LOD Part 1 – What LOD Really Does

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.

About Ben Supnik

Ben is a software engineer who works on X-Plane; he spends most of his days drinking coffee and swearing at the computer -- sometimes at the same time.

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