[x3d-public] beyond Blinn-Phong: PBR

[Andreas Plesch]

Super input. Just a few remarks.

On Fri, Feb 8, 2019 at 5:48 PM Michalis Kamburelis <michalis.kambi at gmail.com>
wrote:
>
> 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)

It is called basecolortexture in glTF but baseTexture would be following
x3d conventions.

> 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:

Perhaps somebody has insight into the underlying reasoning behind the
current approach.

In a way, the new Materials are on the level of the current Appearance.

Structurally, the main question may be what works best for reuse of
Appearance, Material and Texture.

Currently, reusing Material gives you the option to use a different diffuse
texture for the same set colors. This does not seem to be a very common
requirement. With the new design this flexibility disappears since colors
(as factors) and existing maps are reused. This is the common requirement.

> - 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.

Although I agree that normal maps are generated independently of the
lighting model, the use of a particular normal map is strongly coupled to
use of a corresponding set of other maps. I do not see a situation where
you would want to reuse a PhysicalMaterial and its maps with a different
normal map. In fact, reusing a PhysicalMaterial should not require
repeating the use of the same normal map (outside of PhysicalMaterial).

This does not leave much space for Appearance to act in, really only
TextureTransform I believe. This may be appropriate as variations in
TextureTransforms produces variations in the appearance of the same PBR
material.

We may also have to discuss the option to use more than one set of texture
coordinates for the same geometry, for different maps. I think glTF
requires that two UV sets are supported. I think x3dom supports this for
glTF but may not expose it for X3D. Let me check.

Cheers,

Andreas

>
> Sorry for a long train of thought :) Hopefully this is informative.
>
> Regards,
> Michalis



-- 
Andreas Plesch
Waltham, MA 02453
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://web3d.org/pipermail/x3d-public_web3d.org/attachments/20190210/0e390877/attachment-0001.html>