Update behavior of specular_weight, specular_ior_level, coat_ior_level #157

portsmouth · 2024-02-09T17:53:37Z

This implements the changes proposed in

which we agreed to go ahead with in the recent meetings.

The specular_ior_level param is removed, and replaced by specular_weight which works in the same way. I incorporated the facility to increase the specular_weight to values >1, though with a soft-max at 1 -- retaining the facility we had before, though actually more flexible since it doesn't get capped at an arbitrary maximum value of 2 (it just gets capped by the requirement that the Fresnel factor cannot exceed 1). This seems reasonable and potentially convenient (though we may want to verify that it behaves OK in practice).

The specular_weight is now no longer involved in the Fresnel factor multiplier, only specular_color. We state that this is applied only to one specific scattering mode (the initial Fresnel reflection from above), and call it out explicitly as an unphysical convenience.

The coat_ior_level param is also removed, simplifying the coat discussion:

…or_level.

portsmouth · 2024-02-09T18:00:22Z

Corresponding changes will be needed in the MaterialX graph (and our native implementations). I'll leave those for a separate PR though.

virtualzavie

Nice.
With this new parametrisation, I believe we need to remove specular_weight from the metal section.

index.html

portsmouth · 2024-02-20T15:09:19Z

Nice. With this new parametrisation, I believe we need to remove specular_weight from the metal section.

We could do, as it functions currently (for the metal) only as a multiplier of the specular_color edge tint, which is probably not very important to retain, as the specular_color covers the edge-tint perfectly well. Arguably also it's helpful to remove it since if doing e.g. a rusty metal with varying metalness, if the specular_weight is textured it's inconvenient for that to affect the metallic parts and dielectric parts in a different way.

This means we would remove the instances of specular_weight outlined here:

…ular_weight. Disallow specular_weight > 1.

portsmouth · 2024-02-21T14:17:14Z

In bffd9e1 I changed as follows:

Do not allow specular_weight > 1. This seems preferable, as otherwise it will generally cause the Fresnel to saturate at 1 producing an unnatural/discontinuous looking band of white at grazing edges, which I think it is best to prevent. It is also simpler to just think of the specular_weight as a suppression factor of the reflectivity in $[0,1]$.
Have the metal Fresnel multiplied by an overall factor of specular_weight (again in $[0,1]$). This was important to stipulate, as otherwise (as Julien pointed out on Slack), there was no way to avoid the white band at metallic edges as Fresnel always saturates to 1 at grazing (even with black base and specular color), unless suppressed by the weight. In this case, the suppression is "unphysical" (though applied to the Schlick curve which itself is only loosely physically based).

A more physical scheme would amount to somehow lowering the IOR of the metal to 1, as in the dielectric case, producing a narrowing of the Fresnel highlight rather than a suppression, but this is not doable in the standard Schlick representation, so the linear scaling with the weight seems likely the best approach. (Though note that the $F_0$ does actually scale the same way with specular weight, for both metal and dielectric).

…_color_changes # Conflicts: # index.html

…ssion also.

…penPBR into specular_weight_and_color_changes # Conflicts: # index.html

virtualzavie

While it is a good change overall, there are two propositions I'm not comfortable with:

specular_weight affecting the metal lobe, as I strongly suspect it will yield inconsistent behaviour.
specular_weight limited to 1, thus preventing from locally increasing specular reflection. I think a use case would be gemstones on a character outfit. There is also the problem of converting existing assets, given that a modulation up to 2 has been common in several major tools for about a decade.

index.html

virtualzavie · 2024-04-09T00:49:40Z

index.html

 \end{equation}
-This formulation has the useful property that it reduces to the regular Schlick reflectivity at the default values of $\mathtt{specular\_weight}$ and $\mathtt{specular\_color}$. Note that the edge cannot be brighter than the standard Schlick term, but this is generally true in real metals. We consider this a benefit of this parametrization, as it makes it impossible to produce physically implausible metals with excessively bright edges.
+The final metallic Fresnel term we employ is then given by an overall multiplication by **`specular_weight`**, ensuring that entire metallic lobe is suppressed as the weight goes to zero:


I'm concerned about the implications of this description.
Suppose an artist makes a material with a combination of dielectric and metal. Then after the fact, they want to tune the specular reflection with a uniform specular_weight. They are not going to observe the same behaviour on the dielectric and metal parts, which I think could be questionable.

I should also note, given the recent discussions on the metal-rough model, that there is a bit of inconsistency in how the metal layer is parametrised: other layers have a weight that acts as a coverage parameter. One could argue metalness is conceptually close to that role.

More discussion is needed here, both short and longer term.

Suppose an artist makes a material with a combination of dielectric and metal. Then after the fact, they way to tune the specular reflection with a uniform specular_weight. They are not going to observe the same behaviour on the dielectric and metal parts, which I think could be questionable.

I think actually it's more sensible now than it was before, since now the specular_weight directly dials the normal-incidence reflectance (i.e. F0) of both the dielectric and the metal.

Before, we had it only affecting the F82-tint color of the metal, but not the F0. I found that more inconsistent, since the F0 is the dominant effect. (Also, it didn't allow the metal lobe to be zeroed, as you noticed, so we have to have the weight multiply the entire lobe instead, which is more consistent with the dielectric F0 anyway).

The specular_weight multiplies the metallic lobe though, rather than operating via the IOR, so the effect is of course slightly different (suppression at grazing angles, rather than "tightening of the white highlight"). But we don't have an IOR control for the metal, so it's not possible to make them match to that extent.

I should also note, given the recent discussions on the metal-rough model, that there is a bit of inconsistency in how the metal layer is parametrised: other layers have a weight that acts as a coverage parameter. One could argue metalness is conceptually close to that role.

No, the metal is a component of the base, mixed with the "non-metallic base", according to the base_metalness. So base_metalness operates as a mix weight, not a coverage weight. Similarly for transmission_weight and subsurface_weight.

A coverage weight only makes sense for a layer which can optionally "cover" another, i.e. the coat and fuzz layers.

Note that we also decided against trying to interpret specular_weight as the coverage weight of the diffuse base with dielectric (since the dielectric is not really functioning as a layer, it is really the base substrate in which the scattering media of the transmission/subsurface/glossy-diffuse slabs are embedded). Instead, specular_weight operates by modulating the IOR of the unified base dielectric.

Then after the fact, they way to tune the specular reflection with a uniform specular_weight. They are not going to observe the same behaviour on the dielectric and metal parts, which I think could be questionable.

I see what you mean though, if they have a textured metalness, then the single specular_weight affects both the metal and dielectric parts (now in roughly the same way -- by multiplying the F0), which maybe they don't want. They'd have to modulate the weight by the metalness map.

This goes back to our discussions (on Slack and in #140) about having a separate optional parametrization for metal. If you really want this use case of e.g. metal and non-metal bits on the same object, you have to deal with blending of various maps to get that look, and then get issues in the transition regions because you are trying to represent a 50/50 mix of metal and dielectric with blended maps driving both the metal and dielectric BRDFs, instead of independent maps for the two BRDFs.

A solution is something like the proposed additional metal params in #140 (comment). But that adds a fair bit of complexity to our model. My compromise approach is to accept that these issues are real, but treat this as a limitation of our model that we traded off for ease of use and simplicity. I would even propose to say something in the spec about textured metalness being problematic, and that we really intended metalness to be more like a binary selector over the entire surface, or very smoothly varying, and not something which is allowed to have harsh transitions (though I know that in reality, people certainly use metalness to represent areas of metal/dielectric "coat" or "substrate" living in a background of dielectric/metal).

For better quality results, people can always do more elaborate things like use MaterialX to add layers of dielectric stuff on top of OpenPBR metal, say. (To model rust more correctly for example, physically it should be a layer of semi-opaque rust on top of a substrate metal -- which is just not representable in OpenPBR since our only layers are an absorbing dielectric coat and fuzz).

Yes, I agree this is an improvement over the previous proposition, and I don't see a better alternative.
My concern is whether this could become a subject of complain, and if it does, can we still make a better proposal in a future without being locked by this decision.

That being said, if in the future we decide to go toward separate metal parameters, then this one parameter would likely be overridden for metal anyway, so I guess we're safe.

Co-authored-by: Julien Guertault <9511025+virtualzavie@users.noreply.github.com> Signed-off-by: Jamie Portsmouth <jamports@mac.com>

portsmouth · 2024-04-09T21:07:31Z

specular_weight limited to 1, thus preventing from locally increasing specular reflection. I think a use case would be gemstones on a character outfit. There is also the problem of converting existing assets, given that a modulation up to 2 has been common in several major tools for about a decade.

We've discussed this a fair bit before. I think it would be fine to allow specular_weight to exceed 1 (make it a soft-max at 1, say). We would just need to specify appropriate clamping logic (for both metal and dielectric Fresnel) since the resulting F0 cannot be allowed to exceed 1.

I find the proposed use-cases a bit unconvincing though, as if you're doing gemstones on a costume you could just set the IOR to that of the gemstones, and lower the specular_weight accordingly for the non-gem bits. Or just paint the actual IOR, in a tool where you are not restricted to [0,1] maps.

Also, above I noted:

In the latest commit, I removed that.. [specular_weight > 1] (for both dielectric and metal). As I think it generally means that the Fresnel will at some angle near grazing end up saturating to 1 with a C1 discontinuity, which I would expect to look a bit unnatural (as real Fresnel doesn't saturate like this, except when TIR happens).

Which is true, e.g. if at specular_weight = 1 the F0=0.75, then at specular_weight=1.333 the IOR will saturate to infinity (to achieve F0=1, according to Equation 26 below, assuming we added a clamp so that the thing inside the square root is $\le 1$). So then varying specular_weight > 1.333 does nothing, the Fresnel curve is just maxed at 1. That seems a bit weird.

jstone-lucasfilm · 2024-04-11T14:45:04Z

Once we're happy with these proposed changes to the specification, I'd recommend that we include the matching updates to the reference implementation in this same pull request. This should allow us to validate the new user controls before merging, and will also insure that our specification and reference implementation remain in sync as we approach a 1.0 release.

virtualzavie · 2024-04-22T11:16:51Z

index.html


-The Fresnel factor $F(\omega_i, h)$ is determined by the index of refraction of the reflecting material, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].
+The Fresnel factor $F(\omega_i, h)$ is determined by the complex index of refraction of the reflecting material of each microfacet, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].


Why is "masking-shadowing function" emphasized, but the other factors are not?

Fixed up in e6a15d3

virtualzavie · 2024-04-22T11:17:32Z

index.html


-The Fresnel factor $F(\omega_i, h)$ is determined by the index of refraction of the reflecting material, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].
+The Fresnel factor $F(\omega_i, h)$ is determined by the complex index of refraction of the reflecting material of each microfacet, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].


Suggestion: have F, G and D in 3 separate paragraphs.

Suggested change

The Fresnel factor $F(\omega_i, h)$ is determined by the complex index of refraction of the reflecting material of each microfacet, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].

The Fresnel factor $F(\omega_i, h)$ is determined by the complex index of refraction of the reflecting material of each microfacet, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section.

The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].

Done in ec47ee0.

virtualzavie · 2024-04-22T11:23:58Z

index.html


- - The specular lobe shape is controlled by the roughness properties of the surface, parametrized by **`specular_roughness`**, **`specular_anisotropy`**, and **`specular_rotation`** (see the section on the [Microfacet model](index.html#model/microfacetmodel) NDF).
+- The **`specular_ior`** parameter controls the index of refraction (IOR) of the dielectric. The **`specular_weight`** parameter provides a convenient, texturable linear $[0, 1]$ multiplier of the dielectric reflectivity at normal incidence via modulation of this reference IOR. When **`specular_weight`** is $0$, the specular reflection disappears entirely, as the IOR of the dielectric is then equal to that of the surrounding medium.


Note to ourselves: this is where we want to briefly define the behaviour above 1 (how we prevent non sensical values would probably be better explained with equation modulated_ior).

Added changes to explicitly allow specular_weight > 1 in 4361177 and cceeb42

Great, thank you.

virtualzavie · 2024-04-22T11:31:14Z

index.html

-**`specular_weight`**     | Weight     | `float`  | $ [0, 1]      $ |            | $ 1         $ | Scalar multiplier to the dielectric Fresnel factor
+**`specular_weight`**     | Weight     | `float`  | $ [0, 1] $      |            | $ 1         $ | Modulates the dielectric reflectivity at normal incidence
 **`specular_color`**      | Color      | `color3` | $ [0, 1]^3    $ |            | $ (1, 1, 1) $ | Tints the dielectric Fresnel factor


Bit of nitpicking: there's an inconsistency in that specular_weight and specular_color are described by verbs when other parameters are described by nouns.

I don't find it a problem personally.. They are just descriptions, with slightly different phrasing (it's a scalar multiplier, versus it modulates.. etc. Both work).

virtualzavie · 2024-04-22T12:01:10Z

index.html

-where the range of $\xi_c$ is clamped according to $\xi_c \in [0, \mathrm{min}(1, 1/(2 F_c)]$. Inserting this modulated IOR ratio $\eta^\prime_c$ in the coat Fresnel factor then produces the desired reflectivity modulation.
-
-If there is a fractional $\mathtt{coat\_weight}$ $\mathtt{C}$, then the surrounding IOR of the base dielectric or metal varies statistically across the surface depending on whether the coat is locally present (the fuzz layer can be assumed to have the ambient IOR $n_\mathrm{ambient}$). A reasonable approximation of this is to take the surrounding effective coat IOR to be a linear blend [^lerp] between the ambient and modulated coat IORs according to the coat weight:
+The IOR of the coat medium $V_\mathrm{coat}$, controlled by the **`coat_ior`** parameter, will alter the Fresnel factor of both the coat top interface and the underlying metal or dielectric. If there is a fractional $\mathtt{coat\_weight}$ $\mathtt{C}$, then the surrounding IOR of the base dielectric or metal varies statistically across the surface depending on whether the coat is locally present (the fuzz layer can be assumed to have the ambient IOR $n_\mathrm{ambient}$). A reasonable approximation of this is to take the surrounding effective coat IOR to be a linear blend [^lerp] between the ambient and coat IORs according to the coat weight:


Nitpicking: do we know how far the proposed approximation is from ground truth? I'm wondering if "reasonable approximation" is a fair description, or if we might want to use a different adjective like "acceptable".

Actually, the stated approximation is very bad indeed.. Thanks for drawing attention to this.

For example, consider if the ambient IOR is 1, the coat_ior = 2, and specular_ior = 1.5. Then coat weight = 0.5 would (according to this approximation) lead to no base specular reflection (since the "effective" coat IOR is 1.5, same as the base dielectric), i.e. $\eta_s = 1$. But obviously in reality, there is a pretty strong specular reflection (from both the coat boundary $\eta_\mathrm{ca}=2$, and specular boundary $\eta_\mathrm{sa}=1.5$).

Really the integrator should statistically mix between the coated and uncoated configurations. While still approximate, a clearly better formulation is to take:

$$ \eta_s = \mathrm{lerp}(n_\mathrm{specular}/n_\mathrm{ambient}, n_\mathrm{specular}/n_\mathrm{coat}, \mathtt{C}) $$

This produces $\eta_s = 0.5 * (2.0 + 1.5) = 1.75$, which is a lot better.

I've added that in a1c4d90:

virtualzavie

The current state of the PR looks good to me.

…m/portsmouth/OpenPBR into specular_weight_and_color_changes

portsmouth · 2024-04-27T20:45:46Z

In 0e9560a I updated the MaterialX graph to remove specular_ior_level and coat_ior_level, and implemented the various changes required to be consistent with the updates to the spec of this PR.

I verified that this compiles in the MaterialX viewer, and seems to work correctly.

jstone-lucasfilm · 2024-04-28T18:05:37Z

This looks great, thanks @portsmouth, and my only suggestion is to double-check that @AntonPalmqvist's example materials render correctly using this new parameterization.

jstone-lucasfilm · 2024-04-28T18:19:18Z

@portsmouth I've looked at each of the example materials, and the only unusual result I see is that open_pbr_honey.mtlx seems to have lost its transmission color, and now looks more like glass. Let's consider this an issue to address in a future change, though, and I think it's fine to move forward with the current pull request.

portsmouth added 2 commits February 9, 2024 17:42

Changes to description of specular_weight, specular_ior_level, coat_i…

fcfa75c

…or_level.

Delete remaining coat_ior_level reference.

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portsmouth mentioned this pull request Feb 12, 2024

Should non-physical tinting affect lower layers? #81

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Making roughness ranges all [0,1] #160

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Fix up the metal Fresnel to be modulated by an overall factor of spec…

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…ular_weight. Disallow specular_weight > 1.

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Fix descriptions of metal parameters in table

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Merge remote-tracking branch 'upstream/main' into specular_weight_and…

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Clarify that specular_weight IOR modulation should affect the transmi…

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…ssion also.

portsmouth mentioned this pull request Apr 2, 2024

Suggest separate parameters for diffuse and metal colours instead of "base_color" #140

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Merge branch 'main' of https://github.com/AcademySoftwareFoundation/O…

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Co-authored-by: Julien Guertault <9511025+virtualzavie@users.noreply.github.com> Signed-off-by: Jamie Portsmouth <jamports@mac.com>

Merge branch 'main' into specular_weight_and_color_changes

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Add coat darkening and saturation effects #162

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portsmouth added 5 commits April 26, 2024 17:42

Fix up emphasis in Microfacet Model section

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Break into paragraphs.

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Add appropriate wording to allow specular_weight to exceed 1.

4361177

Update range of specular_weight, to allow for soft-max at 1

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…m/portsmouth/OpenPBR into specular_weight_and_color_changes

Update MaterialX graph to match the corresponding changes in the spec

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jstone-lucasfilm changed the title ~~Changes to description of specular_weight, specular_ior_level, coat_ior_level~~ Update behavior of specular_weight, specular_ior_level, coat_ior_level Apr 28, 2024

jstone-lucasfilm merged commit 0f3eea8 into AcademySoftwareFoundation:main Apr 28, 2024
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Purpose of base_weight and specular_weight? #76

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Update behavior of specular_weight, specular_ior_level, coat_ior_level #157

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		The Fresnel factor $F(\omega_i, h)$ is determined by the index of refraction of the reflecting material, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].
		The Fresnel factor $F(\omega_i, h)$ is determined by the complex index of refraction of the reflecting material of each microfacet, and its form differs depending on whether the material is a dielectric or conductor [#Walter2007]. Its parametrization in each case is described in the Dielectric base section and Metal section. The _masking-shadowing function_ $G(\omega_i, \omega_o)$ accounts for the probability that the input and output directions are occluded by the microsurface. It is usually derived using the Smith model which determines $G$ given the NDF, and for the GGX NDF equation [GGX] the masking-shadowing function then has a well-known form [#Heitz2014b].


		- The specular lobe shape is controlled by the roughness properties of the surface, parametrized by `specular_roughness`, `specular_anisotropy`, and `specular_rotation` (see the section on the [Microfacet model](index.html#model/microfacetmodel) NDF).
		- The `specular_ior` parameter controls the index of refraction (IOR) of the dielectric. The `specular_weight` parameter provides a convenient, texturable linear $[0, 1]$ multiplier of the dielectric reflectivity at normal incidence via modulation of this reference IOR. When `specular_weight` is $0$, the specular reflection disappears entirely, as the IOR of the dielectric is then equal to that of the surrounding medium.

Update behavior of specular_weight, specular_ior_level, coat_ior_level #157

Update behavior of specular_weight, specular_ior_level, coat_ior_level #157

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