Category: File Formats

It’s Time to Try Nine

If you create plugins, airplanes, or scenery for X-Plane and haven’t tried your add-ons with X-Plane, please do so soon!! It’s much easier for us to fix backward-compatibility problems while we’re still in beta. Beta 14 introduced some bugs (that should be fixed in beta 15 real soon) but I think we’re reaching the point where you can do compatibility testing.

We’re working on a public Linux beta – see here.

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The Global Scenery is Special (for now)

In X-Plane 8 the global scenery was installed in the “default scenery” folder, which normally contains most of the built-in artwork to make X-Plane (and the global scenery) work.

In the X-Plane 9 beta it is installed in the custom scenery folder. But the sim recognizes it by name and makes sure custom scenery is higher priority than global scenery. This is imperfect but works. In the future we may do something different. Survival tips:

  • The log file tells the priority of all scenery packages.
  • Don’t rename or move the global scenery – leave it where it was installed.
  • Custom scenery will always trump global scenery.
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What is a panel region?

X-Plane 9 introduces a new OBJ feature: panel regions. The basic idea is this:

In X-Plane 8 you could use the 2-d panel as a texture in your 3-d cockpit. This allows a plane to have a moving map or glass display in the 3-d cockpit. However, the panel texture is expensive, particularly in v9 for a few reasons:

  • You have to take the entire panel, even if you don’t use it all. For example, consider all of the “wasted space” from the windows.
  • The real texture is rounded up to a power of 2 (and in x-plane 9 that could mean 2048×2048 for a a 1600×1600 panel.
  • The texture has an alpha channel, which probably isn’t usfeul (model your 3-d cockpit windows in 3-d, not using transparency). The alpha channel increases VRAM use by 33% and requires some pixel shader gymnastic in v9 that slow things down.
  • In X-Plane 9 this is all twice as painful since we have a panel day and and lit texture.

Panel regions address all of these problems. Here’s how they work:

  • A panel region is a rectangle within your 2-d panel. It can be placed in any location as long as it is (1) fully within the 2-d panel and (2) its dimensions are a power of 2.
  • The cockpit object declares up to four panel regions it wants to use.
  • A new attribute lets you use each of the four panel regions as a texture (alpha is not provided – the regions act opaque).

This does exactly what you might expect: it creates between one and four smaller power of 2 textures rather than one huge one and manages those smaller textures. We do have more textures, which is usually bad but we get some big wins:

  • Better VRAM use. The panel texture, being a dynamic texture, puts a lot more pressure on VRAM than regular scenery textures. Without this optimization, we could be paying 25 MB of permanent VRAM use just for the 3-d cockpit. Now we don’t have to pay to round up to a power of 2.
  • Faster updates of the 2-d panel into the 3-d texture, since we have to process a lot fewer pixels.
  • Efficiency – a clever author can cut down the panel use to only the parts that really matter, which might only be the EFIS at 256×256 pixels.

I will try to provide detailed documentation on this in the near future. There will be a new ac3d plugin coming out (hopefully in the next week) that will provide both editing capabilities for key frames and for panel regions.

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ANIM_hide is not a framerate optimization

I’m sure I blogged this before, but the blog’s gotten old enough that I can’t find my old posts. This came up in an internal company discussion, so let me say it again:

ANIM_hide is not a framerate optimization. 99% of the time it doesn’t make your object draw faster.

ANIM_hide is provided to let authors make certain animated effects (like swapping out a prop disc for blades based on prop RPM). These are animated effects and cost fps, because they use the CPU and interrupt the graphics cards from just blasting out triangles at warp speed. An example:

You might have a very complex object attached to a plane that doesn’t need to be drawn at all, like a landing gear assembly. This object could have its own texture, a ton of animations, and high triangle count. Using a giant ANIM_hide around it to prevent drawing when the gear is retracted does you no good. Here’s why it doesn’t help.

  • Before an object can be drawn, X-Plane prepares the geometry and texture to be used by the card. These are atomic operations (we have to prepare the whole texture and all of the geometry no matter what we will actually use) and they are expensive.*
  • X-Plane then must evaluate the animation command on the CPU to decide if the object must be drawn. If the object drawing is simple and the object is drawn, this CPU work is just wasted, so this technique wouldn’t even make sense unless the object is hidden a lot of the time.
  • If the object is hidden, X-Plane will still run through every command in the object, simply skipping drawing. So if the object is heavily animated, we still pay a lot of CPU for the “hidden” object.

You will actually save some performance with ATTR_hide in one situation: if the geometry that is not drawn with ATTR_hide covers a huge amount of the screen (e.g you are really close to the object) then not drawing the pixels saves frame-rate. This would be extremely unlikely – you’d have to hide an entire cockpit from the inside to see any kind of benefit with this.

Why doesn’t ANIM_hide actually skip work? Well, it may someday, but for now it’s a question of how we optimize objects. When we load objects, we evaluate their command sequence and attempt to consolidate and remove unnecessary CPU work. We remove unnecessary state change, combine drawing commands when possible, etc. This works because we know at object-load time exactly how the object will be drawn. But consider this bit of OBJ “code”:

TRIS 0 30
TRIS 30 60
TRIS 90 120

If ANIM_hide really skipped work, is the ATTR_smooth routine necessary? Can the last two TRIS commands be combined into one big TRIS command? If ANIM_hide removes an attribute, we don’t know until we actually draw the object what attributes will be run. But if we say ANIM_hide doesn’t affect state change (which requires us to actually do the state change) we can then realize that none of the state change in this object are actually necessary, saving 3 attributes, and allowing us to combine a triangle batch.

So for now our policy is one of simplicity:

  • ANIM_hide is for artistic purposes, but not optimization purposes. It simply stops drawing, it doesn’t eliminate work.
  • The optimizer then goes to work on your object.
  • I think we will someday provide a new mechanism for the “landing gear case”, one that’s specifically designed to quickly eliminate parts of a plane that are expensive and don’t need to be drawn.

* X-Plane does try to optimize out this cost in some cases. For the purpose of drawing, an object can get “culled” (not drawn at all) by being too far away for LOD or fully off-screen, or it can get “drawn” (meaning we do look inside the commands). Culling is very very fast, and drawing is usually quite slow. The optimization of avoiding using geometry and textures always work in the culling case, but usually do not work in the drawing case. So having a smart LOD on the landing gear is a huge win.

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Airport Layouts and Global Scenery

When we make the global scenery, we pre-process the terrain to make flat grassy areas for the airports, using the latest apt.dat data. We will be recutting the scenery again soon for version 9, so if you have a favorite airport that isn’t flat and grassy in v8, you can submit a layout to Robin Peel – notes here.

I do not know exactly when we will be rendering the scenery, so I don’t know how long you have to submit a layout. I can’t make a promise that “if you submit a layout by X it will be in the scenery” – something can always come up that requires us to not use the layout (especially now that we have apt.dat 850 and layouts can contain all sorts of strange things). So I can only say: please consider submitting your layout soon. If it’s only partly done, finish the boundary and submit it to Robin with a note “this is only for global scenery” – you can always finish it and submit again later.

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Draped Roads – A Can of Worms

I don’t like it when overlay scenery depends on the underlying base mesh…I’ve been telling authors for a while now on the lists, “don’t assume the base mesh is the global scenery”, and while this actually has been a safe assumption for version 8, I suspect that once we get v9 out the door I’ll have some time to write mesh-editing tools, at which point the user will have a choice of base meshes.

Roads are currently built in MSL space (that is, absolute elevation on the planet) and thus don’t change when the base mesh changes. In a previous post I discussed the idea of having them be AGL based (draped) so that they would “hug” whatever base mesh you have.

First I should say: this isn’t coming any time soon and won’t be in 9.0. But also, upon closer examination, there are a number of ugly issues just below the surface. Consider all of the things that the global-scenery generator does with roads:

  • Road type is selected not only from source road data, but from surrounding conditions, some of which are not obviously determined by the final DSF.
  • Road bridging is determined based on nearby roads, water, gradient constraints, and available bridge templates.
  • Road type is further influenced by bridging considerations.
  • Roads are smoothed to try to build the most even access, “hiding” rapid changes in underlying terrain from a high-speed roadway.
  • Powerline tower locations are picked from terrain peak to peak with constrained maximum lengths. (The individual segments also controls the span of wires, so introducing new nodes in the draping process might be undesirable.)

The problem therefore with a draped road is that the decision to bridge (both how high and what kind) are dependent on the underlying terrain.

The eventual solution would be to do all of the road processing in X-Plane while draping – but this implies that the author of a custom road grid doesn’t actually have control over how the roads come out. One advantage of the current system is that total control over the roads is provided in the DSF – given appropriate tools, a modeler could very carefully hand-craft the roads with a lot of detail.

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Key Frames in Version 9 (and datarefs)

X-Plane 9 will feature key-framed animation for the rotation and translation animations (but not the show-hide animations). Basically:

  • You can have more than two key frames.
  • Interpolation between them is linear.
  • Extrapolation off the ends is linear (use two equal-value key frames to clamp).

Key frames will be available for all OBJs: scenery, cockpit and aircraft parts. I will try to post a spec soon – the implementation is very similar to the RFC I posted a while ago.

(If you read the .tcl code in the AC3D plugin, you may have guessed that this was coming.)


Now I know somebody is going to ask me: “will this allow me to make an animation X start when event Y happens in the sim?” The answer is: no! Key frames allow you to map the relationship between an input dataref and your geometry in any way you want. But if the input dataref is not changing, the output position cannot change either.

Example: you can use key frames to make complex things happen as the gear deploys. Gear deployment is a ratio that starts at 0 and slowly changes to 1 as the gear deploys (over the cycle time as set in PlaneMaker). So you can use this to make the wheels move forward first, then down.

But you cannot use key frames in the cockpit to make the gear handle move slowly. The gear handle dataref is really a switch – it instantly changes from 0 to 1. If you set up a nice key framed animation, we will instantly jump from the animation position at 0 to the animation position at 1.

In other words, whether or not you have key frames, the timing of an animation comes from datarefs, and datarefs alone. I am not planning any extensions to the OBJ system any time soon that will let you trigger animations from events. In other words, you can’t synthesize timed sequences from an OBJ. (The purpose of OBJs is to visualize things, not model things.)

If you want to create timing data, write a plugin. If you don’t know how to code, I have seen at least one plugin that lets authors “invent” simple datarefs using a text file.

No Spare Datarefs

And while I’m going off on a tangent about authoring and datarefs, we will not be adding any “spare” datarefs. Again, datarefs should be made with plugins, and non-programmers should use a plugin that creates datarefs from text files. We will not be creating any “unused” datarefs in the sim.

Why not? Think of the integration problems. Even if we make 100, what happens if Joe uses “spare dataref 93” in the cockpit of his great 747 and Fred uses “spare dataref 93” for the windmills turning in his scenery pack? Splat.

The naming convention of datarefs start with the company or author who created it – or “sim” for X-Plane itself. This is intentional – it assures that two independent authors won’t use the same dataref name and create conflicts between add-ons!

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X-Plane 9: The Absurdity of Pretending

There have been plenty of rumors and semi-official posts regarding the upcoming major revision of X-Plane (X-Plane 9). I have been trying to keep my mouth shut about it…the problem with pre-announcing anything is that the upside to us is small (at best we do what we said) and the downside is large (at worst we don’t do what we said and people get grumpy). No one complains if XP9 turns out to have no-pause scenery load and it’s a surprise…but plenty of people complain if we say “there won’t be pauses” and then they are.

But…the situation is becoming mildly absurd…plenty of info is out there, and saying “the upcoming major release”, etc. just feels political and weaselly. Austin would be disgusted.

So listen: I am going to try to provide some info on X-Plane 9. This info is subject to change. This is what we think is going to happen to the best of our knowledge. The release is still a ways away and enormous changes will happen. When things change, do not bitch to me that “you promised X” would happen. I do not promise anything. This info is provided to try to help those making add-ons for X-Plane plan appropriately.

With that in mind…I will try to post some more details on the authoring environment in the next few days. For now, here’s some very basic guidance on compatibility and hardware requirements:

  • The hardware requirements will be at least as high as X-Plane 8. If your machine is gasping and wheezing on 8, it’s not going to be any better on 9.
  • X-Plane 9 will depend more heavily on pixel shaders. If your hardware doesn’t have pixel shaders (GeForce 2-4, Radeon 7000-9200) or has really crappy shaders (GeForce FX series) you will miss out on a lot of the cool stuff in v9, and possibly have the scenery look worse (but faster) than v8 (as we move features from the CPU to pixel shaders).
  • Scenery that opens in x-plane should open in X-Plane 9 unmodified – if the scenery works in 8 but not 9, report it as a bug!
  • Plugins that work in v8 should work in v9 without modification.

Finally, we are trying to finish up X-Plane 861…this is a bug-fix patch for version 8 – it contains no new features, but it does fix a few nasty bugs, some of which cause crashes. So if there is any new feature, it’s coming in 9, not 861. Version 8 has been out for a very long time, so I will accept no argument that v8 should have more features than it does now. It’s been a long run!

(One of my main goals with 861 is to try to fix any weird behavior for third party scenery add-ons, so that a scenery add-on looks the same in v8 and v9. If we left the bug in 861 and fixed it in v9, authors would have to hack the scenery to make it work with v8, and then remove the hack and republish for v9. By trying to fix the authoring bugs in v8, at least when possible, it lets authors publish one package for both versions. Of course, v9 will have new features, so I expect some v9-only scenery will emerge pretty quickly.)

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ATTR_no_depth deprecation

I am going to take Marginal’s suggestion: in the future, ATTR_no_depth will be mapped to ATTR_poly_os 1, and ATTR_depth will be mapped to ATTR_poly_os 0. As far as I can tell, historically the ability to turn off all depth buffering was a misguided attempt to do the kind of things that ATTR_poly_os is meant for.

This implementation will hopefully help any content that is (for some reason) still using ATTR_depth and ATTR_no_depth…modern OBJ generators like the Blender plugin and the AC3D plugin never use this old attribute.

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Flat Shading is Evil (and other OBJ sins)

Almost two years ago, I posted this blog entry, pointing out that some legacy OBJ commands are, well, evil.

My inclusion of ATTR_poly_os in that blog post was a little strange – when used correctly, ATTR_poly_os is the right way to overlay decals on the ground, and it is fully supported. (The message is just, I suppose, that if you need a huge number for your poly_os, something is really wrong.)

Now ignoring ATTR_poly_os, you might notice that the AC3D plugin exports almost none of those dubious attributes. But what about flat shading? Flat shading isn’t necessary, but you were allowed to use it.

So I came to my senses and modified the AC3D exporter – the exporter will now never use ATTR_shade_flat – instead it uses smooth shading and per-vertex normals, avoiding an unnecessary attribute no matter what you do in AC3D. 99.99% of the time this will yield fastest framerate?

Why wasn’t the plugin always like this? I thought it was, and only discovered today that I hadn’t finished this optimization.

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