Selecting mesh LODs and impostors by visible error

Optimize what reaches the image

Level of detail (LOD) changes an object's representation as its visible importance changes. Distant geometry often needs fewer triangles, but triangle count alone misses silhouette, material, and animation errors. An original crowd-rendering implementation illustrates per-instance LOD choices that also affect animation work. [3]

Define acceptable error using actual gameplay views. A distant landmark's outline may matter more than a nearby hidden surface, even when both meshes contain similar amounts of geometry.

Compare representations and error

Mesh simplification reduces geometry under an error target. The meshoptimizer documentation explains that topology or error limits can prevent reaching a requested triangle count. [1] Treat the achieved result as something to inspect, not merely a percentage to accept.

An impostor substitutes image-based data for detailed geometry. Traditional cards have view-dependent limitations; more elaborate impostors reconstruct depth or intersections but increase pixel work. [2] The decision therefore trades geometry processing against texture storage, overdraw, and shading.

Work through a screen-size example

For an illustrative front-facing object 2 metres high, at camera-space depth 20 metres, a 1080-pixel-high view with a 60-degree vertical field of view gives a projected height of approximately 93.5 pixels. At 40 metres, that becomes approximately 46.8 pixels. The calculation uses perspective projection with unchanged orientation and field of view.

Try switching to a coarse representation below 50 pixels and returning to the finer one above 60 pixels. This example hysteresis band avoids repeatedly switching around one threshold. The values are proposed test settings, not engine defaults. A different field of view or resolution requires reevaluation.

Review transitions as a sequence

Generate candidate LODs and compare their silhouettes, UV distortion, normals, and material boundaries. Traverse each transition forward and backward, then rotate the camera around the asset. Check shadow and reflection views as well as the main image.

If using cross-fades or dithering, measure their overlap cost and inspect interaction with temporal antialiasing. For impostors, inspect viewing-angle changes, apparent depth, lighting consistency, and alpha edges. Include a crowd of instances so simultaneous transitions reveal worst-case work rather than only one object's cost.

Preserve the actual benefit

A reduced mesh with the same expensive material may remain shading-bound. Large impostor cards can replace triangle work with substantial empty-pixel processing. Do not assume that the representation with the fewest vertices wins.

Record measured frame cost alongside visual acceptance and storage. Keep collision, gameplay identity, and rendering representation coordinated without requiring identical geometry. Revisit thresholds when camera behaviour, output resolution, or the asset's materials change.

Sources

• [1] meshoptimizer simplification documentation — Arseny Kapoulkine and meshoptimizer contributors.
• [2] True Impostors — Eric Risser; NVIDIA GPU Gems 3.
• [3] Animated Crowd Rendering — Bryan Dudash, NVIDIA.