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README.md

KHR_materials_ior

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

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

The dielectric BRDF of the metallic-roughness material in glTF uses a fixed value of 1.5 for the index of refraction. This is a good fit for many plastics and glass, but not for other materials like water or asphalt, sapphire or diamond. This extension allows users to set the index of refraction to a certain value.

Extending Materials

The index of refraction of a material is configured by adding the KHR_materials_ior extension to any glTF material.

{
    "materials": [
        {
            "extensions": {
                "KHR_materials_ior": {
                    "ior": 1.4,
                }
            }
        }
    ]
}
Type Description Required
ior number The index of refraction. No, default: 1.5

Typical values for the index of refraction range from 1 to 2. In rare cases values greater than 2 are possible. The following table shows some materials and their index of refraction:

Material Index of Refraction
Air 1.0
Water 1.33
Eyes 1.38
Window Glass 1.52
Sapphire 1.76
Diamond 2.42

The ior parameter affects the dielectric_brdf of the glTF 2.0 metallic-roughness material. The new dielectric_brdf that includes the ior parameter is given as follows:

dielectric_brdf =
  fresnel_mix(
    ior = ior,
    base = diffuse_brdf(
      color = baseColor),
    layer = specular_brdf(
      α = roughness^2))

Valid values for ior are numbers greater than or equal to one. As a special case, a value of zero is allowed as described below.

Specular-Glossiness Backwards Compatibility Mode

Setting IOR to zero permanently switches the material into a special specular-glossiness backwards compatibility mode designed to ease content transition from the legacy specular-glossiness model (previously available via KHR_materials_pbrSpecularGlossiness extension) to the glTF 2.0 core metallic-roughness PBR model.

This mode has the following implications:

  • The effective IOR becomes positive infinity and the Fresnel term MUST evaluate to 1.0 independently of the view or light direction.

  • All material features MUST treat IOR as having a very large value representing positive infinity, subject to numerical precision. For example, this would cause the dispersion property (as defined in KHR_materials_dispersion) to have no effect on the material appearance.

  • This mode cannot be toggled dynamically, e.g., with KHR_animation_pointer or KHR_interactivity extensions. If the IOR property is set to zero in JSON, glTF Asset Object Model updates of it MUST be ignored.

  • A value of zero (as well as any other value less than one) MUST NOT be used in an animation sampler targeting the IOR property, even if the IOR is set to zero in glTF JSON.

Implementation

This section is non-normative

The extension changes the computation of the Fresnel term defined in Appendix B to the following:

dielectric_f0 = ((ior - 1)/(ior + 1))^2

Note that for the default index of refraction ior = 1.5 this term evaluates to dielectric_f0 = 0.04.

Interaction with other extensions

If KHR_materials_specular is used in combination with KHR_materials_ior, the Fresnel term is additionally scaled by the values given specular and specularColor. See KHR_materials_specular for more information.

If KHR_materials_transmission is used in combination with KHR_materials_ior, the ior affects the strength of the transmission effect according to the Fresnel term. See KHR_materials_transmission for more information. If KHR_materials_specular is used in addition, the Fresnel term is additionally scaled by the values given in specular and specularColor.

If KHR_materials_volume is used in combination with KHR_materials_ior, the ior affects the refraction effect. See KHR_materials_volume for more information.

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.

Vulkan is a registered trademark and Khronos, OpenXR, SPIR, SPIR-V, SYCL, WebGL, WebCL, OpenVX, OpenVG, EGL, COLLADA, glTF, NNEF, OpenKODE, OpenKCAM, StreamInput, OpenWF, OpenSL ES, OpenMAX, OpenMAX AL, OpenMAX IL, OpenMAX DL, OpenML and DevU are trademarks of The Khronos Group Inc. ASTC is a trademark of ARM Holdings PLC, OpenCL is a trademark of Apple Inc. and OpenGL and OpenML are registered trademarks and the OpenGL ES and OpenGL SC logos are trademarks of Silicon Graphics International used under license by Khronos. All other product names, trademarks, and/or company names are used solely for identification and belong to their respective owners.