Category: Aircraft & Modeling

How Many Aircraft Does It Take to Change a _LIT Bulb?

500.  1 to write the new lighting code and 499 to work out the gajillion ways that backward compatibility gets broken!

Suffice it to say, there are a number of major bugs with aircraft lighting and drawing in 10.10 beta 6.  Beta 7 is now out and attempts to fix most of them, but I fear there may be at least one more round of fixing backward compatibility bugs in airplanes.

If you have an aircraft (built-in, third party, or yours) that looks good in either 10.05r1 or 9.70 and looks borked in 10.10b7, please report a bug, particularly if the panel and panel lighting is not working.

Why is debugging aircraft drawing so difficult?  It’s a bit of a perfect storm.

  • The code supports a huge number of individual behaviors.  A lot of the airplane drawing code was done incrementally, so even supporting ‘What v9 does’ means supporting the presence or absence of a large number of small features in every combination.  You may or may not be using regions, may or may not be using translucency, may or may not be using interior lighting, may or may not be using the 3-d texture, may or may not be using generics, may or may not be using ATTR_lit_level, may or may not be using additive instrument lighting, may or may not be using auto-adjust levels, and I haven’t even started on the per-object check boxes and global OBJ attributes!  (This list is a combination of flexibility, combining two complicated systems, panels and objects, and the “little at a time” approach which introduces a number of intermediate authoring modes into the airplane SDK.)
  • A number of these options are very quirky.  Sometimes they have long running bugs, sometimes they’re limited by hardware, and often using two together produces weird behavior.
  • Often the quirks are useful.  Rather than avoiding quirky weird behavior, lots of planes take advantage of it.  Bugs become new features.
  • We didn’t do much in X-Plane 9 to flag, warn, or prepare authors for X-Plane 10.  While we knew HDR was coming, X-Plane 9 never flagged or squawked about old authoring techniques, so a “works in v9” plane might work because X-Plane 9 supports lots of old weird stuff.
  • HDR rendering (with the deferred renderer) is fundamentally different from standard “forward” rendering (what we had in v9) and thus everything above has to be coded twice.

Ironically, the generous backward compatibility of the panel/cockpit authoring system in the past (including the ability to use all of the intermediate use cases of these features) has made backward compatibility worse now by making the problem space much more complex.

Unity For 10.10

As I have posted before, one of my goals is to make X-Plane 10.10 a stable and final authoring platform for v10 planes – that is, fix all of the rendering bugs so that if your plane looks good in v10, it won’t need any more updates or check-boxes changed.  10.05r1 did not meet this criteria because there are problems with airplane rendering in HDR that an author can’t work around – they are bugs in the sim.

As part of that process I am trying to create a single “right” path for authors to use the panel texture in an HDR compatible way.  This consists of:

  • GLOBAL_cockpit_lit gives you the newest lighting path.  When this attribute is present, lighting is the same for regions and no-region panels, the panel texture is fully lit by HDR, and transparency works as well with a panel as it does with a regular texture.*
  • With 10.10 the cockpit object has a full set of plane-maker check-boxes for control over shadowing, LOD, glass, lighting, etc., to make it consistent with the attached objects.
  • In 10.10 any attached object on an airplane can use ATTR_cockpit or ATTR_cockpit_region as long as the cockpit texture does too.  (This means you can mae your panel texture interior for lighting but reuse the cockpit texture in a glass object for a HUD.)

These features provide for all of the authoring techniques I have seen and work with both HDR and non-HDR.  Most legacy airplanes can be updated simply by adding GLOBAL_cockpit_lit to the cockpit object and confirming that check-boxes are correct in Plane-Maker.

Logging Problems
With this beta I modified the non-fatal error reporter to handle airplanes.  What is the non-fatal error reporter?  You know it as:

Error loading the scenery package:
/Custom Scenery/my_awesome_scenery/
The scenery may not look correct.
Please see the log.txt file for detailed error information.

The idea of this message is that it puts up a single user-readable warning that something has gone wrong, while providing details on every error in the log.  Authors can see all of their problems in one load of the sim, and the single dialog box is annoying enough to users that authors can’t ignore the problems.

A few things have changed:

  • Airplanes can now participate in these dialog boxes, so we can give you one message that there are several problems with your airplane.
  • There is now a “quiet” version of this that logs without the annoying dialog box.  This lets us put warnings in that aren’t annoying yet but may become annoying in the future.
  • The log output goes through the dev console, part of an effort to unify our logging efforts.  You can reopen the log file without quitting or browse the dev console on screen.

I was asked a few weeks ago whether warnings in the log hurt performance, and my unsatisfying answer was “it depends on the warning.”  But I can tell you this: any problem in your add-on that causes one of these “non-fatal error” dialog boxes should be fixed!  We only use it when (1) there is a definite error in your file and thus it is not parsing properly or (2) you are using a very old technique in X-Plane for which a better replacement has existed for a while and the old technique is going away.

Don’t Overuse Translucency

One last note from the * above; this came up when Tom was working on the Baron, but I see it in third party airplanes too.  While you can use translucency with the panel, I do not recommend it for building non-translucent elements.

Example: if you have an annuciator panel with warning lights that can flash then…

  • I do recommend that you build the panel in Plane-Maker using panel texture and multiple instruments layered on top of each other.  With GLOBAL_cockpit_lit (or regions) you will get correct 3-d and HDR lighting on this panel, and it will color match the rest of your cockpit object perfect.y
  • I do not recommend building the annunciators out of clear areas in the panel and layering two polygons in the 3-d object (a back polygon for the back-plate and the panel texture for the front).

Why not overlap 3-d polygons?  First, the OBJ format doesn’t provide a good way to overlap near geometry without risking Z errors.  Second, and more importantly, you will not get correct lighting by using alpha and multiple layers.  The annunciator panel should be affected by 3-d light including the flashlight and shadows from the sun; that won’t work unless you produce a single “baked” annunciator panel in your panel texture.  Finally, any time you use alpha in HDR mode, there’s fine print and issues with the lighting, so use it when you really need alpha, not when you can get the job done with a panel texture!

Posted in Aircraft & Modeling, News by | 15 Comments

Light Groups for Fun and Profit

Conventional 3-d games like first-person-shooters and racing games are just full of cheats in the rendering engine.  And that is not a bad thing!  By cheat I mean they find clever ways to optimize performance that make the game look good while doing less work.  The result: better framerate, better graphics.

I look at these games with a bit of envy because those cheats are often inapplicable to general purpose flight simulators for two reasons:

  • The unrestricted viewpoint of a flight simulator is incompatible with a number of optimizations that on-the-ground or movement-constricted games can apply.
  • Since a lot of our content comes from a third party, we can’t apply restrictions and optimizations across the entire set of content for the sim.

Despite these limitations, rendering-engine specific optimizations are beginning to creep into X-Plane as the engine becomes more powerful.  These options and optimizations are unsurprising to anyone used to making 3-d game content but new to X-Plane itself.

What Is a Light Group?

Light groups are a cheatoptimization available in many 3-d rendering engines.  The basic idea of a light group is that the author of 3-d content can create 3-d lights, meshes, and then specify which lights apply to which meshes.

There are two big advantages to light groups:

  1. Lights are expensive in traditional (non-deferred) rendering engines; by restricting which lights affect which objects, an artist can reduce the average number of lights applied to their 3-d meshes, which is good for framerate.  (Using light groups to remove lights from objects that are out of the light’s range can be done automatically by a pre-processing tool.)
  2. Lights in games often do not cast shadows, either because the engine can’t support this (this is the case for X-Plane now) or just because shadows are expensive to generate.  With a light group, artists can get correct results without shadows by simply marking geometry that is shadowed by the light as not in the light group.

As an example of the second case, imagine that you have two rooms separated by an internal wall and a powerful light source in one room.  If the light source casts shadows, the unlit room will be dark because the entire room is shadowed by the internal wall.  But if the lights do not shadow, the light from the lit room will “leak out” into the second room.

With light groups, an artist simply marks the second room as not being part of the light’s group, and the rendering engine doesn’t even consider the light.  The dark room renders faster and has no incorrect light leakage.

Light Groups in X-Plane

X-Plane implicitly has two light groups: the exterior of the user’s aircraft, all AI aircraft and the entire world, and the interior of the user’s aircraft.  In X-Plane 9, landing lights don’t light up the cabin interior, and the 3-d lights inside the cockpit don’t light up the runway or the landing gear.

In X-Plane 10.05r1 HDR mode was not supporting these light groups correctly; one obvious example was the Piaggio tail landing light spilling light on the cabin seats.  This is now fixed in X-Plane 10.10.  Just like X-Plane 9, your choice of “interior” or “exterior” for Plane-Maker objects matters for lighting!

Light Groups and Spill

What if you include spill manually by attaching a named light that casts spill to an object?  What light group should the spill cast light on?

My idea to resolve this is:

  • Spill lights attached to “exterior” Plane-Maker objects will be part of the exterior light group and only light up the aircraft exterior and surrounding scenery.
  • Spill lights attached to “interior” Plane-Maker objects will be part of the interior light group and only light up the aircraft interior.
  • Scenery-attached spill lights will light up everything.  (Perhaps this should be “exterior” instead?  I am not sure!)

For a non-enclosed aircraft (E.g. an old biplane with no cabin) you would not use the interior light group at all – everything would be exterior and all lights would affect everything.

If an aircraft has two light groups (interior and exterior) and a light affects both, you can simply include the spill light in two objects; the spills are cheap.  This also means you could make effects.  For example, if the spill light does actually light up the interior a little bit, rather than having the actual landing light blast the cabin with directional light, you could include an exterior spill for the landing lights themselves and a second interior spill that is omnidirectional, dimmer, and tinted, to represent the reflected light that makes it into the cabin.

Posted in Aircraft, Development by | Comments Off on Light Groups for Fun and Profit

Named Spill is Not Automagic

I’m way behind on documentation, and there isn’t any documentation for this yet, but to clarify:

In the named lights list, spill (casting light on the ground and other stuff in HDR mode) and billboards (a square that faces the camera with a picture of the light from a light bulb) are always separate!

Typically the spill light has the same name as the billboard with an _sp suffix, or the billboard has _bb.

Why did we do it this way?

The lights are kept separate because:

  1. We do not have enough information from the billboards to know how to make a spill.  For example, you have a v9 parameterized landing light billboard on your aircraft OBJ.  How big (in meters) should the spill be?  We don’t know!  The billboard params never included enough information to know things like the size and cone ark for a spill light.
  2. There may not even be a 1:1 correspondence of spill to billboards.  For example, any time there is a multi-lightbulb enclosure, it can be a win to use more billboards than spills; overlapping spills hurt fill rate.

Spills are therefore “opt-in” at the named light level; you opt in by adding a _sp variant.

Note that if you use Plane-Maker’s light level, you get spill for free; that interface is a higher level, simpler “I want this thing” type of interface.  It only knows about the basic airplane light types, but it is fairly powerful. For example, you can create multiple landing lights (controlled by multiple switches), and you can use the “generic” lights for utility purposes, like inlet ice lights, leading edge lights, logo lights, etc.

Posted in Aircraft by | 6 Comments

New OBJ Commands

I have updated the OBJ8 specification with the new X-Plane 10.10 commands.  This blog post explains why we added some of these commands.

Pixel-Space Drag Manipulator

The pixel-space drag manipulator is a new axis manipulator whose drag length is measured in screen pixels instead of meters; its drag axes are always screen aligned, but it works both horizontally and vertically.

The goal of this manipulator is to make draggable knobs that:

  • Have a good response speed for both slow and fast drags via a non-linear curve and
  • Have the same drag sensitivity no matter what camera angle.

The axis manipulator has the problem that it works in 3-d; depending on how you look at the manipulation target (both position and rotation) the sensitivity of the drag can change radically.

Recommendation: use the regular drag-axis manipulator for levers and other physically moving items.  Use the pixel-space drag manipulator for drags where the underlying target does not move, like knobs.  Use button-type clicks for anything that just toggles – it’s easier on the user.

No Shadows

Three new attributes (GLOBAL_no_shadow, ATTR_no_shadow, and ATTR_shadow) allow you to exclude part or all of your object from shadow casting.  Shadows can make certain types of geometry, like grass billboards, look silly; excluding them from shadows fixes artifacts.

Note that ATTR_no_shadow is different from ATTR_shadow_blend..

  • ATTR_no_shadow: the geometry simply does not cast a shadow!  This is meant to exclude objects like vegetation.
  • ATTR_shadow_blend: the geometry does cast a shadow, but only if the alpha is greater than a certain ratio.  This is meant to make windows with tint cast shadows correctly.

Recommendation: fix shadowing bugs using ATTR_no_shadow for non-instanced objects, and GLOBAL_no_shadow for instanced objects.  Use the Plane-Maker check-box for parts on aircraft.


This attribute lets you have your cake and eat it.  In X-Plane 9, ATTR_cockpit gives you alpha blending, but ATTR_cockpit_region gives you correct 3-d lighting.  You have to pick one or the other.

In X-Plane 10.10, GLOBAL_cockpit_lit gives you both.  It makes ATTR_cockpit use 3-d lighting (while maintaining translucency) and it makes ATTR_cockpit_region respect alpha translucency (while maintaining cockpit lighting).

You can add this attribute to any airplane.  This attribute should make it easier for authors to adopt correct 3-d lighting in their airplanes.

Recommendation: use GLOBAL_cockpit_lit on any airplane that uses ATTR_cockpit to change to 3-d lighting for your panel texture.  Also see here for more guidance.

Posted in Cockpits, Development, File Formats, Modeling by | 11 Comments

Viewing the Inside of Plane-Maker Geometry is Deprecated

If you haven’t run beta 3 and already been told so, X-Plane 10.10 beta 4 is out.  Hopefully we have a beta stable enough for it to last more than 48 hours.

The ability to draw the inside of your Plane-Maker fuselage is going away.  In X-Plane 10.10 beta 4 you will still see this geometry if your airplane uses it, but:

  • There is no option to enable this in Plane-Maker anymore and
  • If you save your airplane in Plane-Maker, the feature is turned off.

I’ve blogged about the path to removing a feature before; the basic idea is that we are not forcing you to update your airplane now, but the next time you go into Plane-Maker to do work on your plane, you will have to fix your use of interior geometry too.

If you really want your interior to look exactly like your exterior, but inside out, it’s not hard to get this effect:

  • Export your airplane as OBJs from Plane-Maker.
  • Open the fuselage in a 3-d editor and flip the faces.
  • Attach your new “interior” in Plane-Maker.

Here’s the overall LR viewpoint on Plane-Maker and drawing:

  • OBJs are the way to draw an airplane.  OBJ provides all of X-Plane’s rendering features, really good performance, a stable file format, and good tools to edit.
  • Visualization of the built-in Plane-Maker geometry is good for simple planes and experimentation, but it is not at all meant for serious graphic work.
  • We will not be adding any of the new rendering features to Plane-Maker’s “built-in” drawing.
  • We don’t think “I can’t draw X without an OBJ” is a problem; use an OBJ.

In other words, we’d rather focus our effort on a single drawing path, OBJ, and not invent a parallel, inferior second drawing system using Plane-Maker beyond what’s needed to simply say “there is my airplane.”   Thus we are not adding new drawing features to the Plane-Maker geometry.

Posted in Aircraft by | 11 Comments

Consistency and HDR

At this point the only thing holding up a public beta of 10.10 is, well, me.  I am currently working on a set of related aircraft rendering bugs, and as soon as I can get the car off of jacks, we can go beta.

One of my goals or 10.10 is to close out the issues that stop payware authors from releasing final conversions of their aircraft to 10.10.  By final, I mean conversions that won’t need any additional future editing.  Right now X-Plane 10.05r1 has a few bugs that cause v10 to look different from v9, different depending on rendering settings, or just wrong.  I want to fix those problems in a permanent manner so that authors can release aircraft and not worry about having to update.

Here are my goals for 10.10, roughly in priority order:

  1. Rendering should be consistent from X-Plane 10.10 onward through the rest of the v10 version run.
  2. Rendering should be consistent between HDR and non-HDR mode.  Authors should not have to create two versions of textures where the HDR and non-HDRs offer the same capabilities.  (In other words, while you might have to make two textures to bake lights when in non-HDR mode, you shouldn’t have to make two textures to correct for differences in color-correction between the two renderers.)
  3. Where possible, non-HDR mode should match X-Plane 9.

The switch of priority between item 2 and 3 is a fundamental change in priority from what I originally intended for X-Plane 10.

Originally I thought that the best thing to do would be to keep non-HDR rendering as close to v9 as possible, so that at least content would look correct with HDR off.

My new thinking for 10.10 is that we should aim for consistency between HDR and non-HDR modes.  Realistically, an author is going to have to make at least a few adjustments in moving v9 content to v10; better to have the cost of rendering engine changes be borne out in a v9->v10 upgrade than to have every v10 airplane require double authoring to cope with HDR vs. non-HDR differences.  In the long term, it’s best to not have v9 haunt v10 like a ghost years after authors are making exclusive v10 content.

Why is HDR Mode So Weird

This begs the question: why is HDR rendering so weird?  Why does it look different from non-HDR rendering, and why doesn’t it look the same as v9?  What did you guys do?

There are a number of changes to how we render in X-Plane 10, some specific to HDR rendering, and some sim-wide.

  • The entire sim now works with a linear lighting equation.  Basically when the sim performs lighting  calculations on the GPU, it thinks in terms of photons and not colors, which produces more realistic results in most cases.  With lots of light sources, linear lighting is absolutely necessary for combining those lights.
  • The order of rendering operations is very different between HDR and non-HDR mode, and the formats that they render into (24-bit RGB vs. floating point, etc.) are very different.  HDR is fundamentally a two-pass format.

X-Plane maintains two separate rendering paths for HDR vs. non-HDR and they are quite different in both what happens at each stage and when the stages occur.

Fixing some of the authoring bugs has required further changing the HDR pipeline to allow for correct rendering.  The up-side of this change in pipeline is that the new one supports better HDR tone mapping and possibly even better fill rate performance.  The down-side is that it’s a lot of complex code to touch and it may take a few betas to work out the bugs.

Posted in Aircraft, Development, Modeling by | 43 Comments

Manipulators and the Scroll Wheel

One complaint we hear a lot from tech support is that the 747 knobs are hard to control in the 3-d cockpit.  Javier and I did some investigation into this; this post describes what we found, what we are changing, and what why I don’t think the scroll wheel probably shouldn’t be used to affect the 3-d cockpit.

Hard To Drag

The fundamental problem is that it’s hard to control the autopilot knobs in the 747 by dragging with the mouse.  Large drags make only a small change in the knob, so it takes forever to dial in an autopilot altitude.  You’d think the solution is simple: change the scale for dragging on the knobs, right?  Well, not quite.

It turns out that the “sensitivity” of the knobs to dragging is a function of the way you turn your head in the cockpit.  Sit in the default 3-d position, turn your head 30 degrees to the right and drag and the knobs turn quickly.  But look straight ahead, slide to your right, and drag and they are very slow.

The problem is one of perspective, and this is where it gets interesting for authors.  The drag axis manipulator (which lets you make the 3-d cockpit respond to dragging the mouse) measures its drag in meters.  But the distance on the screen that the drag distance takes up (in meters) depends on where the camera is placed and at what angle it is turned.  This can lead to some very strange effects: in some views, a 500 pixel drag moves the altimeter only a few hundred feet, while in other views, 500 pixels moves tens of thousands of feet.

Screen Space Dragging

For real physical parts like a lever (a part that moves as you drag it), dragging in meters makes perfect sense; it lets authors match their animations to their manipulations.

But for a drag that doesn’t have a real-world correlation (e.g. you drag on the knob and the knob spins but it doesn’t move) having the camera angle affect the drag distance results in panels that can be used only from certain viewpoints.

To fix this, we are introducing a new “pixel” drag axis – unlike the current drag axis, the distance over which the user can drag is specified in pixels, so that the “sensitivity” to the mouse is the same no matter what view angle the user has.  I will post details on this when we go beta.

The Mouse Wheel

While I was working on pixel drag axis, I looked at using the mouse wheel to turn knobs, something our users asked for.  And while the prototype seemed ‘clever’, after some arguments with Chris I came to a bit of an inescapable conclusion: the mouse wheel for changing parts of the panel is the wrong tool for the job.

The problem with the wheel is that in the rest of the universe it is use to manipulate view information.  This is true in X-Plane 10.05 as well (and it works well), but things get quite tricky once the mouse wheel is added in.  Some of the problems:

  • Making the mouse wheel zoom and manipulate (e.g. if you are over a knob it manipulates the knob, otherwise it zooms) risks surprising results.  A user who wants to zoom might accidentally “bump” a cockpit knob, something that’s pretty frightening to a real pilot.
  • We looked at requiring once of the buttons to be held down while mouse-wheeling, but that’s not a gesture you see anywhere else in the universe – effectively one of the two uses of the wheel is “buried” and we might as well only use the other.  Furthermore, if we are going to require a click, the user might as well just drag on the knob itself.
  • If we have to pick one or the other (zoom or manipulate), zoom is by far the most consistent thing, the thing that fits with the host OS.
  • If we make the option a preference (e.g optional mouse-wheel on knobs) so few users will enable it that authors won’t be consistent in adding support to their cockpits, and the system will never get momentum.  (We can’t just add “mouse wheel automatically” because the sim doesn’t know how much one click of the wheel should change a given dataref.)

We tossed the mouse wheel idea around, but in the end we concluded that the wheel should be a view/zoom/scroll function, not a data change function – we couldn’t find any example apps that used the wheel to change the contents of the screen.  In the end, authors need to make clicking work well, and we need to provide manipulators (like the screen-space drag manipulator) to make that possible.

Posted in Aircraft & Modeling, Development, File Formats by | 78 Comments

Three Plane-Maker Tricks You Should Know

Here are three obscure Plane-Maker/X-Plane features that can save you time if you developer complex aircraft.

Plane-Maker Will Copy Your Instruments

You may know that in Plane-Maker, you make your own copies of X-Plane’s PNG files to customize the look of the instruments.  But did you know that Plane-Maker will copy the images for you?

Select an instrument and type Control-P (which is the default for the command binding “pln/panel/copy_instrument_png”).  Plane-Maker will copy all PNGs for that instrument into your aircraft’s cockpit or cockpit_3d folder.  This can save you the time spent wading through X-Plane’s cockpit folder to find the right PNG files.

X-Plane Can Make a Panel Image for UV-Mapping

When you are making a 3-d cockpit, you use the 3-d panel as a texture.  But how do you know how to UV-map this texture in your cockpit?  Often the panel background (panel.png) is blank.

X-Plane can make a snapshot of your panel for you, in the exact size you need to UV map.  Use Control-Alt-Shift-Space (Command-Option-Shift-Space for Mac nerds) to run the “sim/operation/make_panel_previews” command in X-Plane.  It will make a PNG file in your aircraft called Panel_preview.png – it’s just like Panel.png but with the instruments drawn in – perfect for UV mapping.

Plane-Maker Will Tell You What’s Wrong

That sad face icon in top bar of the Plane-Maker panel editor enables “warning” mode.  In warning mode, every instrument that has a potential problem gets a red outline.  Select one instrument with a red outline and in the upper left corner of the panel you’ll see a description of what’s wrong.

This picture on the left is from Plane-Maker editing a 3-d panel. (That’s why it is just a “packed” set of instruments with no background; this panel is used as a texture for a 3-d cockpit – each instrument is UV-mapped into the right location.)

The air-speed indicator has been flagged as having a problem, and the text shows it.  In this case, the lit texture has an alpha channel, which causes the lit texture to draw incorrectly.  Fix the texture and the warning will go away.

I strongly recommend checking all Plane-Maker “red boxes” on your plane – most of the warnings will tell you about problems that would otherwise be very hard to detect.

Posted in Aircraft, Aircraft & Modeling, Cockpits, Panels, Tools by | 1 Comment

What’s New in OBJects?

I finally finished up an update to the OBJ8 specification, as well as the forest specification – see here for the documents.  These specifications are mostly of interest only to developers who are working on scenery exporters.

The OBJ specification is very thick – here are some of the hilights of what’s new.

Global Lighting

In X-Plane 10, you create global spill lights by attaching a light to an object.  (Thus, spill can come from any object-bearing part of the sim – an airplane, custom scenery, etc.)

One way to do this is with the existing named and parameterized lights – these features existed in version 9, but in version 10 there are some spill lights added to the light list.

What may be of more interest to custom authors is the new LIGHT_SPILL_CUSTOM command.  This lets you build a completely customized spill light. You control its size, color, shape and direction.  You can even optionally run the light through a dataref, giving a plugin custom control of the light.


In X-Plane 10, an object can contain geometry that is “draped” on the ground – that is, X-plane will subdivide, bend and modify part of your object mesh so it sits perfectly on the ground even if the ground is sloped.  ATTR_draped makes this happen.

This feature is a much better alternative to using ATTR_poly_os to make marks on the ground.  Draping produces objects with better performance, and the geometry always sits on the ground with no artifacts.

As an added bonus, you can optionally use a second, different texture for the draped part of your object from your regular object.  (Internally the draped geometry actually becomes something like a .pol file – this is why it can have its own texture.)

Draped geometry makes it much easier to make airport markings.  If you want a parking spot, simply draw it on a quad, make it draped, and drop it into place with WED.  Tom uses this heavily in our airport library.

Draped geometry also makes it easier to have ground markings that match the object they come with.  For example, if you want a house and the house comes with a driveway texture, you can make the driveway texture a draped quad and when you place the object, the draped driveway is always in the right place.

Global Attributes and Instancing

In X-Plane 10, it is possible to set a few key attributes (blending and shininess, among others) globally for the entire OBJ, rather than using an ATTR_ to change part of the object.

These global attributes make hardware instancing possible.  In hardware instancing, X-Plane draws many objects with a single instruction to the CPU.  In order for this to happen, X-Plane must be able to draw the entire object without needing CPU intervention mid-object.  This means an instanced object has to be free of animation, attributes, and a few other features.

The global attributes let you set things like shininess and still have a single-call draw object, ready for instancing.  Alex uses these heavily in the urban autogen, and it really helps performance.

My fear is that global attributes are going to be a source of confusion for authors.  When should you use them?  How do you add them?  This is my thinking:

  • Modeling program exporters should allow an author to identify an object as “for instancing” or not.
  • Authors should check “for instancing” for any object that is heavily repeated.  (A car or a single tree or a static airplane, for example.)
  • The modeling program can then try to prefer global attributes for instancing objects but not for regular ones, which should come very close to optimal behavior.


Conditionals are simple logic statements that include or ignore parts of an art file based on the rendering settings.  In particular, they let you change an OBJ based on whether HDR is on or shadows are on.  For example:

TEXTURE_LIT my_airplane_hdr_LIT.png
TEXTURE_LIT my_airplane_LIT.png

In this example, which LIT texture the OBJ uses will depend on HDR.

Because the conditionals can be used anywhere in the OBJ, you can change any aspect of the OBJ to customize for HDR.  You can replace a texture, remove lights, add more geometry, etc.

I don’t know how heavily people will use conditionals, but they give authors the option to make one file tuned for both HDR and non-HDR, shadows and non-shadows.

I think the two most common uses of conditionals will be:

  • Providing alternative LIT textures when HDR is on or off.  Note that only one texture is ever loaded (when the HDR rendering setting is changed, X-Plane unloads one and reloads the other) so this does not increase VRAM.
  • Removing drop shadows that are baked into a model when shadows are on.

That second case would look like:

# This is the shadow geometry
TRIS 300 6

When global shadowing is turned on, the entire set of draped geometry disappears, removing baked vs. real shaodw conflicts.

Posted in Development, File Formats, Modeling, Scenery by | 31 Comments

Plugins: Check the OpenGL Error (in Debug Mode Only)

A quick OpenGL note for plugin authors: if your plugin uses OpenGL to draw (e.g. for a plugin in an airplane that draws a custom glass display) you should:

  • In debug mode: always check the OpenGL error state (using glGetError) at the end of each draw callback that you handle.  X-Plane will not leave OpenGL errors around, so any error is yours.  (To verify that it’s not another plugin, just add a glGetError at the beginning of the callback.)
  • In release mode: never check the OpenGL error state, as it can slow down the driver.

An OpenGL error often implies that an OpenGL command did not complete as expected (because a param is bad, etc.) and this can mean that you are not cleaning up OpenGL state for X-Plane!

(And of course you should always use XPLMSetErrorCallback in debug mode to catch plugin errors. Set a break point in your callback to see exactly where the error occurred.)

Posted in Aircraft, Development by | 2 Comments