GLTFExporter sets a default metalness of 0.5

[schwyzl]

Description

If an object material is not a MeshStandardMaterial the GLTFExporter sets a default metallicFactor of 0.5.

		if ( material.isMeshStandardMaterial ) {

			materialDef.pbrMetallicRoughness.metallicFactor = material.metalness;
			materialDef.pbrMetallicRoughness.roughnessFactor = material.roughness;

		} else {

			materialDef.pbrMetallicRoughness.metallicFactor = 0.5;
			materialDef.pbrMetallicRoughness.roughnessFactor = 0.5;

		}

three.js/examples/jsm/exporters/GLTFExporter.js

Line 1497 in 752400c

materialDef.pbrMetallicRoughness.metallicFactor = 0.5;

This should probably not happen. There is no such thing as semi-metalness in the real world so Three.js should probably choose 1 or 0 in this case. Seeing as this is a generic object exporter I would assume not adding any metalness is the more desirable behaviour.

[Mugen87]

How about metallicFactor = 0; and roughnessFactor = 1;? That would correspond to the default values of MeshStandardMaterial.

[schwyzl]

Yep, sounds like a good idea to match the standard material defaults.

[Mugen87]

Would you like to file a PR with the suggested change?

It seems these values exist since the beginning of the exporter and they were never discussed (see #11917).

[Mugen87]

In the meanwhile, I've filed a PR with the suggested defaults.