Tuesday, March 31, 2009

The New Ac3d Export Plugin (Beta 1) - You Must Update Your .AC file!

I just posted the new X-Plane AC3D plugin (3.2 beta 1). For the info, please subscribe to the x-plane-scenery yahoo mailing list. I will post links on the scenery website once the plugin has undergone more testing; during early beta I only need a few testers to tell me I broke things.

Please read the README that comes with the download completely!

An important note for anyone using an existing .ac file to make airplanes with panels:

The new plugin gives you direct control over manipulations. But older .ac files don't have the manipulator set on any of the objects. Thus if you export your airplane, your panel texture will work, but the panel will not be clickable.

To fix this, for each object in the hierarchy that has panel texture, select the object, open the X-Plane Object Properties... dialog box, and change the manipulator from "None" to "Panel". (If you don't see this option, make your properties window a bit taller.)

Note that if you don't need your panel to be clickable, setting the manipulator to "none" is slightly faster in X-Plane 930 and a lot faster in X-Plane 922.

Other details: you'll need the Commands.txt and DataRefs.txt file from X-Plane's Resources/plugins folder.

Panel sub-regions are now handled quite a bit differently - please be sure to read the README completely. If you were using the 3.1 plugin with panel regions, you may need to update your .ac file a bit.

Friday, March 27, 2009

A Bit More Open Source

Thanks to Janos ("sothis" on the .org) we now have a GIT repository of the scenery tools, with public browsing of the scenery tools code. 

(Non-programmers - this basically means that source code updates for the scenery tools will now be available every hour, rather than every now and then when I get around to it.  The rest of this post is for programmers.)

The X-Plane tools code have always been open source, in that the LR-created code is distributed under the MIT/X11 license (which basically says "do whatever you want, don't sue us").  The public repository makes the process of getting the code a lot simpler:
  • The master code is actually in CVS, but this public GIT repository is updated from CVS once an hour, so this code is very close to the latest we have.
  • The full version history, tags, and other information that might be useful to a programmer are all present.
  • The web interface supports online browsing of the code, as well as downloading a "snapshot" of the entire tree (as a zip, gz, or bz2 file).
Git is, to put it mildly, a confusing tool if you don't already use it.  However, the web interface allows you to simply fetch the code from a given date.  If you are a git user, git cloning is supported via http, and we are working on getting the git daemon running too.  The repo is read-only; if you want to send us a patch, contact me.

(Git users will note that most of my checkin comments are really lame.  This is a bad habit that comes from using CVS too much.  CVS's checkin comments are per-file, not per-group, which makes them somewhat useless to search on.  Typically CVS users rely heavily on tags.  The bridge from CVS to git tries to group them into a single commit, which helps reveal the actions taken on the source code.)

Thursday, March 26, 2009

Standards Aren't Standards During Beta

In particular, when a new spec is being developed, during beta it may change.  (During a beta of a future version, old specs won't change.)

So...please do not release non-beta aircraft and scenery based on beta builds like 930.

Here's an example: the current spec for attached objects is that the draw order is based first on lighting mode, then on the order listed in Plane-Maker.

It turns out that if we do that, polygon offset can't be used in a number of weird cases.  So the rules will have to change.  I'm not sure what they will change to, but the decision will be finalized when 930 is finalized.

Wednesday, March 25, 2009

Why Can't I Mark My Object As "Extreme Resolution"

I receive a number of requests from authors for an attribute to tag an object as "needs maximum texture resolution" or "needs compression disabled" or "needs maximum anisotropic filtering". The general idea is that the author wants to ensure a viewing environment that looks good.

For the most part, I am against these ideas - think of the two cases:
  1. If the attribute on the content is request for a relative improvement in resolution (e.g. set my object to one texture res higher than the rest of the world) then what we'll have is an arms race - every author will set their content with this flag, and the result will be that the entire sim tends to run at one res setting higher than expected.  The result: users without enough VRAM will turn their res settings down another notch and all the content will look like it did before.
  2. If the attribute on the content is a request for an absolute setting (e.g. load this texture at the highest resolution possible) some content will simply not run on some computers that do run X-Plane.
My general point is this: users run X-Plane with texture resolution, anisotropic filtering, and compression set to lower settings for a reason - because their hardware isn't very fast!  Forcing the sim to ignore the settings and run at a higher res won't make the user's video card any better - it will just take the framerate vs. visual quality tradeoff out of the hands of the user.

That's a simplification of the issue - in fact I am sympathetic to the notion of differential settings - that is, we need to use more texture resolution for art elements that are closer to the viewer.  The sim already improves airplane resolution a bit and cockpit resolution a lot. We set anisotropic filtering a bit higher on runways because they are viewed from a shallow view angle pretty much all the time during normal flight.

At this point I am looking at some more specific overrides for cockpit objects.  In particular, modern cockpits are built out of many attached objects, and not just the "cockpit object" itself - reducing the resolution of these objects can make cockpit labels illegible.

If we do get extensions to improve resolution I can only say this: use them very, very sparingly! Adding the extension doesn't improve the user's hardware. If the user had the ability to run your airplane at extreme res without compression and 16x anisotropic filtering, he'd already be doing that!

Friday, March 20, 2009

The Mathematics of Field of View and Vanishing Points

In order to understand the vanishing point in Plane-Maker, we first have to look at field of view and the process by which X-Plane simulates a 3-d world on a 2-d monitor.

Field of View

Field of View is the angle that you get if you go from the left edge of your vision to your eye, then back up the right edge.  In the case of a monitor, we can calculate this (depending on how far back I am sitting).  For example, my 19" LCD is 14.8 inches across the top; to have a 45 degree FOV I need to sit about 17.8 inches away from the monitor.

X-Plane lets you set the field of view.  Imagine that you were sitting in front of a window on an airplane.  As you put your face closer to the window, you can see more of the world outside. Effectively you are increasing your field of view.  X-Plane works the same way - turning up the field of view parameter will increase the amount of "stuff" you can see.

Where Is The Horizon?

So where is the horizon?  The answer is: it depends.  Assuming you are looking straight forward, the most logical place to put the horizon is exactly half-way up the monitor.  And this is what X-Plane does in any external view.

As you rotate your head up and down, the center of your vision changes relative to the horizon. But if you simply move your head up and down, the horizon doesn't move.  This is due to parallax.  The closer an object is, the more it moves as you move your head.  This is what lets me look "over" the dashboard of the car by sitting on a phone book: as my head goes higher, the dash board (close) appears a lot lower but the road (far) appears only a little bit lower.  The horizon (very, very far away) doesn't move at all.

This effect works in X-Plane.  Try moving the view point up and down in a plane with a full 3-d cockpit, like the Cessna 172.  As you move your head up and down, your ability to see the runway out the window will change.

2-D Panels

Things get weirder when we have a 2-d panel.  A 2-d panel is sort of a flat image of what a 3-d cockpit might look like.  We need some kind of correlation between the 2-d world and 3-d world...that is, where does the horizon appear through this 2-d panel.  That location is the "vanishing point" in Plane-Maker.

Here's where things get strange: what do we do when we scroll the panel?  Do we move our head or tilt our head?  The answer is: neither.  Scrolling the 2-d panel simply scrolls the "window" within the 3-d world that we look through.  This has the effect of moving the horizon (by the exact number of pixels the panel scrolled) without rotating your view point.

This isn't necessarily the best way to scroll the panel, but it looks pretty good, and anything we do with 2-d panels is going to be an approximation.

And Now The Bug

Of course, there must be a bug in here somewhere...these blog posts are usually the result of an investigation into an edge-case in the sim.  In X-Plane 930b6, we pick a vanishing point based on the 2-d panel when we are in 3-d cockpit mode.

Why would we do such a silly thing?  Originally it was to keep the horizon from jumping when there is no 3-d cockpit object.  This behavior is okay in that case, but here's how we get burned: if the 2-d cockpit has to scroll, the vanishing point might be off the top of the screen.  Authors who have made very large 2-d panels and separate 3-d cockpits see this as the 3-d viewpoint being stuck straight down.  What's happening is the vanishing point (and thus the center for the mouse) are off the top of the screen.

For beta 7 I am fixing this:
  • If there is a 3-d cockpit object, the vanishing point will be the center of the screen, which is almost certainly the right thing to do for a real fully 3-d view.
  • If there is no 3-d cockpit object (but instead X-Plane's default of the 2-d panel floating in space) the vanishing point will match the 2-d view, but taking the default scroll position into account. This should keep the horizon at a reasonably sane point.
As a final note, it is possible to specify a 3-d panel without a 3-d cockpit object in X-Plane. Don't make a plane like this.  It's a silly thing to do!

Thursday, March 19, 2009

Beta 6: Cockpit Chaos

The code that decides what parts of the plane get drawn in what views is, to put it midly, byzantine.  The code evolved, and in this process became more complex and convoluted.

One of the side effects was a series of bugs that couldn't easily be fixed - typically it was a case where some new piece of code had to do some drawing, but the decisions on how to draw were being made by legacy code that had no business making those decisions.

So for beta 6 I rewrote that code.

Here's what this means: if your airplane starts showing the wrong "stuff" in various views in beta 6, please send me the plane, and a description of what it does in 922 and 930b6.

The risk is that I may have missed some of the quirkier behaviors that 922 was capable of.  My goal is to have 922's behavior, but with clean code.  So if things changed, please let me know!

Object Materials - The Crayon Rule Lives On Forever

Let me just clarify one thing about yesterday's post on materials: the new material system will let authors edit material parameters like shininess; it will not let the user change textures.

Objects will continue to require texture atlasing (that is, the use of one texture set with the individual images combined). Inside the company, we call this the "crayon rule" - Austin's original analogy to explain atlasing to our art team was that you draw slower if you spend all your time putting a crayon back in the box and taking a new one out.

So materials will let you change some macro-parameters of how the object is drawn, but not swap out the texture.

Wednesday, March 18, 2009

Object Materials - Some Rough Thoughts

First, we're not going to get an OBJ materials model in 930.  930 is crammed full of stuff, and I don't want to add anything more!  I am not sure when we'll get OBJ materials, or even if this is how it will go down.  But this is what I am thinking:
  • Materials will be controlled on the "batch" level, that is, by using ATTRibutes.  You'll be able to change materials mid-object.
  • Materials will be grouped into "material classes" - the material class will be mutually exclusive.  So if we have a metal material class and a "plastic" material class, you won't be able to combine them.
  • There will be some kind of material class that corresponds to the current behavior, and all of the existing attributes will keep working; probably there will be a newer, cleaner way to state in an OBJ what you can already do now.
  • You will set all material class parameters at once.  This simplifies the syntax and assures that we don't have any strange mix & match combinations.
To this last point, consider that OBJ currently has a number of strange cases where you can set attributes that don't have any effect.  For example, you can set the shininess while drawing the cockpit texture (which is unlit).  Code like this has historically been buggy, partly because it's not necessarily obvious to a human reader what the object should do.

I do not yet know how normal and gloss maps fit in.  Normal maps may be useful to multiple material classes; gloss maps are somewhat material class specific (in that if a material class has no concept of gloss, the gloss map is moot).  It is definitely advantageous to pack a normal map and gloss map into a single texture (a la blender).

The main point of material classes is to create the infrastructure for complex extensions to the material system in a clean way.  Adding additional attributes creates an NxN problem - for each new attribute I add, I have to consider how it interacts with every other attribute.  By comparison, a new material class by definition shuts all other material classes off.

Wednesday, March 04, 2009

Back-Lighting For All Instruments

X-Plane 930 beta 4 will be out real soon, and it has a somewhat significant new instrument feature: all instruments (not just generic instruments) can have additive lighting.

But wait - what is additive lighting?  Additive lighting is the basic equation X-Plane uses for "lit" stuff.  Basically it says that the _LIT texture is added to the daytime texture to simulate the effect of an object that reflects sun light and emits its own light.  Here's a more detailed description.

It used to be that the _LIT textures for instrument overlays were used instead of the daytime ones at night.  Generic instruments introduced a new lighting mode, "back-lit", where the LIT texture is added to the daytime texture.  This lets you make an instrument that has a light behind it to illuminate the markings at night.

In particular, when an instrument is back-lit, the amount of LIT texture added in is a function of the instrument light levels (the pilot can turn this up and down) while the amount of day time texture is a function of the sun and spot light shining on the area.

Originally I did not want to extend the legacy instruments to support back-lit lighting.  What finally made me change my mind was the amount of detail in some of the standard mechanical instruments.  As an exercise, I converted the six pack and nav instruments of Max's default Cessna to generic instruments.  

The conversion required a lot of new (and very weird) datarefs, some esoteric extensions to the generic needle instrument, and it lacked some of the finesse of Austin's built-in instruments. All of these problems stem from one limitation: the animation action of generic instruments cannot be nested.

(Of course there is a practical consideration too - for an author with legacy instruments, rebuilding with generics takes time. Typically it took about 4-6 generics to model each built-in instrument in the six pack. The conversion only took me about eight hours, but I have access to the source code of the built-in instruments, a luxury authors would not be able to leverage.)

So in the long term, I am at least investigating the notion of nested animations and movements for generic instruments; I think that this would be the final flexibility needed to model just about anything with generic instruments.

But for the short term, you can back-light your built-in instruments; just set the lighting mode to "additive" and create the LIT textures.