Reintroduce MeshPhysicalMaterial

[WestLangley]

As suggested the discussion in #8505, this PR adds MeshPhysicalMaterial by extending MeshStandardMaterial.

Also as proposed by @bhouston in #8505, the new MeshPhysicalMaterial adds the reflectivity parameter, which models the specular reflectance of the material for non-metals.

reflectivity takes values in [ 0, 1 ], and has a default value of 0.5, which maps to an F0 of 0.04.

/ping @bhouston

[bhouston]

We can move this to someplace common to remove the duplication?

[WestLangley]

I agree. Some refactoring/cleanup is needed. I'll gladly take your suggestions on that. Can we do it in the next PR?

[bhouston]

Thank you @WestLangley for this! Sweet!

I love the fact that we can add more material parameters but I am a little concerned that we have added 300 lines of code to be able to do that, and most of that code is duplicated code.- now we have to maintain two code paths that are nearly identical. It would be cool after this PR is merged to figure out some way to further share glsl code between MeshStandardMaterial and MeshPhysicalMaterial.

[bhouston]

could potentially merge in the parameters from 'standard'? Reduce deuplication?

[WestLangley]

It would be cool after this PR is merged to figure out some way to further share glsl code

Agreed. I was thinking that we could hack in the advanced MeshPhysicalMaterial features fairly quickly, and then refactor when we were done, rather than trying to refactor first -- unless it is clear to you already what will be common and what will not. Either approach is fine with me.

[bhouston]

I wonder if we could create a "meshbsdf_vert.glsl", and a "meshbsdf_frag.glsl" that could be shared between Phong, Standard and Physical. I'd go even further and create a MeshBRDFMaterial that could serve as the base class between all three, and it would have the huge number of shared parameters:

https://github.com/mrdoob/three.js/blob/master/src/materials/MeshStandardMaterial.js
https://github.com/mrdoob/three.js/blob/master/src/materials/MeshPhongMaterial.js

[mrdoob]

These checks aren't needed because THREE.MeshPhysicalMaterial extends THREE.MeshStandardMaterial so the check is already true.

[WestLangley]

Correct. Will fix.

[mrdoob]

It would be cool after this PR is merged to figure out some way to further share glsl code

Agreed. I was thinking that we could hack in the advanced MeshPhysicalMaterial features fairly quickly, and then refactor when we were done, rather than trying to refactor first -- unless it is clear to you already what will be common and what will not. Either approach is fine with me.

I think the duplicating approach is a bit confusing...

How about we add features on the glsl code, and maybe rename that code to PHYSICAL.
Then on the javascript side we just make the MeshPhysicalMaterial extension as this PR already does.

So both materials use the same glsl code, but MeshStandardMaterial exposes less of it.

[WestLangley]

I think the duplicating approach is a bit confusing...
So both materials use the same glsl code, but MeshStandardMaterial exposes less of it.

Yes, we could add the physical.glsl files here, and then remove the standard.glsl files in the next PR. Then standard would use the physical shader, and we would use if defined STANDARD, etc, as needed.

[mrdoob]

Yes, we could add the physical.glsl files here, and then remove the standard.glsl files in the next PR. Then standard would use the physical shader, and we would use if defined STANDARD, etc, as needed.

Sounds good!

[mrdoob]

Thanks!