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Monday, October 17, 2016

Slaying the LOD monster

LOD, or Levels of Detail, is a technique for managing very large and dense scenes like we have today in open world games. The general idea is you can have multiple copies of the same content at different resolutions. Then you switch which copy to use depending on how much detail is necessary for the scene.

LOD switches can be implemented as plain swaps, morphing between states as shown in this earlier post, or even as completely adaptive mesh representations, as shown here.

Sometimes we get fixated on LOD swaps and we drift away devising tricks to make them harder to see. I feel this is not the right angle to look at this problem. While LOD swapping techniques are very important, and progressive swaps are much nicer, etc., the real goal is to have as much detail as possible in the first place. If you find yourself masquerading LOD swaps, it is probably because your scenes still include levels of detail that are too coarse.

How much detail do you need? Just enough to make LOD transitions close to being sub-pixel. Once LOD swaps are sub-pixel, it does not matter what swapping technique you use. It also means your rendering of the content is as detailed as the screen resolution will allow.

This sounds like a Holy Grail, is it feasible with the current hardware generation? We believe it is. Take a look at the following video:

As you can see in the video, buildings preserve their detail, even when viewed from afar. LOD changes, while they do happen, are virtually impossible to detect. All this while keeping interactive frame-rates.

It turns out the same techniques that prevent buildings in the distance from looking like melted crayons also give you smooth LOD transitions. I will be covering how all this works in my next post.