If you have a dual core Intel Mac, OS X 10.4.9, X-Plane 8.60 and an Intel Graphics chip, try running the sim with this command-line option:
–use_threaded_ogl
You can read about how to use X-Plane with command-line options here.
This series of posts will discuss how the LOD system works for objects. Understanding LOD is very important for tuning the performance of custom scenery that uses either very complex objects, or a very large number of objects.
Authors View vs. X-Plane’s View
A “level of detail” for an object is a complete representation of the model with complexity changed to fit a viewing distance. In other words, LODs are copies of your objects, some of which have been simplified so they will draw faster when far away.
Each LOD for an object contains a viewing range, indicating for which range it is appropriate to use this LOD.
An X-Plane object can have LOD information, via the ATTR_LOD attribute. If it does, it may have one or more “LODs” – that is, separate representations of the same object.
If an X-Plane object has no ATTR_LOD command, X-Plane turns the entire object into one LOD by automatically calculating the viewing range based on the size of the object. (I’ll discuss this in more detail in later posts.)
This last point is important because it means that, to X-Plane, every object has at least one LOD. If an LOD is not provided by the author, one is created!
Picking an LOD
When X-Plane draws an object, it measures roughly how far away it is and picks the LOD within the object to draw. This can save some frame-rate but isn’t a cure-all.
– The cost of animation and attributes is based on the LOD drawn. So if you have an object with a lot of animation that is not visible a long way away, using a second, simpler LOD with no animation will help framerate because X-Plane won’t evaluate the animation when the complex LOD is not drawn.
– The cost of moving the texture to the graphics card is incurred no matter what LOD is picked, so you don’t save on “working set” (that is, the effective amount of VRAM X-Plane needs) with LOD. If your object has a huge texture, you pay that cost any time it is drawn.
– The cost of the number of triangles in the model is partly saved by using a simpler LOD, but it is partly global to the entire object. The problem is that all of the geometry coordinate data for objects is kept together in one giant glob. So when the tiny simple version of your model is being drawn, X-Plane may be shoveling the complex version to the card anyway.
This last point is important: making more LODs may actually increase the total amount of geometry data and make performance worse. Therefore while it may help a lot to have a second LOD that is much simpler, having 10 LODs, each slightly simpler than the last will probably hurt performance. Be sure to try your objects on a computer that doesn’t have a lot of free VRAM to see the effect of more LOD!
Don’t Draw That Object
While you can save some frame-rate by having X-Plane draw a simpler LOD, the true power of the LOD system happens when an object is so far away that none of the LODs apply. For example, if my object has 3 LODs from range 0-1000 meters, 1000-3000 meters, and 3000-6000 meters, then once the object is over 6 km away, X-Plane simply doesn’t draw it at all!
Not drawing an object is great for framerate. Not only do we save the time on the animations and attributes, but X-Plane never has to process the texture or geometry data, and that’s where the real framerate win comes from. Therefore the most important question for object LODs is “what is the maximum LOD” – that is, what is the farthest distance before X-Plane can stop drawing your object. X-Plane is pretty efficient at recognizing cases where objects, or even whole classes of objects, are simply too far away to draw.
(Consider the runway lights with the new “high detail world” setting…X-Plane places an object for every single runway/taxiway light, of which there are almost 10,000 at Chicago O’hare. But since the maximum LOD for these tiny objects is only 500m, X-Plane then eliminates the vast majority of the objects very early on and doesn’t bother to draw them, keeping framerate up.)
To draw a stupid analogy: the shorter the grocery list, the quicker your shopping. But if your grocery list has NO items and is totally EMPTY, you don’t even have to drive to the store, which really saves you time.
In the next part I will discuss world level of detail and maximum LOD didstances.
Here’s one of those little details that probably isn’t in the scenery docs but needs to be: if you are making base DSF meshes and they use water (the special terrain type “water”) it must be on the bottom of the stack of terrains – it can’t be used as an overlay!
I just finished an example scenery package that shows how to place orthophotos using .polygon files. Check out the scenery website (look under tutorials) to download the package and also read the illustrations. (This is an experiment: this is more of an annotated scenery example tha a true step-by-step tutorial.)
The basic anatomy of an orthophoto-in-a-polygon is:
This pattern is actually the same design pattern that is used everywhere in the X-Plane scenery system:
One decision I made early on in my work on the scenery system was to stop using “magic file names”. Prior to X-Plane 8, in a number of cases it was enough to add _LIT to the end of a texture file and a lighting mask would appear. In all modern file scenery format, there is a texture command (usually TEXTURE_LIT) to add this information.
Now this is probably more confusing to new users, and certainly a little bit confusing to anyone who was used to X-Plane 7. To some extent my goal is to have most scenery files be generated automatically – we’re still a long way away from that, so my decisions to prefer extensibility to ease-of-use in the low level formats cause problemes for now. (But try the new AC3D X-Plane plugin version 3.0 – with UI to edit all X-Plane properties, does it matter anymore whether OBJ8 files are easy to read? We now have a real UI for editing X-Plane specific properties.)
In the case of file formats my concern was extensibility. There are only so many “tricks” that we can possibly cram into a file name, and the more we try to make the file name do, the less simple it becomes. When textures can only be “lit”, tagging _LIT on the end is simple, and haivng to add TEXTURE_LIT can be annoying. But since then we’ve also added layering information and physics information. Imagine ksbd_LIT_AIRPORTS_-1_GRASS.png. Now we’re getting into the domain of confusing file names.
Take a look here at some of the things we’d like to someday do for textures…seasonal textures would really make the texture names crazy. (MSFS9 accomplishes this using four seasons, which keeps the names simple…but what if we want to customize the time range for seasons?)
And dataref-controlled textures? Well, you can’t encode a dataref in a file name – the / is reserved!
So all of these text files (.OBJ, .pol, .for, .ter, etc.) provide extensibility – they let us add new features to the way that DSF content is viewed without requiring cahnges to the DSF file format itself.
Since this blog post has gone off into a philosophical rant and the real info is in the tutorial, let me drift totally off subject by mentioning that while you’re looking at tutorials, take a look at Kriss’s tutorial on OBJ8 animation using the new AC3D plugin. If you’ve tried using the old system of animation (by typing cryptic goo into the AC3D object naes) you’ll find the new plugin will let you work a lot faster!
Before I do this, an important disclaimer: we want you to use the .pol files that are found in the default scenery using the library system, not to copy the files! I encourage you to look inside the X-Plane resources folder to see how the system works, but don’t copy a file if you can use it via the library; the library system saves loading time and VRAM.*
With that in mind, here’s a line-by-line of what we might find in a .pol file. This one came from my X-System folder in Resources/default scenery/820 world terrain/pol/apt_desert_hot.pol. We have:
A
850
DRAPED_POLYGON
This is a standard header for a .pol file, which defines the look of a draped polygon.
TEXTURE ../textures/apt/apt_desert.png
This defines what texture the polygon will be drawn with. In this case, we are using our apt_desert.png file, found elsewhere in our package. (The double periods, .. move us up to a higher level directory. This is legal within a single scenery package.)
SCALE 500 500
This defines how large the texture appears in the X-Plane world. In this case, the horizontal span of the entire texture (before it repeats) is 500 meters (the first number), and the vertical span is also 500 meters.
NO_ALPHA
This tells X-Plane to ignore the alpha channel on the PNG file. Often PNG files used for base textures have alpha channels consisting of noise, for the purpose of creating terrain transitions. Since seeing this alpha isn’t desirable in an overlay polygon, this command can be used to strip out the alpha. This lets us use one PNG file for two purposes.
SURFACE dirt
This defines the physical surface properties of the polygon. In this case, it is “dirt”, which for X-Plane means some dust puffs will come off the wheels and your plane will clonk around. See the OBJ8 specification for a list of all possible surface types. (These names match the ATTR_hard command in X-Plane 8 objects.)
LAYER_GROUP airports -1
This defines where in the drawing stack to draw. In this case, we will draw one layer under airports (-1 means under, +1 would mean over). This means that this surface will be seen right below all aspects of the airport, but above regular terrain, which is what we want. The names here correspond to the same names you’d find in an OBJ8 in ATTR_layer_group – see the OBJ8 spec for the complete list of names.
* Basically any time there are two PNG files on disk, the VRAM is allocated twice, but any time one PNG file is referenced multiple times, the VRAM is shared. Since the library system lets you share other packages’ resources, using it almost always means a savings in VRAM.
In my previous post I discussed airports using the apt.dat and polygons and when to use custom artwork in a DSF.
One problem with the X-Plane scenery system (and a topic for another blog) is that, because scenery is fully pre-processed*, introducing a new airport won’t put grass underneath the airport surface area. (Only airports that existed at the time the global scenery was created have grass underneath them.)
To make an airport there are two types of processing you need to do to the underlying area:
With X-Plane 8.60 (in RC as of this writing) you can do both! I’ve blogged about airport flattening (and why it isn’t very good) before; today I’ll try to describe how you can fix the base terrain.
You can cover the base mesh with a draped polygon in a DSF overlay. All you need to do is:
In X-Plane 8.60 we provide .pol files for you for all of the basic airport terrain textures. If you open the file Resources/default scenery/820 world terrain/library.txt you will find the library listing for the global scenery terrain. Toward the bottom you will see a section like this:
EXPORT lib/g8/pol/apt_sdry_cold.pol pol/apt_sdry_cold.pol
What this means is: the library path lib/g8/pol/apt_sdry_cold.pol is apped to the file pol/apt_sdry_cold.pol in this scenery package. You will see a fairly large number of .pol files on the left. Basically these represent the basic “environments” that an airport can exist in.
So to make an overlay DSF that has wet cold grass underneath it, make an overlay DSF with a .pol file using the path “lib/g8/pol/apt_wet_cold.pol”.
EDIT: Important — there are actually four .pol files in this library that do not work!! They are:
apt_desert.pol
apt_dry.pol
apt_grass.pol
apt_sdesert.pol
You will know they don’t work because your scenery will not load with them. Basically, they are accidental hold-overs from a previous naming scheme for airport terrain, but were never deleted. Note how they name only an amount of rainfall (except for apt_grass, which tells us nothing). The rest of the files are named things like apt_sdry_cool, which gives us both temperature and rainfall. This 2-dimensional climate grid (with rainfall and temperature) is how most of the X-Plane terrains are organized.
Why use the library system to do this? Three reasons: copywrite and VRAM, and compatibility.
One advantage of using our library is that it gives you a way to use our artwork without copying our artwork. Remember that you cannot copy the X-Plane PNG files into a custom package and then sell it – it’s a violation of X-Plane’s EULA. But you don’t need to! You can simply use the .pol file and X-Plane will load up the appropriate PNG files.
This is also a win for VRAM. If the wet grass texture is being used in the base terrain (for an airport that has wet grass underneath it from the global scenery) and in your overlay DSF (via the library .pol file) X-Plane will only load one copy of the texture, saving VRAM, speeding up load time, etc. If you copied the PNG file, besides being a EULA violation under some circumstances, you’d also be wasting VRAM. So the library system is more efficient!
Finally, by using our polygons, you ensure that your base grass will match the rest of the scenery system. If we update the look of our grass, because the library .pol files reference our artwork, you will see the same thing in your airport that we have in the default ones.
(Of course my warning from last week applies: if you use a wet airport grass polygon for something other than wet airport grass, then in the future our improvements on the artwork may make your scenery look strange. With these airport polygons we are aiming for “as much like an airport as possible”, not “as much like 850 used to look as possible”!!)
* By this I mean, all the decisions about how scenery will look are made ahead of time, so the look of scenery does not respond to the combining of multiple packages by changing the underlying packages – only by superimposing the new ones on top. Thus a new apt.dat layout does not change the underlying terrain from urban and forest to grass. This is a design trade-off; full pre-processing has both good and bad aspects.
With the new version of DSF2Text and X-Plane 8.60 it is possible to overlay orthophotos on top of DSFs. I’ll get into this in more detail in a future blog post…the purpose of this post is to be a wet blanket.
Using polygons to overlay orthophotos on the base mesh might seem perfect. It’s easy to do and you can clip polygons to any shape. Perfect, right?
WRONG!!!!
There are a few major problems with overlay polygons that make them well suited to some cases but not a general purpose cure-all for orthophoto-based scenery. Here’s the fine print:
Basically overlay polygons are great for texturing small areas like airports or special landmarks within a city, but are not appropriate for making wide-scale orthophoto base meshes. For that you really need to replace the base mesh DSF!
Sergio and I were discussing the future of facades today. Facades are DSF polygons that are extruded into buildings by pushing up the polygon into a roof and texturing the roof and walls from a single texture using some simple formulas. The rules for facades are very simple – we originally thought them up for the purpose of creating buildings that precisely fit a city grid no matter what the block spacing.
The problem with facades is that the rules make very simple buildings – especially the roofs. So we want more powerful facades and the question becomes: how best to do this? There are two possibilities and I mention them here because they underscore what I think is the largest overarching design decision when creating a flight simulator scenery system.
1. We could make the facade builder in X-Plane smarter (write new rules).
2. We could write a stand-alone tool that converts the DSF polygons (plus a rule file) into OBJs that are then placed directly into the overlay.
This second strategy would turn the facade from a type of scenery element into a tool for making OBJs. We would be “pre-compiling” our facades.
Here are some of the considerations:
SPACE VS SPEED
Precompiling primitives is a space-vs-speed trade-off. For any given algorithm, it’s usually faster to pre-build the entities and load them from disk than to build them in the sim. But pre-building means more files on disk, meaning bigger downloads, more DVDs, etc. Building up buildings from facades inside X-Plane is actually a form of compression.
EASE OF DEVELOPMENT
This is one criteria where I think pre-compiling scenery is a clear win: it’s easier to build a stand-alone facade-to-OBJ converter than to implement it inside X-Plane. X-Plane is a relatively unfriendly place to build scenery algorithms because it’s busy being a flight sim.
STABILITY VS UPGRADABILITY
If we make OBJs out of facades using a tool, the objects will look the same even if we create a new version of the tool. This is good in that it means that custom scenery won’t change how it looks, but it is also a limitation because improvements to facade technology require rebuilding a lot of objects to take effect.
(Another aspect of this: if the objects made from the tool use a texture, that texture can’t have its shape changed even if we make new rules, because some objects will be on users machines that were built with the old rules.)
X-Plane mostly errs on the side of pre-building scenery, and this is the one area where we really take it on the chin for that decision: the inability to easily create a new composite look from changes to some parts of the scenery. (Airports that don’t cause grass to appear under them is an example of this.)
Perhaps some good questions for weighing these options are:
1. Would an author care whether the facade looks exactly the same in the future?
2. Are there enhancements coming along in this technological area that we would not want to miss out on for older scenery?
OTHER CONSIDERATIONS
Information loss: when we turn a facade into an object, the original polygon information is lost. Is it important that we ship the actual base polygons that make up buildings, or are the OBJs that represent them good enough?
Contextual Information: often we know more about a given situation when making scenery than when displaying it. Could we do better at building up a facade inside the sim (where we have context from other scenery packages), or when creating the sim (where we have a much larger dataset)?
Just a gentle reminder: if you are having problems with X-Plane (especially the installer), please do not post a comment to this blog. Instead email Randy and Jack, or tech support guys, at info at x-plane dot com. This is the official way to get tech support, and they will work hard to make sure you can use X-Plane.
In a previous post I discussed polygons, how they can be used, and a little bit about how they relate to X-Plane 850 and the new apt.dat system. I have been working on some demo scenery that will make this all clear, but the great is the enemy of the good, so rather than wait I’ll post more on this now and get the demos done as soon as I can.
There are essentially two ways to get at the new polygon code: via the apt.dat system or via an overlay DSF. When should you use apt.dat and when should you use an overlay DSF?
This last point is important: do not “abuse” the definitions of apt.dat files just because they look similar.
There are essentially two kinds of features in the scenery system:
This second type of feature is the one where I issue caution: if a feature in apt.dat or the sim is meant to emulate how the real world looks, we will change how it looks to improve rendering! This is why it is important not to use apt.dat feature for purposes other than they were intended for. It might be that in X-Plane 8.50 the blue taxi lights look just like some other feature you want to code. But in a future version, we might make them more realistic for an airport and they will look worse for your other application. By using apt.dat features only for their true purpose, you help us ensure that our changes to the base artwork make scenery better, not worse.
(This division of all scenery features between ones that are “stable” and ones that are “based on the real world” can also be seen in most parts of the sim. In particular, the flight model is designed with a “based on the real world” philosophy, a very controversial decision I’ll have to blog about some other time.)
Next: fixing airport terrain with polygons.