What does "LOD" mean anyway?
"LOD" stands for Level Of Detail. In FS this refers to aircraft (and sometimes scenery) models that are actually made up of several models, built with varying degrees of detail. The most detailed model is displayed when the aircraft (or scenery object) that is being displayed is very large on your screen (in other words, it appears to be very close to you, the viewer). When the aircraft is much further away a model with much less detail can be used because you can't make out any details since the model appears so small to you.


Why use LOD models?
In a word: FRAMERATE. The framerate (frames displayed per second) determines how "smooth" your FS display is going to be. FS is displayed as individual still frames on your monitor, that when displayed fast enough, give the illusion of motion. The more frames being displayed per second, the smoother your FS experience will be. (Note: Once you get to 20-25 frames per second, the human eye can't discern any real difference, it all looks smooth)

We've all probably had the experience of flying along in FS, and everything is going quite well, right up to the point where you turn your plane towards a major airport, and suddenly FS becomes very "jerky". This happens because your FPS (Frames Per Second) abruptly dropped to a level where the individual frames aren't being displayed fast enough. The FPS dropped because suddenly FS had to deal with a lot more objects than it could handle without slowing down. Of course this varies from computer to computer, the faster your computer and video card is, the more "stuff" your FS can handle before it starts to slow down.

There are other things that will slow FS down that you might not think of as "models", such as clouds, but here we're going to concern ourselves with aircraft models.

The reason FS slows down when having to display a busy scene, such as a crowded airport, is because it is having to make calculations for every polygon within the field of view. The more polygons FS is trying to display, the longer it takes to complete a single frame and display it. If you can lower the number of polygons FS has to calculate, you can increase your framerate.

LOD models allow you to do this by displaying lower polygon models when an aircraft is further away from you.

This is particularly important for AI models, since at a busy airport, you will have many planes in view. If you are seeing 20-30 planes, and none of them use LOD models, your framerates will probably suffer for it. This is why for AI use, you should seek out planes that use LOD models. If you use a very detailed, 32,000 polygon model that has no LOD models (no LOD models are also sometimes referred to as single LOD models) as AI, FS will be calculating textures and lighting on 32,000 polygons no matter if the plane is filling the screen, or if it is a tiny dot on the horizon.

Won't building LOD models take a lot of time?
If you've built a single LOD model, you're probably thinking of all the time you spent trying to get that model to look just right, and you're thinking building LOD models is going to take almost as much time.

Well, it shouldn't, because you've already done all the hard work on the LOD1 model. On the subsequent LOD models, you're just making the less detailed models match up as closely as possible to the model that appears before it. Since you've already spent all the time getting the LOD1 model to look right, there's no need to do it again, just freeze or place in the background your original model and then build the LOD models on top of it. Once you get the hang of it, you can make enough parts in a day to have 10-15 LOD models on your completed aircraft. (The aia 727-200 initially had enough parts for 17 LOD models, all of which were built in less than a day)

LOD models are also a lot easier to build than your original LOD1 model, because they aren't as detailed, and they can't be seen as closely either. Once you get to the far off LOD models, you can build models that look like paper airplanes and get away with it.


How many LOD models should you use?
The number of LOD models you use can vary from aircraft to aircraft, a general rule is that the larger the aircraft is, the more LOD models it could use.

You can use as few as 2 or 3 LOD models if you wish, but with so few LOD's you'll end up having to make a compromise somewhere. If you're trying to be as framerate friendly as possible, you may end up having to let the less detailed model appear too soon, meaning that you can actually see that is a less detailed model. This would also mean very abrupt LOD shifts where it is quite easy to see where one LOD model shifts to another. On the other hand, if you're more concerned with the visual appearance of the model, you may have to let your higher poly more detailed model remain visible for much longer that you can actually make out much of the detail on it.

3 LOD models is pretty much the absolute minimum of LOD models you should use on an AI model. A 3 LOD model is going to have to make one of the compromises described above though.

The goal when building LOD models should be to reduce the number of polygons that are being used as quickly as possible as the model gets further away from you, without having obvious LOD shifts where one can see when one LOD shifts to another.


Where's the "Make LOD's" button?
(simulation of what such a button might look like if it actually existed)

There isn't one! You've got to build the LOD models yourself.
You can use the 'optimize' command to do some limited polygon reduction, but it's not very effective. For really good LOD's, you've got to roll up your sleeves and build them yourself.


You mean I have to build a completely new model for each LOD?
Absolutely not. Building LOD models does not mean you have to build brand new models for each LOD. You only need to build replacements for the parts that are polygon intensive. Generally, this means any part that is round or curved, such as: fuselage, engines and wheels.

Parts that don't use a lot of polygons to begin with like a rectangular gear door, can probably be reused on all your LOD models up to the point that the plane is so far away you can't see the gear doors. Once you can't see them, you can stop using them for all subsequent LOD's.

Some parts like wings, might not use a lot of polygons, even at LOD1, but they can usually be replaced by one or two simpler versions as the plane gets further away.

There does not have to be a particular part that changes on every LOD either. The fuselage might get replaced on one LOD, and then remain the same for the next few LOD's. Sometimes it's good not to have too many parts all changing on a particular LOD as this makes it more likely that there will be a visual "jump" at the point the LOD's shift. The fuselage and engines might each have a slight visual change as the LOD's shift, but you can make the shift less objectionable by having the fuselage change at one LOD shift, and the engines change at the next LOD shift.

Here's a line up of ai-aardvark 757-200's. This is a very early aardvark model and it only uses 5 LOD models. The closest 757 is the LOD1 model, and the one the furthest away is the LOD5 model. Can you tell where the LOD2 model starts appearing? Or the LOD3 model?

As you can see below, the LOD1 model is only being used once in this shot. The LOD2 model also only appears once. The textures have been removed from these models and the parts of the model have been assigned colors so that you can more easily see where the shifts are occurring, as well as what parts are being duplicated on more than one LOD model.

The LOD2 model retains the wing, tail surfaces, and most of the gear parts of the LOD1 model. These parts are all relatively low poly to begin with since they are not curved.
The yellow LOD3 model is almost completely new, but is still using some of the landing gear parts from the LOD1 model.
The orange LOD4 model retains the yellow tail section that was originally used on the LOD3 model.

The LOD shifts are very obvious when the parts are colored this way, but on a textured model, they can be very difficult to spot.

The 16 757's in this shot are adding 9725 polys to this scene. That may seem like a lot, but if this model did not use LOD's, if it were just the 2188 poly LOD1 model, the 757's would be contributing a total of 35,008 polys that FS would have to make calculations for.

If only 3 LOD models had been used, the LOD1 model would probably have needed to be visible where the 4 closest planes in this shot are so as to make the LOD shift harder to spot. The first 4 planes alone would have been displaying 8752 polys! As you can see LOD models make a big difference in reducing the total number of polygons present in a scene, which contributes significantly to keeping the framerate as high as possible.

1909

LOD 2

This model has less polygons on the lower half of the fuselage, on the inboard sides of the engines, and wheels that have less sides than the LOD1 wheels.

Because this model is still seen fairly close, we're limited in what can be simplified.

LOD 3

This model is basically still the previous LOD 2 model, but with lower poly wheels, which is why the total poly count has not dropped very much.

LOD 4

Still the same fuselage as LOD3, but with a lower poly tail section, as well as new engines.

The reason the same fuselage is still being used is because the specular highlights make the LOD shifts on the fuselage particularly noticeable.

LOD 5

Finally we have gotten far enough away from the model so that the fuselage can be replaced with a lower poly version without the LOD shift being very noticeable.

The tail section is replaced again, and a new lower poly wing appears.

The engine pylons are also replaced with a simpler version.

LOD 6

The fuselage is replaced, along with the engines, wing and horizontal tail.

The wheels, which have been the same since LOD3 are now replaced.

LOD 7

The fuselage is replaced again, as are the engines. The vertical tail is also simplified.

The wheels are replaced again.

Spoilers, gear struts and flap canoes have been removed.

LOD 8

We're obviously a long distance from the model now.

The fuselage is replaced with a very simple 4 sided version. The engines and vertical tail are also replaced

The wheels, wings and horizontal tail are replaced.

Flaps, slats and gear doors have been removed.

LOD 9

The ultra simple model!

Basically just a flat silhouette that in the far distance will let you see that there is a fuselage and wings.

Because this model has no volume at all, when viewed from either directly in front or behind, it is practically invisible, so you would use this for only the far, far away model.

People often wonder what the best way to build the less detailed LOD models is. Basically, you have two options:

Make a copy of an existing part and start merging polygons together on that to reduce the poly count.

Start from scratch, and build a new lower poly model of the part.

Like all things there is no right answer to this question, in actuality, you will probably find yourself doing both.
So, let's take a look at both methods.

Starting with an existing model.
The initial advantage of this method is obvious, you're starting with a part that is already built. The part is already the right size and shape, you just need to reduce the amount of polys used. This method is probably best used on more complex shapes that would take longer to rebuild from scratch. A possible disadvantage of this method is that it may result in a more substantial polygon loss than you may want since it tends to reduce the number of sides in an object by half.

Here's one half of the engine after having half the polys removed from it.
The advantage of this method is that it is relatively quick, the disadvantage is the polygon reduction may be more severe than you wish.

Just to make the contour slightly more accurate from the side, we've added a slice to the lower side of the nacelle.
This is an important technique to remember, if you're having to adjust the contour of a fuselage, and the area you need to adjust is just on the top or bottom of the fuselage, you don't have to slice the fuselage from top to bottom to do it. Just make a slice in the area you're working on to add some vertices.

The shift to the LOD2 model was going to occur fairly quickly, while the model would still be quite large in the screen, which limited the amount of severe polygon reduction that we could do. In addition to the underside of the fuselage, we also eliminated polygons on the inboard sides of the engines, since this area of the engines is very difficult to see, especially from any distance.

In addition to the fuselage and engines, all of the wheels were replaced with lower poly versions. All told, resulting in a LOD2 model that was 363 polys less than the LOD 1 model.

There are some parts that viewers eyes are drawn to more than others. These are parts you need to be particularly careful of when the aircraft shift from one LOD to another LOD.
One such part is the cockpit windows. On the 727-100 the cockpit windows stayed the same for the first 5 LOD models, before shifting to a new fuselage where the cockpit windows were not actually modeled, but instead just painted on the fuselage.

For this particular model, the shift from modeled windscreen to one that was just painted on isn't very noticeable, but that's not always the case.

You may find yourself working with a LOD fuselage where the painted on windscreen may look very distorted because of the fuselage geometry. If you can't see any noticeable shift in FS because of the distance that the shift happens, you don't need to do anything.

If, on the other hand the windscreen makes a dramatic change in appearance when the LOD's shift in FS, you'll need to resort to this cheap trick. Instead of remodeling the fuselage by adding lots of polygons, just make a copy of the polygons that make up the windshield and place them in exactly the same position as they are on a LOD where the modeled windshield appears. You may have to adjust some of the fuselage vertices slightly so that the fuselage does not protrude through the windscreen. Now you can reduce the fuselage polygon count but the windscreen will not appear to shift at all as the model switches from one LOD to another.

We refer to this as the "cheap sunglasses technique".

There are a few areas where you can lose some parts of an assembly without losing the illusion that the group of parts are still in place. Two obvious areas are landing gear and flaps.

On most aircraft the landing gear and flaps are actually an assembly of parts moving in unison. At some point as the aircraft gets further away from you, some of the smaller parts become too small to see. At this point you can delete them from the LOD model you're seeing at that distance.

Landing gear are made up of fairly large parts like wheels and gear doors, and thin parts like gear struts. At some point in your LOD progression you should be able to stop using the gear struts because the plane is too far away to discern the struts. On a main gear set, the struts may be made up of a main strut and one or more smaller struts. The smaller struts can stop being used in your LOD models before you get the to the LOD where you no longer need the main strut either.

Depending on the type of aircraft being modeled, you may be able to lose some gear bay doors, if for example there are wing mounted engines that are obscuring the landing gear.

LOD 1	LOD 5	LOD 7

Flaps are also made up of a collection of parts, the big obvious slab-like sections of the flaps themselves, and the housings that cover the mechanisms that actually move the flaps. These housings are often referred to as "flap canoes".

Some flap canoes can be fairly complex, consisting of one or more hinged parts that have to be animated with the flap. This probably means they had to be built with a fair amount of polygons.

The good thing about flap canoes is that they are buried under the wings, and often obscured by engines and landing gear. This means you can lose the flap canoes pretty quickly.

But wait! If you have a hinged canoe, there is one section you should not be in a hurry to get rid of. That part is the very last bit, the pointy section that protrudes out beyond the trailing edge of the wing. The reason for this is that you can often see this part of the canoe silhouetted against the sky long before you can make out the rest of the canoe parts. So you may need to let the trailing section of a hinged canoe stay visible for longer than you would expect. But you don't need the rest of the canoes till you get fairly close to the model.

LOD 1	LOD 3	LOD 4

Notice that the LOD3 flap canoes, while still representing the complete canoe, are simpler versions of the LOD1 canoes. LOD modeling is often a combination of simplifying the parts as well as removing parts that would no longer be visible.

The easiest LOD model of all to build is the last one. The one that's so far away you can barely see it. This is the plane that will be making your framerates very happy when you're 5 miles out from a busy airport, because the polygon load on your computer will be next to nothing.

This model only needs to bear a passing resemblance to the actual aircraft. It's primary purpose is to hold a texture up and catch the light so that you think there's a plane sitting out there over a mile away. As you'd expect it doesn't take a lot to do this. This model doesn't have to have any thickness at all, it can just be flat polys in the rough shape of the aircraft.

One thing you may be tempted to do with this aircraft is to use a double sided polygon. There's a couple of reasons you shouldn't do this though.

1) Using a double sided poly isn't going to save you any polygons. It may look like one polygon in Gmax, but once you export it out as a .mdl, it gets turned into 2 polys.

2) Lighting/shading errors. A double sided poly in FS only has one side as far as the light in FS is concerned. FS figures out what the lighting should be on one side of the double sided polygon and then just repeats it on the other side. This is really noticeable at dawn or dusk when the sun is low on the horizon. For example, at dusk in FS, a double sided polygon arranged so that it is facing the sun, will appear to be lit by the setting sun on both sides of the polygon on both sides, even the side not facing the sun. If you rotate the polygon 180° both sides will be in deep shadow. You should also keep this in mind whenever you're modeling thin parts that you might be thinking you could just make double sided. Parts like antennas, gear doors, strakes, etc.

So, you might as well take the time and do it right.

Close observers of the LOD9 model of the 727 shown earlier might wonder why the 2 planes representing the fuselage are tilted at an angle. The sides were tilted so that the light could hit them better. After initially building the sides so that they were perpendicular to the ground, it was noticed that at noon that where this model shifted to this LOD there was an extremely noticeable LOD shift. This was happening because the preceding LOD model had some dimension and the sun was hitting the angled face of the fuselage. Then the shift to the last LOD would occur and the model would suddenly go dark because the sun was no longer hitting it. Angling the fuselage sides slightly fixed this problem.