Measuring Mesh Decimation Quality

Fewer triangles, same shape — prove it

A decimation (or LOD) tool trades triangles for speed. The whole game is keeping the shape while dropping the count, and "looks the same" is not a measurement. To test a simplifier you need the original dense mesh and a way to quantify how far the simplified version drifted from it.

Pair dense with low-poly

Each case in the mesh-decimation set ships a dense ground-truth mesh and a low-poly simplification of the same shape, with triangle counts recorded in the spec. The dense sphere and its low-poly twin are a clean starting pair: same geometry, very different budgets.

The metrics that matter

• Hausdorff / RMS surface distance between the two meshes — sample points on each surface and measure the distance to the other. This is the primary "how far did the surface move" number; report both the max (Hausdorff) and the average (RMS).
• Normal deviation — decimation flattens curvature, so compare surface normals at corresponding points. A simplifier that preserves the silhouette but wrecks the shading normals will look faceted.
• Silhouette / boundary error — render both from several angles and compare the outlines. Silhouette is what the eye locks onto, so a tool that protects it can drop far more interior triangles without looking wrong.

Track surface distance against the triangle-reduction ratio. The honest way to compare two simplifiers is at equal triangle budgets: whichever holds a lower surface distance for the same count wins.

Sample the surface, not the vertices

The most common way to get these numbers wrong is to measure vertex-to-vertex distance. Decimation removes vertices by definition, so there is no correspondence to measure — and every surviving vertex usually sits exactly on the original surface, which makes a vertex-based distance look suspiciously excellent while the surface between them has sagged badly.

Measure surface-to-surface instead:

• Sample points uniformly by area across each mesh, not per triangle. Per-triangle sampling over-weights the dense regions, which is precisely where decimation did the least damage, and flatters the result.
• Measure point-to-triangle distance, not point-to-nearest-vertex. On a coarse mesh the nearest vertex can be far away while the nearest surface point is directly underfoot.
• Measure both directions and take the worse. One-directional Hausdorff misses whole categories of error: a simplifier that deletes a protruding feature entirely scores well in the direction that maps simplified onto original, because every remaining point still has a close match.
• Use enough samples that the number is stable. Re-run with a different sample count; if the result moves, you are measuring your sampler.

Report the distance as a percentage of the model's bounding-box diagonal rather than in raw units. That makes results comparable across models of different sizes, which is the only way a threshold in CI means anything.

Watch what breaks first

Decimation tends to fail at high curvature and at feature edges — the rim of a cup, the teeth of a gear. Keep varied shapes in your suite (a sphere, a torus, a surface-of-revolution vase, a cylinder) so you're not tuning to one topology. A tool that's great on smooth blobs can mangle sharp features.

Reproducible geometry

The meshes are built deterministically with no randomness, so vertex positions are identical everywhere and the GLB is self-contained. Your surface-distance and normal-deviation numbers are comparable across tool versions and CI runs, and a regression reproduces on the exact same mesh.

Get the fixtures

Start with the mesh-decimation set, or the wider mesh-processing fixtures and the Models library. Free to use, no attribution required.