Simplification with `ratio: 0 error: 1` does not simplify enough about gltf-transform HOT 1 CLOSED

Testing with this script in https://gltf.report ...

import { MeshoptSimplifier } from 'meshoptimizer';
import { weld, simplify } from '@gltf-transform/functions';

await document.transform(
	weld({tolerance: 0.0001}),
	simplify({simplifier: MeshoptSimplifier, ratio: 0.0, error: 0.45, lockBorder: true})
);

... I get pretty much the same result. Note that MeshoptSimplifier does not see all of a mesh's attributes, just the vertex positions and their connectivity (via indices). In general, and particularly with lockBorder, it's going to try to preserve topology, so I think collapsing a 3D shape to a single triangle is probably something the algorithm explicitly avoids.

What we can do to give Meshopt the best possible chance of success, would be to minimize seams and maximize connectivity. Welding tries to do that, but extra vertex attributes make it hard. I'm not sure which attributes you need at higher LODs, but removing everything but POSITION resolves the issues you're seeing...

import { MeshoptSimplifier } from 'meshoptimizer';
import { weld, simplify, prune } from '@gltf-transform/functions';


for (const mesh of document.getRoot().listMeshes()) {
	for (const prim of mesh.listPrimitives()) {
		for (const semantic of prim.listSemantics()) {
			if (semantic !== 'POSITION') {
				prim.setAttribute(semantic, null);
			}
		}
	}
}

await document.transform(
	prune(),
	weld({tolerance: 0.0001}),
	simplify({simplifier: MeshoptSimplifier, ratio: 0.0, error: 0.45, lockBorder: true})
);

... with that simplification gets the result you're probably expecting:

• error = 0.45 -> tetrahedron
• error = .50 -> pair of back-to-back triangles
• error = .55 -> empty primitive

Possibly a weld mode that explicitly ignores vertex attributes other than position would help here (but doesn't exist in glTF Transform now) if you don't want to delete these attributes. There's also an experimental meshopt simplification function that includes vertex attributes — with custom weights — but that isn't integrated into glTF Transform yet, see #1153.

from gltf-transform.