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KHR_materials_unlit

Contributors

Copyright (C) 2017-2018 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

Dependencies

Written against the glTF 2.0 spec.

Overview

This extension defines an unlit shading model for use in glTF 2.0 materials, as an alternative to the Physically Based Rendering (PBR) shading models provided by the core specification. Three motivating uses cases for unlit materials include:

  • Mobile devices with limited resources, where unlit materials offer a performant alternative to higher-quality shading models.
  • Photogrammetry, in which lighting information is already present and additional lighting should not be applied.
  • Stylized materials (such as "anime" or "hand-drawn" looks) in which lighting is undesirable for aesthetic reasons.

These use cases are not mutually exclusive: artists may pick an unlit material for performance reasons, and make aesthetic decisions to complement that choice. As a result, client implementations capable of rendering PBR should not automatically "upgrade" to fully-shaded PBR. Any core PBR properties specified on an unlit material (except baseColor) are meant only as fallbacks for clients that do not support the KHR_materials_unlit extension. The extension, whether required or optional within the asset, indicates preference for an unlit visual style.

Extending Materials

The common Unlit material is defined by adding the KHR_materials_unlit extension to any glTF material. When present, the extension indicates that a material should be unlit and use available baseColor values, alpha values, and vertex colors while ignoring all properties of the default PBR model related to lighting or color. Alpha coverage and doubleSided still apply to unlit materials.

{
    "materials": [
        {
            "name": "MyUnlitMaterial",
            "pbrMetallicRoughness": {
                "baseColorFactor": [ 0.5, 0.8, 0.0, 1.0 ]
            },
            "extensions": {
                "KHR_materials_unlit": {}
            }
        }
    ]
}

Definition

The Unlit material model describes a constantly shaded surface that is independent of lighting. The material is defined only by properties already present in the glTF 2.0 material specification. No new properties are added by this extension — it is effectively a boolean flag indicating use of an unlit shading model. Additional properties on the extension object are allowed, but may lead to undefined behaviour in conforming viewers.

Color is calculated as:

color = <baseColorTerm>

<baseColorTerm> is the product of baseColorFactor, baseColorTexture, and vertex color (if any), as defined by the core glTF material specification.

Example

The following example defines an unlit material with a constant red color.

"materials": [
    {
        "name": "red_unlit_material",
        "pbrMetallicRoughness": {
            "baseColorFactor": [ 1.0, 0.0, 0.0, 1.0 ]
        },
        "extensions": {
            "KHR_materials_unlit": {}
        }
    }
]

Extension compatibility and fallback materials

When possible, authoring tools should provide a fallback material definition and mark the KHR_materials_unlit extension as optional. Models including the extension optionally will still render in all clients that support the core glTF 2.0 specification, falling back to a PBR metal/rough material. In clients that do not support the extension, direct lighting will be applied and performance may be degraded.

Defining a fallback to the default PBR shading model is achieved by setting appropriate parameters on the default material.

"materials": [
    {
        "name": "unlit_with_fallback",
        "pbrMetallicRoughness": {
            "baseColorFactor": [ 1.0, 1.0, 1.0, 1.0 ],
            "baseColorTexture": { "index": 0 },
            "roughnessFactor": 0.9,
            "metallicFactor": 0.0
        },
        "extensions": {
            "KHR_materials_unlit": {}
        }
    }
]

Implementation Note: For best fallback behavior in clients that do not implement the KHR_materials_unlit extension, authoring tools may use:

  • metallicFactor is 0 and emissiveFactor is [0, 0, 0].
  • roughnessFactor is greater than 0.5.
  • Omit metallicRoughnessTexture, occlusionTexture, emissiveTexture, and normalTexture.

Implementation Note: When KHR_materials_unlit is included with another extension specifying a shading model on the same material, the result is undefined.

Appendix: Full Khronos Copyright Statement

Copyright 2017-2018 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.