[x3d-public] beyond Blinn-Phong: PBR

[Michalis Kamburelis]

Thanks for many good ideas! I agree with everything, I only want to
add a note to the below paragraph:

Andreas Plesch <andreasplesch at gmail.com> wrote:
> A node design choice was to add the texture maps as fields to
> PhyicalMaterial rather than Appearance. I think Timo's reasoning was
> that this way all new functionality can be contained in one new node.
> But this is a bit of a departure. Other designs are certainly
> possible.

I see advantages of this design, where a material node allows to
configure every property by a constant factor (scalar, vector) and
optionally to multiply it by a texture.

CommonSurfaceShader also uses this design, CGE docs (linking to X3DOM
and InstantReality docs):
https://castle-engine.io/x3d_implementation_texturing_extensions.php#section_ext_common_surface_shader

E.g. in CommonSurfaceShader you have

  SFVec3f diffuseFactor
  SFNode  diffuseTexture
  SFVec3f emissiveFactor
  SFNode  emissiveTexture
  SFVec3f specularFactor
  SFNode  specularTexture
  .. and so on

(I'm simplifying a bit, in reality you need at least a way to provide
texture coordinate index to each texture slot, so there are more
fields.)

In my view, we should follow this approach to the consistent end :) So
PhysicalMaterial would have

  baseColorFactor
  baseColorTexture
  .. and so on
  (or maybe just baseColor, baseTexture)

and to the regular Material we add

  diffuseTexture
  specularTexture
  emissiveTexture
  .. and so on

So all factors (scalars/vectors) have a counterpart texture, and the
texture field is present right next to the non-texture (scalar/vector)
value.

Advantages:

- This is simple for authors. """Question: What does the texture in
slot xxxTexture do? Answer: The same thing as xxxFactor, but it's a
texture so it allows to vary this material property over a surface.
See the xxx treatment in lighting equations."""

- It is simple to implement. Plug the texture xxxTexture into the
shader at the same place where you use xxxFactor.

Compare this to the current approach of X3D:

- The lighting equations say to use the Apperance.texture for diffuse,
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/lighting.html#Lightingmodel
. (And to treat RGB and grayscale textures differently, which is
another point I find bad -- RGB textures should "replace" by default,
while RGB should "modulate", according to the spec.)

- The multi-texturing specification says that when MultiTexture is
present, the "MultiTexture.mode" rules (and the default is "modulate",
regardless of RGB or grayscale,
http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/texturing.html#MultiTexture
). And the "MultiTexture.source" determines whether the texture
affects the diffuse or specular calculation.

There are a couple of inconsistencies here. And it's not implemented
completely by X3D browsers, as far as I tested.

And it still doesn't provide all the necessary flexibility. E.g. you
can only modify "diffuse" and "specular" by "MultiTexture.source", you
cannot modify "emissive" color, and it's unclear what modifies the
"transparency". Although some of these things could be easily fixed in
the multi-texturing spec, but I think that overall the approach of
PhysicalMaterial/CommonSurfaceShader is better.

I discussed some of the above in """How does this relate to the
existing X3D multi-texturing nodes?""" on
https://github.com/michaliskambi/x3d-tests/wiki/How-to-add-PBR-to-X3D%3F
. In short, I think we have to keep "Appearance.texture" supported,
and when it's present it should override "Material.diffuseTexture" or
"PhysicalMaterial.baseColorTexture" (depending whether
"Appearance.material" is Material or PhysicalMaterial). But new models
should be advised to instead add textures inside Material or
PhysicalMaterial.

And the normalmap texture (that is not related to lighting equations)
should be specified outside of the material, in "Appearance.normalMap"
field. This is also consistent with glTF, that places "normalTexture"
outside of the "pbrMetallicRoughness" block, recognizing that
normalmaps make sense for all lighting models, since "varying normal
vectors over a surface" makes sense for all algorithms that look at
surface normals. CGE has "Appearance.normalMap" already.

Sorry for a long train of thought :) Hopefully this is informative.

Regards,
Michalis