GLTFExporter: Support metallicRoughnessTexture

[takahirox]

This PR adds metallicRoughnessTexture support to GLTFExporter.

In Three.js metalnessMap and roughnessMap can be different textures but can't be in glTF 2.0. So exporting only if they are the same texture.

[mrdoob]

Thanks!

[donmccurdy]

Oh just realized, ideally if occlusionMap is the same texture we shouldn’t process it again.

[mrdoob]

Ah yeah... aoMap in three.js terms.

[takahirox]

Ah, you meant that's from OcclusionRoughnessMetallic compatibility. I'll fix.

[mrdoob]

We have to revisit this...

If you only use a roughnessMap texture the exporter ignores it because roughnessMap !== metalnessMap.

Also... Maybe the exporter could build the composite texture for the user?

[takahirox]

In glTF metallic-roughness texture, B channel is for metalness and G channel is for roughness.

If you only use a roughnessMap texture and the exporter exports it as metallic-roughness texture, its B channel will be unexpectedly (for exporter user) used. So I let the exporter ignore the texture in that case.

Probably we could composite and, I need to review the metallic-roughness calculation but, in the case even one of them is used perhaps we could make up by filling 0 or 1 in a certain channel?

[donmccurdy]

Also... Maybe the exporter could build the composite texture for the user?

If we want to get into image manipulation, we could also see about converting alphaMap to the alpha channel of baseColorTexture... maybe worth putting that code into a helper method:

var blob = ImageUtils.composite({
  r: 1,
  g: roughnessTexture,
  b: metalnessTexture
});

[takahirox]

I let the exporter ignore the textures in those cases because I wanted to quickly implement. I think compositing is good for users.

And, as Don mentioned, doing that in helper method is nicer than doing in the exporter. It can be reusable from other modules and users (could be used for optimization?).

[mrdoob]

👍👍👍

[takahirox]

we could also see about converting alphaMap to the alpha channel of baseColorTexture

That could be nice, map A channel * alphaMap => baseColorTexture A channel. User won't lose alphaMap info.

[takahirox]

BTW tho I said it's good to users, do you think there're many models using separated metalness/roughnessMap? I suppose (O)RM (occlusion/roughness/metal) is packed into one texture (called ORM texture) in many cases.

[mrdoob]

BTW tho I said it's good to users, do you think there're many models using separated metalness/roughnessMap?

I was trying to build my own. I was trying to add this texture (albeit is inverted) as roughnessMap to the world test. But I can't do it because the exporter expects me to add a metalnessMap too.

[takahirox]

I see. Yeah seems better to compose/make up not to lose info as much as possible.