Category: Aircraft & Modeling

DataRefs, the Cirrus, and Opt-In

There’s a bug in X-Plane 900 RC2 that will be fixed in RC3.  Basically when you’re flying in the Cirrus with a friend (also using the Cirrus via multiplayer) you’ll see your own control deflections on his or her plane.

Understanding this bug gets into the way datarefs and object animation work.  In simple terms, a dataref is an access point to data provided by some other part of the sim…the flight model, or another plugin.  An object references a dataref as its source for animation values.  So if you want to animate the wings, you use sim/flightmodel/controls/lail1def or sim/flightmodel2/wing/aileron1_deg.
Now the question is: when you access this dataref in a multiplayer context, which aircraft’s ailerons are you talking about?  It turns out the answer depends.
  • For all datarefs except sim/flightmodel2/ and sim/cockpit2/, if the dataref needs to access a plane, the access is to the user’s plane (flight 0), always.
  • For sim/flightmodel2/ and sim/cockpit2/, the access is usually to flight 0.  But if we are drawing an aircraft, these datarefs refer to the aircraft being drawn!

(Note: this second feature only works correctly in RC3.)

So when you want to animate the wings of your plane in X-Plane 9, by using the sim/flightmodel2/ datarefs in your object, you’ll get the behavior you want: animation with the first plane’s flightmodel when your plane is loaded as plane one, animation with the second plane’s flightmodel when your plane is loaded as plane two, etc.
Of course the rule of thumb is simple: if working on v9, always prefer sim/flightmodel2/ and sim/cockpit2/ when possible for animation work.
Could we have simply made the sim/flightmodel/ and sim/cockpit/ datarefs have this “pick the right airplane” behavior?  Possibly, but I took the more conservative opt-in approach.  Basically any time we want to change the behavior of the sim we can do one of three things:
  1. Change it globally – all third party content is affected; don’t like it, too bad.  This is how pixel shaders work; if shaders are on you can’t disable the different fogging they provide for your airplane or object.
  2. Opt-in – you have to change your content to get the new behavior; old content defaults to unchanged.  This is how the new datarefs work.  You have to edit your content to use the new datarefs to get the new behavior.
  3. Opt-out – you have to change your content to avoid the new behavior.  For a while cockpit lighting was like this, but I received so many reports of version 8 airplanes looking totally strange that I changed the cockpit lighting choices to opt-in.

Generally opt-in is the safest thing we can do to leave previous content working; if we make a change globally then we break any content that doesn’t work well with our changes.  Datarefs are used so heavily that a fundamental change in how they work would probably not be safe.

Given a choice between opt-in and opt-out I’ve come to prefer opt-in; both require the same amount of work for LR (we have to write code to support the old and new behavior) so why not do the thing that provides better compatibility?
Finally a general historical note: sim/cockpit2/ and sim/flightmodel2 are not just there to provide new behavior with objects; they are designed to give us a clean start in providing only authoring-related datarefs in an easier-to-understand format.  The original datarefs were meant only for programmers, so we didn’t worry too much about usability.  Since then, datarefs have become the communications medium for panels and a lot of 3-d animation.
So not only do sim/cockpit2 and sim/flightmodel2/ provide correct animation behavior, but they’re also easier to read.
An important distinction between sim/cockpit2 and sim/flightmodel2 is that they partition the datarefs based on perception vs. reality:
  • Generally sim/cockpit2 shows datarefs as the pilot sees them in the cockpit, and actions as the pilot commands them.
  • Generally sim/flightmodel2 shows the plane’s behavior as it’s really simulated, and actions as they actually happen.

In other words, when the pitot tube ices up, sim/flightmodel2 shows the real airspeed and sim/cockpit2 shows the incorrect airspeed.  When the hydraulic system fails, sim/cockpit2 shows that the pilot wants to bank, and sim/flightmodel2 shows that the ailerons aren’t really moving much.

What this means is that you should use sim/cockpit2 for cockpit objects and generic instruments and sim/flightmodel2 for objects attached to the airplane exterior.  This will assure that failure simulation works correctly (e.g. failures are perceived correctly by the pilot) and the external control surfaces match the plane’s physical behavior.
If you’ve studied sim/cockpit2 a lot, you know it’s pretty incomplete.  We’ll be adding the rest of the cockpit systems in 910.  I would have liked to get the entire set of airplane changes into 9.0 but there’s a lot more to do – not just more datarefs, but also manipulators for 3-d objects and a new lighting model for aircraft interiors.  I think 910 will finish what 900 has started in terms of new airplane features.
Final random note: RC3 will allow you to pick any dataref for a generic instrument, not just sim/cockpit2 and sim/flightmodel2.  This is mainly so people writing plugins can easily hook their datarefs up to the panel by editing datarefs.txt instead of typing them by hand.
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You Can’t Replace Objects in Liveries

The livery system lets you replace the image files used by an aircraft’s exterior paint, and it lets you replace the image files used by the objects that are attached to the plane via the “objects” folder.

But the livery system does not let you replace the objects themselves.

(It also doesn’t let you replace or modify the actual ACF file or generally change the functional behavior of the plane. The livery system is just for paint, not airplane mods.)

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X-Plane 9: What Comes Next

X-Plane 9.00 is going to be going final pretty soon – we’re on RCs right now. But this is hardly the end of the road; rather it’s the first step for a number of new development areas. What comes next?

Tools: I am working on tools releases now; a few users already have alpha copies of MeshTool. My hope is to have some new tools posted tomorrow, with more by the end of the week

More rendering engine/shader work: X-Plane 9 only begins some of the shader work we want to do; more engine enhancements will be coming in 910. Like 9.00, more advanced rendering will consume more hardware, and will be scalable via rendering settings.

Systems and Airplane SDK: X-Plane 9 introduced a lot of new features for airplane modeling, but there’s still more to do. I hope to have the panel region system and advanced lighting options all fleshed out for 9.10. Austin is doing a lot of work on systems modeling. We’ll be making more new airplane-related datarefs to tie it all together.

Orthophoto Scenery: I don’t know if this is going to happen or not, but we get more and more requests for large-orthophoto-scenery support in X-Plane; MeshTool is only going to increase that desire by making it possible to cover large areas with orthophotos without sacrificing framerate. So I would like to do some work on the texture pager, but I do not know if I’ll be able to do that in time for 9.1.

My hope is to get 9.1 into beta early so that we can start getting third party aircraft authors involved with trying new features. If you are working on an advanced airplane, keep in touch!

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Version 9 – Probably Too Late

If you have a third party add-on and you haven’t tested it against X-Plane 9, well…honestly it’s probably a little bit too late to fix compatibility bugs now. We’re in RC, and only changes to fix critical bugs are going into the sim; everything else will have to wait for 9.1.

So if you have an add-on and you haven’t tested it against 9.0, for crying out loud, do it now! Submit the bug anyway – if we can’t fix it for 9, we can fix it for 9.1. But…after almost 14 weeks in beta, we’ve got to close this build up.

BTW a minor side note on fuel pumps…

In X-Plane 864, an airplane will run even if the electric fuel pump is off. (However, if the fuel pump failure is triggered and the electric fuel pump is off, either by switch or electrical-system failure, then you’re out of fuel.)

X-Plane 900RC1 restores this behavior; in beta 25, an electrical failure would turn off all sources of fuel. This is wrong for a plane like the C172 where gravity can feed the engine.

In the long term X-Plane does need a more complex abstraction (e.g. does this plane have gravity feed or engine drawn fuel, or engine driven pumps, etc. etc.) but for now the 864 model, while inaccurate and incomplete, causes less problems than beta 25, which was simply wrong for a number of planes.

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Don’t use ATTR_cockpit outside the cockpit objects

I’ve blogged about this before, but…let me be totally clear:

Don’t use ATTR_cockpit in objects that are not one of the two cockpit objects for your airplane.
Don’t use ATTR_cockpit in the attached misc. objects for your plane – move the parts of the mesh that require ATTR_cockpit into the cockpit object.
Don’t use ATTR_cockpit in scenery objects.
The OBJ spec basically says as much when it says “don’t use cockpit features” outside of cockpits.
Now what goes wrong if you violate this varies with the betas vs. X-Plane 8, but I can tell you this: no version of X-Plane has ever shipped that will correctly handle ATTR_cockpit in attached objects for all cases.  There’s always been bugs in this not-such-a-good-idea code path; it’s just the severity has varied over time.
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Airplanes – How it Fits Together

Here’s a summary of the new airplane features in 9.0 (and some coming). Hopefully this will give you an idea of what new capabilities are available for modeling planes in X-Plane 9. This list will sound like a broken record – virtually all of these features are optional; you don’t have to recut your finished airplanes to use them in version 9.

2-d vs. 3-d Panel

You may have noticed the new “3-d panel” option in PlaneMaker 9. This allows you to build a separate panel for the purpose of providing the texture to ATTR_cockpit (or ATTR_cockpit_region). You can then:

  • Provide alternate instrument artwork in a cockpit_3d folder. (This lets you have perspective artwork for the 2-d cockpit and orthogonal artwork for the 3-d cockpit.)
  • Pack your instruments together tightly to save space. (There is a real cost to large panels, so using a 1024×1024 panel for the cockpit object is a lot better.)

The 3-d panel is strictly optional, fully replaces the 2-d panel only for cockpit objects, and is activated by providing a custom panel background in a cockpit_3d folder. (See the “Example Plane-Widescreen+objects” plane in beta 19.)

ATTR_cockpit_region

Cockpit regions are an alternative to using the entire 2-d panel to texture your objects. They provide a few advantages:

  • Performance. By requiring a power of 2 and allowing you to use a sub-area of the panel, cockpit regions avoid a lot of wasted computing that ATTR_cockpit can cause.
  • Next-gen lighting. Unlike ATTR_cockpit, real 3-d lighting is applied to the panel when you use this attribute. This means that you will get a gradual decrease in light on your geometry (correct based on the angle of the sun) that matches the rest of the object.

Please note that you can mix and match which way you get your cockpit texture and whether you use the 2-d or 3-d panel feature (above) independently. However, you can only use ATTR_cockit or ATTR_cockpit_region in your airplane, not boht. ATTR_cockpit is still supported.

Generic Instruments

Generic instruments let you build instruments that follow some basic shapes (needles, tapes, etc.) that can be tied to any dataref. This both lets you customize particular instruments very precisely or create an instrument driven by a plugin dataref. These instruments are optional in version 9 – the old “premade” instruments are still supported.

New Datarefs

X-Plane 9 provides new datarefs targeted at airplane authors. The datarefs are better organized and have clearer names. But the old datarefs still exist, so legacy planes do not have to be updated.

Generally the entire cockpit should use only sim/cockpit2/ datarefs, and the plane exterior should use only sim/flightmodel2/ datarefs.

One special feature of these two sections: if your plane is used as an AI plane, these datarefs will animate the plane with the AI plane’s control deflections, not the user’s control deflections. So using these datarefs fixes the “AI animation” problem.

Plugins in Aircraft Folder

Version 9 airplanes may have a plugins folder (inside the ACF package) with fat plugins inside them. If you develop a plugin for your airplane, consider packaging it this way — this will allow your users to install the airplane with a single unzip for all platforms and no extra “drag-this-file-here”.

Plugins in the airplane folder is optional – you don’t have to provide a plugin, and plugins that are installed in the main Resources/plugins folder will still work. Still, I encourage you to use this feature because it makes the install process a lot simpler. The X-Plane SDK website will have documentation on fat plugins.

Liveries Folder

X-Plane 9 features a new “liveries” folder. Liveries (replacement exterior paint for airplanes and their attached objects) can be placed in packages in the liveries folder to greatly simplify the process of repainting an aircraft. See the “Example Plane-Widescreen+Objects” for an example.

While the liveries feature is optional, I strongly encourage anyone doing repaints to adopt it. Liveries can be switched by the user in the sim without any file manipulation; there is thus no risk of accidentally deleting or breaking an aircraft.

Large 2-d Panels

In X-Plane 9, a panel can be up to 2048×2048 in size. You pick the dimensions. The panel will scroll horizontally if necessary.

Note that if you use the new 3-d panel feature, the 2-d and 3-d panel do not have to be the same size. I would recommend a large 2-d panel (to fill large monitors) and a smaller 1024×1024 3-d panel (for performance).

Hiding Parts

X-Plane 9 will allow you to hide aircraft parts. Many v8 planes use OBJs to model the plane geometry, and use a transparent ACF texture to hide the ACF. Setting the parts to “not drawn” saves the CPU time that X-Plane would spend drawing the airplane, and is thus more efficient.

Keyframes

X-Plane 9 supports key-framed animation; this is useful for the scenery system, but for airplanes it allows for much more complex and realistic animation. OBJs that don’t have key frames still work.

Manipulators

This is a feature coming in the future: the ability to control how the user clicks and interacts with the cockpit object in detail. In X-Plane 9.0 we only support clicking on cockpit-textured geometry; manipulators will make features like draggable handles a lot more workable.

Global Illumination

X-Plane 9 does not yet offer a lot of control of the in-cockpit lighting environment; we’ll be working on this in future versions. These features will be opt-in…that is, you’ll have to change your model to get the new features, and old planes will work the way they always used to. It is likely that you’ll have to use “modern” airplane-building techniques to use these new features (meaning OBJs, named or custom lights, lego brick instruments ,etc.).

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When Can You Not Use DDS?

There are a few cases where you cannot use DDS files in X-Plane:

  1. Airplane 2-d panels (any layer – base, lit, -1 shadow layer, 2-d or 3-d).
  2. Airplane instrument images.
  3. Bitmap-based region specification referenced in a library.txt file.
  4. Any gray-scale/alpha-only texture (e.g. mask files in the scenery system).

Beta 17 is treating cases 1 & 2 as an error; beta 18 will simply stop looking for DDS files in those cases.

Please note that airplane panels and instruments are not compressed right now, so there would be no performance benefit to using DDS in these cases. (If anything, PNG has smaller file size when compression is not used.) If we ever allow compressed panel textures, we’ll probably allow DDS panels at the same time.

Case 3 is just a particular version of case 4 – that is, the region bitmap is black and white (1 channel) so DDS provides no benefit. Use a gray-scale no-alpha PNG!

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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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Obects and Liveries

In X-Plane 9 betas 2, you can use the livery system to provide alternate textures for misc objects attached to your plane, but not the cockpit objects. Beta 3 will address this, allowing cockpit objects to reference the livery system too. We’ll get some examples posted for how this all works soon.

Austin and I were discussing this last night – here’s a few thoughts on the difference between the misc. objects and the cockpit object:

  • Because you can have multiple misc. objects, you can effectively use more than one texture for your airplane. These days plane designers want a lot more than one texture.
  • We’re working on optimizations for attached objects – basically by breaking your plane carefully into a few objects, you may be able to optimize frame-rate. A lot of this code is not in the sim yet, but will go into a later beta.
  • Using multiple objects with different LODs is a much more efficient way to improve fps than simply having multiple LODs of the entire plane.

For example, make an interior object with a low LOD (0-500 meters) and an interior texture. Attach it. Then make an exterior object with an exterior texture and a longer LOD (0-50000 meters). When you back away from the plane, X-Plane can entirely skip the interior object, which means that neither the geometry nor texture have to be moved to VRAM. That’s a big performance win.

The cockpit object is also different:

  • The cockpit object induces the sim to make a texture out of the 2-d panel. (And it is the only object where the panel texture is legal.)
  • Mouse-click analysis is only done on the cockpit object. If you load up the cockpit object with all of the animation for your landing gear, X-Plane has to wade through all of that animation to update the mouse cursor in the 3-d cockpit.
  • The cockpit object can be swapped for an alternate object in the exterior view (we’ll provide more control of this for misc objects soon).

So my general advice is: use the cockpit object only for the real panel itself!

However, we have provided livery support. You can’t livery the 2-d panel, and the livery system is not meant to reskin cockpits, but there are probably some planes out there that used the cockpit objet to model exterior plane features (from before we had misc objects). The livery system in beta 4 will let you reskin these planes without changing how your obects work.

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Don’t use the panel texture in a non-cockpit object.

Panels and airplanes aren’t really my thing. But we’re using the OBJ engine more and more in aircraft, blurring the lines between scenery code and aircraft code.

In X-Plane you can actually use the panel texture in any object. Pleaes don’t do this! There are two cases:

Airplane Objects

With X-Plane 860 you can attach many objects to an aircraft. But…you should only use the panel texture in the cockpit object.

The cockpit object is special! It is the only object that gets mouse-click tested. In the future we will decide how much of the 2-d panel to render to a texure (for the 3-d cockpit) based on the cockpit object. So…don’t use the panel texture in your other aircraft objects. Put your panel-textured triangles in the panel object.

(You should do this anyway; switching to the panel texture creates a new batch*, so using it in a lot of separate objects is bad for framerate.)

Scenery Objects

Technically you can also use the panel texture in scenery objects. This is really just a big hack — all of the issues with optimzation apply to this case, plus: how can you even know the layout of the panel texture in scenery? Don’t do this!

* “Batch” is the technical term in the game development and authoring world for a set of triangles that can be drawn by the graphics card via single CPU command. While graphics cards can process tons and tons of triangles, the number of batches they can process is limited by CPU and bus speed, which advance much more slowly. Generally the number of batches is the limiting factor in X-Plane’s framerate.

It’s easy to see how many batches your OBJ8 contains: open the file and count the number of “TRIS” and “LINES” commands at the end. Ignoring lights, the sum of the TRIS and LINES commands is the number of batches! (HINT: fewer is better, and only one batch is great!)

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