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KHR_materials_specular

Contributors

Copyright 2018-2021 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of The Khronos Group Inc. See Appendix for full Khronos Copyright Statement.

Status

Complete, Ratified by the Khronos Group

Supersedes KHR_materials_pbrSpecularGlossiness

Dependencies

Written against the glTF 2.0 spec.

Exclusions

  • This extension must not be used on a material that also uses KHR_materials_pbrSpecularGlossiness.
  • This extension must not be used on a material that also uses KHR_materials_unlit.

Overview

This extension adds two parameters to the metallic-roughness material: specular and specularColor.

specular allows users to configure the strength of the specular reflection in the dielectric BRDF. A value of zero disables the specular reflection, resulting in a pure diffuse material. The metal BRDF is not affected by the parameter.

specularColor changes the F0 color of the specular reflection in the dielectric BRDF, allowing artists to use effects known from the specular-glossiness material (KHR_materials_pbrSpecularGlossiness) in the metallic-roughness material.

Extending Materials

The strength of the specular reflection is defined by adding the KHR_materials_specular extension to any glTF material.

{
    "materials": [
        {
            "extensions": {
                "KHR_materials_specular": {
                    "specularFactor": 1.0,
                    "specularColorFactor": [1.0, 1.0, 1.0],
                }
            }
        }
    ]
}

Factor and texture are combined by multiplication to describe a single value.

Type Description Required
specularFactor number The strength of the specular reflection. No, default: 1.0
specularTexture textureInfo A texture that defines the strength of the specular reflection, stored in the alpha (A) channel. This will be multiplied by specularFactor. No
specularColorFactor number[3] The F0 color of the specular reflection (linear RGB). No, default: [1.0, 1.0, 1.0]
specularColorTexture textureInfo A texture that defines the F0 color of the specular reflection, stored in the RGB channels and encoded in sRGB. This texture will be multiplied by specularColorFactor. No

The specular and specularColor parameters affect the dielectric_brdf of the glTF 2.0 metallic-roughness material.

dielectric_brdf =
  fresnel_mix(
    f0_color = specularColor.rgb,
    ior = 1.5,
    weight = specular,
    base = diffuse_brdf(color = baseColor),
    layer = specular_brdf(α = roughness^2))

The fresnel_mix function mixes two BSDFs according to a Fresnel term. The layer is weighted with weight * fresnel(ior, f0_color). The base is weighted with 1 - weight * max_value(fresnel(ior, f0_color)).

The specular factor used as weight scales layer and base. The less energy is reflected by the layer (specular_brdf), the more can be shifted to the base (diffuse_brdf). The following image shows specular factor increasing from 0 to 1.

The specular color is a directional-dependent weight included in the Fresnel term. At normal incidence (f0), specularColor scales the F0 reflectance f0_color. At grazing incidence (f90), the reflectance remains at 1. In between the scale factor is smoothly interpolated.

As with specular factor, base will be weighted with the directional-dependent remaining energy according to the Fresnel term. f0_color is an RGB color, involving the complementary to specular color. To avoid inverse colors and ensure energy conservation, the RGB color is converted to scalar via max(r, g, b):

function max_value(vec3 color) {
    return max(color.r, color.g, color.b)
}

The following images show specular color increasing from [0,0,0] to [1,1,1] (top) and from [0,0,0] to [1,0,0] (bottom).

The specular color factor is allowed to be set to values greater than [1, 1, 1]. Thus, the reflection amount can go beyond what is determined by the index of refraction (IOR). To still ensure energy conservation, the product of specular color factor, specular color texture, and f0 reflectance from IOR is clamped to 1. Please refer to Implementation for an example on where to place the clamping operation.

Implementation

This section is non-normative.

Appendix B defines the function fresnel_mix. In this extension, we add two additional arguments called weight and f0_color. It scales f0 computed inside the function:

function fresnel_mix(f0_color, ior, weight, base, layer) {
  f0 = ((1-ior)/(1+ior))^2 * f0_color
  f0 = min(f0, float3(1.0))
  fr = f0 + (1 - f0)*(1 - abs(VdotH))^5
  return (1 - weight * max_value(fr)) * base + weight * fr * layer
}

Therefore, the Fresnel term F in the final BRDF of the material changes to

dielectricSpecularF0  = min(0.04 * specularColorFactor * specularColorTexture.rgb, float3(1.0)) *
                        specularFactor * specularTexture.a
dielectricSpecularF90 = specularFactor * specularTexture.a

F0  = lerp(dielectricSpecularF0, baseColor.rgb, metallic)
F90 = lerp(dielectricSpecularF90, 1, metallic)

F = F0 + (F90 - F0) * (1 - VdotH)^5

Note that in dielectricSpecularF0 we clamp the product of specular color and f0 reflectance from IOR (0.04), before multiplying by specular.

In the diffuse component we have to account for the fact that F is now an RGB value.

c_diff = lerp(baseColor.rgb, black, metallic)
diffuse = c_diff / PI
f_diffuse = (1 - max(F.r, F.g, F.b)) * diffuse

Interaction with other extensions

If KHR_materials_ior is used in combination with KHR_materials_specular, the constant 0.04 is replaced by the value computed from the IOR.

dielectricSpecularF0 = min(((ior - outside_ior) / (ior + outside_ior))^2 * specularColorFactor * specularColorTexture.rgb, float3(1.0)) * specularFactor * specularTexture.a
dielectricSpecularF90 = specularFactor * specularTexture.a

outside_ior is typically set to 1.0, the index of refraction of air.

If KHR_materials_transmission is used in combination with KHR_materials_specular, the ratio of transmission and reflection computed from the Fresnel term also depends on dielectricSpecularF0 and dielectricSpecularF90. The following images show a thin, transmissive material.

Specular from 0 to 1:

Specular color from [0,0,0] to [1,1,1] (top) and [0,0,0] to [1,0,0]:

If KHR_materials_transmission and KHR_materials_volume are used in combination with KHR_materials_specular, specular factor and specular color have no effect on the refraction angle. The direction of the refracted light ray is only based on the index of refraction defined in KHR_materials_ior. The ratio of transmission and reflection computed from the Fresnel term still depends on dielectricSpecularF0 and dielectricSpecularF90. The following images show a refractive material.

Specular from 0 to 1:

Specular color from [0,0,0] to [1,1,1] (top) and [0,0,0] to [1,0,0]:

Conversions

Materials with reflectance parameter

Material models that define F0 in terms of reflectance at normal incidence can be converted by encoding the reflectance in the specular color parameters. Typically, the reflectance ranges from 0% to 8%, given as a value in range [0,1], with 0.5 (=4%) being the default. F0 is computed from reflectance in the following way:

dielectricSpecularF0 = 0.08 * reflectance

In contrast, KHR_materials_specular defines a constant factor of 0.04 to compute F0, as this corresponds to glTF's default IOR of 1.5. Therefore, by encoding an additional constant factor of 2 in specularColorFactor, we can convert from reflectance to specular color without any loss.

The following JSON snippets shows the conversion from reflectanceFactor and reflectanceTexture to specularColorFactor and specularColorTexture:

{
    "materials": [
        {
            "extensions": {
                "KHR_materials_specular": {
                    "specularColorFactor": [2 * reflectanceFactor],
                    "specularColorTexture": [reflectanceTexture]
                }
            }
        }
    ]
}

Specular-glossiness materials

Materials that use the specular-glossiness workflow (KHR_materials_pbrSpecularGlossiness) can be converted with help of the KHR_materials_ior. The ior parameter has to be set to 0. In JSON:

{
    "materials": [
        {
            "extensions": {
                "KHR_materials_specular": {
                    "specularColorFactor":
                        [KHR_materials_pbrSpecularGlossiness__specularFactor],
                },
                "KHR_materials_ior": {
                    "ior": 0
                }
            }
        }
    ]
}

This makes it possible to add advanced effects like clearcoat (KHR_materials_clearcoat) and sheen (KHR_materials_sheen) to traditional specular-glossiness materials.

NOTE

As the ior also affects the refraction effect, this conversion is not compatible with volumetric materials (KHR_materials_volume). We do not recommended to use this conversion when creating new materials from scratch.

Why does it work?

There is no clear separation between dielectrics and metals in the specular-glossiness workflow. Thus, it is possible to create materials that do not fall into either of the categories. This doesn't have to be an explicit decision in authoring (although it can be for various artistic styles), it is often just the result of baking several materials into a single texture. Due to anti-aliasing at the borders some texels contain a mix of different material types. This mix may not map to metallic-roughness parameters that are in a realistic range.

We achieve an easy, lossless mapping by treating any specular-glossiness material, even pure metals, as dielectric materials in the metallic-roughness workflow. KHR_materials_ior gives us the means to do so. As the ior determines the upper bound of the specular reflection's strength, we can increase it to its maximum, making it large enough to hold all possible specular-glossiness materials. Looking at the formula to compute F0 from IOR f0 = ((ior - outside_ior) / (ior + outside_ior))^2, we can see that both ior = 0 and ior = inf will result in f0 = 1. As f0 is multiplied by specular color, the value 1 will give us full control over the specular reflection via the specular color.

Schema

Appendix: Full Khronos Copyright Statement

Copyright 2018-2021 The Khronos Group Inc.

Some parts of this Specification are purely informative and do not define requirements necessary for compliance and so are outside the Scope of this Specification. These parts of the Specification are marked as being non-normative, or identified as Implementation Notes.

Where this Specification includes normative references to external documents, only the specifically identified sections and functionality of those external documents are in Scope. Requirements defined by external documents not created by Khronos may contain contributions from non-members of Khronos not covered by the Khronos Intellectual Property Rights Policy.

This specification is protected by copyright laws and contains material proprietary to Khronos. Except as described by these terms, it or any components may not be reproduced, republished, distributed, transmitted, displayed, broadcast or otherwise exploited in any manner without the express prior written permission of Khronos.

This specification has been created under the Khronos Intellectual Property Rights Policy, which is Attachment A of the Khronos Group Membership Agreement available at www.khronos.org/files/member_agreement.pdf. Khronos grants a conditional copyright license to use and reproduce the unmodified specification for any purpose, without fee or royalty, EXCEPT no licenses to any patent, trademark or other intellectual property rights are granted under these terms. Parties desiring to implement the specification and make use of Khronos trademarks in relation to that implementation, and receive reciprocal patent license protection under the Khronos IP Policy must become Adopters and confirm the implementation as conformant under the process defined by Khronos for this specification; see https://www.khronos.org/adopters.

Khronos makes no, and expressly disclaims any, representations or warranties, express or implied, regarding this specification, including, without limitation: merchantability, fitness for a particular purpose, non-infringement of any intellectual property, correctness, accuracy, completeness, timeliness, and reliability. Under no circumstances will Khronos, or any of its Promoters, Contributors or Members, or their respective partners, officers, directors, employees, agents or representatives be liable for any damages, whether direct, indirect, special or consequential damages for lost revenues, lost profits, or otherwise, arising from or in connection with these materials.

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