes - the bug might seem harmless but who knows if that will always be true). XSquawkBox now quits the sim with an ugly alert box because it reads off the end of a piece of the airplane data structure via the plugin system. This hasn't hurt things in the past, but it's not really correct. Beta 5 should fix the underlying problem, letting you run XSquawkBox again. (The fix will be in X-Plane, not XSquawkBox.)

A Tip For Updating Electrical Systems

X-Plane 940 has a new electrical systems model and it has a few important differences from 930:

1. You must specify exactly how many buses your plane has. 930 provided two buses but then did a bunch of cross-tying behind the scenes in case you didn't have enough power sources.
2. X-Plane 940 requires that each battery and generator be on exactly one bus.
3. X-Plane 940 will not allow systems to be powered by non-existent buses.

Now this can have some strange side effects. Consider the default king-air with two generators and one battery:

• In 930 it has two buses, battery feeds both buses, and each generator feeds one. You could have systems split by bus and they would work unless you lost one generator and the battery.
• 940 defaults this plane to one bus, because on battery power only one bus will be fed.
• This means that in 940 all of your systems will be reset to bus 1.

But wait...if you import into Plane-Maker, the import will trash your system bus selections before you can increase the number of buses to 2. What can you do?

Here's a work-around: before you update your plane, make sure you have two battery and two generator switches on your panel. Then open in 940. The import will set 2 buses and your systems will be preserved.

Of course, by the next few betas this may all be moot because we may get something less crazy in there.

Driver Thrash

I'm seeing a number of bug reports where weird artifacts are showing up in 940...missing pieces of runway, flickering triangles...all sorts of good stuff!

I believe that this is due to some kind of bug relating to threading, X-Plane and the video drivers. I won't say whose fault it is because I really don't know. I do know that the bug appears to not happen on OS X. (But this could simply be because the threads time out differently on OS X.)

The changes to the rendering engine for 940 from 930 are substantial and aggressive - it'll take us a little bit to fix these things.

When you wonder how come programs don't use all 8 of your cores yet, well...this is why...multi-core programming is complex, tricky, tedious to debug, and often involves substantial changes from the original code.

12 GB For the RenderFarm

The global scenery gets cut on "the renderfarm", which is our name for the cluster of computers, each loaded with all of the input data for the global scenery. These computers chug for a few days to churn out the whole planet.

With the next global render I am trying an experiment: using one 8-core Mac Pro as the RenderFarm. We've never had more than 8 processors in the farm at a time; in the past to get 5 processors might have taken 3 machines. The appeal of a single machine is ease of setup; no data to sync between machines, no sorting out which machine did which tile and merging it all back.

Today I upgraded the Mac Pro's memory (again) to 12 GB. I thought the logic of why I did this might be of interest to X-Plane users who are trying to figure out "should I have more memory"?

Basically there are three memory limits we care about:

1. The virtual memory limit per process - generally 3 GB per process for a 32-bit application. If an application wants more memory tan this, regardless of what you have, it is dead.
2. The virtual memory limit for the whole machine. Since the machine virtual memory limit is a function of hard disk space, normal users will never care about this - we can have a huge amount of virtual memory.
3. The physical memory actually used by the sum of all programs actually running. Once all programs need more physical memory than you have, they start using hard drive, and they get really, really, really slow.

In the case of the render farm, our processing program runs on one DSF tile at a time. But with 18,000+ tiles we can take advantage of more cores by processing 8 tiles (using 8 copies of the program running at once) on 8 cores.

This is where memory comes in. Before the upgrade my machine had 4 GB of memory, allowing each of the 8 tiles to use a little bit less than 512 MB of RAM before we ran out of physical memory and started paging. (The OS takes a little bit for itself.)

Normally this is all good -- a typical run might only use 300 MB of RAM. But every now and then we hit New York City or Boston and the RAM use spikes out to 1500 MB or more.

This is okay if just one process hits New York, but what if a bunch of processes hit a big city at once? We blow past the physical RAM of the machine and every tile becomes slow at once. And because they are all slow at once, they take a very long time (hours) to clear this state.

(This state of thrash due to many processes is like 8 people trying to go through a doorway at once. If they would just take turns, they'd be through in a second. But by forcing 8 at once they get stuck. The operating system won't pause a few of the high-memory programs to let the others complete.)

Since the RenderFarm has to run overnight unattended, I upgraded RAM to 12 GB, for 1500 MB per rpocess before thrash. My hope is that for this investment, we'll be able to run the processing through without a human to unjam it.

What About X-Plane

So would a RAM upgrade for X-Plane help? Well we can apply what we know from above to figure it out. Generally, you need enough RAM that X-Plane + all other programs won't run out of physical memory. Since X-Plane is 32-bit (and can only use 3 GB of RAM) you are likely to be fine with 4 GB when running X-Plane + the OS. Any more and X-Plane can't take advantage.

The exception might be if you need to run X-Plane and another big program like PhotoShop at the same time. At that point you might want enough RAM for both to run at the same time.

Lego Brick Code

When Sergio first proposed generic instruments, his model was "lego bricks". The idea was to provide a number of very basic parts for panel makers and let panel makers mix and match. The result would be huge flexibility for airplane authors without code bloat.

The problem with non-generic instruments is that there is such a huge variety of behavior among airplanes...if Plane-Maker were to have options for every possible plane, the "special equipment" screen would require 3000 tabs and be completely unusable. Hence the need for a smaller unit of modeling: the lego brick.

The prop disc is the first feature I have done that is meant to be used only by a plugin, e.g. "lego brick" code. The X-Plane systems code sometimes suffers from the same "code bloat" problem as the instruments: a ton of very specific, very tweaky behaviors that interact in strange ways and become very difficult to manage. It's not that the systems code is bad code - it's that the scope of the problem is simply too large. That is, you can't expect X-Plane to cleanly simulate the systems of every airplane ever in a ton of detail through an a la carte menu of check-boxes.

The idea of the prop disc is: someone (LR or otherwise) can write a plugin that encodes a certain style of prop disc. That plugin can then be picked up and moved around like a generic instrument between planes (perhaps with a corresponding text file to control it).

If someone else comes up with a different/better prop-disc algorithm, compatibility isn't an issue...that person writes a new prop disc plugin and the airplane author selects the one desired. Think of it as sort of a portable flight model that stays with your plane.

So we win in three ways:

• Anyone can write the prop disc algorithm, not just LR.
• The code lives with the plane, to avoid compatibility problems.
• More than one plugin can exist, giving authors an a la carte menu.

That's the theory at least.

Tweaking the Prop Disc in 940

X-Plane 940 allows plugins to customize the prop disc. Details here on the wiki.

I put these datarefs in so that modelers wouldn't have to try to model prop discs with OBJs. The problems with prop discs are many:

• They need to be billboarded, and X-Plane does not provide datarefs for manual billboarding inside an airplane (particularly not to the engine's coordinate system, which can be transformed by all sorts of fun stuff).
• They often need variable translucency, which OBJ does not have.
• They cause all sorts of depth buffer errors, which OBJs cannot manage.

In short, prop discs are weirdly special-cased enough that I thought it would be better to provide a general set of parameter datarefs for prop discs and let X-Plane do the drawing.

These options are not available in Plane-Maker. Why not? That'll be my next post.