Level of detail (LOD)

An LOD chain is a set of versions of one model, numbered from most to least detailed. LOD0 is the full asset. LOD1 might carry half the polygons, LOD2 a quarter, and the last level can be a handful of faces or even a flat billboard. The engine measures how large the object is on screen each frame and draws the level that matches.

Engines switch on screen size rather than raw distance, because size is what determines whether detail is visible. A building at 100 metres still fills the view; a coin at 100 metres is a single pixel. Unity's LOD Group uses the object's screen-relative height, and Godot's mesh LOD uses a screen-space metric that accounts for field of view and resolution.

There are three common routes:

• By hand. A modeller builds each level, often by removing edge loops from a clean quad mesh. It takes longest and gives the best silhouettes, so it is usually kept for hero assets.
• Decimation. A tool such as Blender's Decimate modifier collapses edges until a target polygon count is reached. It is fast, but it ignores edge flow, so it is used for lower levels where nobody sees the topology.
• Automatically in the engine. Godot 4 generates LODs on import. Unity builds a LOD Group from meshes named _LOD0, _LOD1 and so on in one FBX file. Unreal Engine's Nanite renders supported meshes at an automatically adjusted detail level with no manual LOD setup.

Good LODs start from a good LOD0. A dense, irregular mesh decimates into lumpy lower levels, which is one reason retopology comes first.

The switch between levels is the weak point. If LOD1 changes the silhouette, players see the model "pop". Three habits keep transitions quiet:

• Preserve the silhouette and remove interior detail first. Outlines are what the eye tracks.
• Keep UV seams and material boundaries in the same places across levels, so textures do not shift.
• Use the engine's fade or cross-dither option where it exists, so one level blends into the next over a few frames.

LODs pay off in open scenes with many repeated assets: forests, cities, crowds, racing tracks. They matter much less in small interiors where every object stays close to the camera. If performance is poor and LODs are already in place, the cost is usually elsewhere: draw calls, shadows, transparency or scripts. That is why profiling comes before optimising.

Exports also need to carry the chain: FBX can hold several LOD meshes in one file, which is how Unity's automatic LOD Group setup works. See 3D file formats for what each format can store.